Early Human Development 90 (2014) 755–760
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Secular trends in newborn sex ratios Victor Grech ⁎ Academic Department of Paediatrics, Medical School, Mater Dei Hospital, Malta
a r t i c l e
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Keywords: Birth rate/*trends Sex ratio Chernobyl nuclear accident Infant, newborn Radiation, ionizing Toxins, biological
a b s t r a c t A wide variety of factors have been shown to influence the male to female ratio at birth, which invariably displays a male excess. This paper will review and amplify recent work by the author, with specific references to individual countries, regions and entire continents in order to provide a global overview of this subject. It will be shown that stress, including stress related to political events, influences this ratio. Man-made radiation is also shown to have played a significant role in relation to the Windscale fire (1957) and Chernobyl (1986). © 2014 Elsevier Ireland Ltd. All rights reserved.
Contents 1. 2. 3. 4. 5. 6.
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Introduction . . . . . . . . . . . . . . . . . . . Definitions . . . . . . . . . . . . . . . . . . . Historical aspects . . . . . . . . . . . . . . . . Broad epidemiological aspects of human M/F . . . . M/F physiology . . . . . . . . . . . . . . . . . Factors known to influence M/F . . . . . . . . . . 6.1. Hormones . . . . . . . . . . . . . . . . 6.2. Races . . . . . . . . . . . . . . . . . . 6.3. Warfare . . . . . . . . . . . . . . . . . 6.4. Stress . . . . . . . . . . . . . . . . . . 6.5. Environmental toxins and occupations . . . Continent-wide secular trends and latitude gradients 7.1. Latitude gradients in M/F . . . . . . . . . More recent work . . . . . . . . . . . . . . . . 8.1. Data sources . . . . . . . . . . . . . . . 8.2. Statistics . . . . . . . . . . . . . . . . . The United Kingdom . . . . . . . . . . . . . . . Germany . . . . . . . . . . . . . . . . . . . . Yugoslavia . . . . . . . . . . . . . . . . . . . Czechoslovakia and the post-Czechoslovakian states . Scandinavia . . . . . . . . . . . . . . . . . . . The former USSR, Europe and Chernobyl . . . . . . The Middle East: Israel, Egypt and Kuwait . . . . . South America . . . . . . . . . . . . . . . . . . 16.1. Chile . . . . . . . . . . . . . . . . . . . 16.2. Argentina . . . . . . . . . . . . . . . . 16.3. Brazil . . . . . . . . . . . . . . . . . . Australasia . . . . . . . . . . . . . . . . . . . Asia . . . . . . . . . . . . . . . . . . . . . . Cuba . . . . . . . . . . . . . . . . . . . . . .
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⁎ Corresponding author at: Department of Paediatrics, Mater Dei Hospital, Malta. Tel.: +356 99495813. E-mail address: [email protected].
http://dx.doi.org/10.1016/j.earlhumdev.2014.08.018 0378-3782/© 2014 Elsevier Ireland Ltd. All rights reserved.
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20. Conclusion . . . . . . Conflict of interest statement Acknowledgements . . . . . References . . . . . . . . .
V. Grech / Early Human Development 90 (2014) 755–760
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1. Introduction The male to female ratio at birth invariably yields an excess of males. However, this ratio has been shown to be influenced by a very wide number of factors. This review will enlarge on recent work by the author and will not only provide a general history of and an introduction to this topic, but also provide a global overview. It will also be shown that stress related to political events has influenced this ratio, as has radiation from accidents including nuclear facilities. 2. Definitions The male to female ratio at birth (technically known as the secondary sex ratio) is commonly abbreviated as M/F, a potentially misleading term as this defines the ratio of male to total births (M/T). M/F will be used in this paper, denoting male live births divided by total live births. 3. Historical aspects In ancient times, it was widely believed that an infant's gender was determined by the degree of heat that a man's ejaculate was exposed to during insemination. It was only until much later that formal analysis of M/F was undertaken. Such a statistical study requires not only raw data but also statistical tools for calculations that provide probabilities of deviation from preset values. The collection of data from London in the 1600s allowed John Graunt (1620–74) to publish the first descriptive statistical analysis of M/F data [1]. Graunt's work included an analysis of annual variation of M/F in London and Romsey. He noted that male births exceeded female births and that this excess was greater in urban London than in rural Romsey. His findings were statistically non-significant but he noted secular variation in M/F which is significant with modern day testing. John Arbuthnott (1667–1735) was a mathematics teacher in London who went on to study medicine [2]. He demonstrated that M/F is significantly in excess of 0.5, the first use of inferential statistics [2]. Ronald Fisher (1890–1962) was an English statistician, evolutionary biologist, geneticist, and eugenicist who popularised the theories of Carl Düsing of Jena [3]. The Fisherian explanation for the skew in M/F is that were male births less common than female births, a male would have better mating prospects and would sire more offspring. Thus, parents genetically disposed to produce males would have more offspring and this tendency would spread within the community, increasing male births, such that this advantage disappears when M/F of 0.5 is reached. The converse would apply were there a dearth of females [4]. These studies comprised the first application of mathematical methods and models to evolutionary biology. 4. Broad epidemiological aspects of human M/F Random meiosis would lead to a mean (Mendelian) M/F of 0.5, with binomial variation around this value. However, this is based on the following assumptions: 1. Males produce equal numbers of X- and Y-bearing sperm in mammalian species. 2. X- and Y-bearing sperm stand equal chances of achieving conception. 3. Equal numbers of male and female zygotes are conceived.
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Thus, any M/F variation would be due to sex-selective foetal wastage. In humans, M/F exhibits a male excess and is expected to approximate 0.515 with a range of 0.505 to 0.520 [5]. The excess of male births may be nature's compensatory mechanism for increased postnatal male mortality. A veritable legion of factors has been proposed for this disparity potentially influencing it [5,6]. 5. M/F physiology Evolutionary theory proposes that mutations may produce individuals who are fitter in a given environment and who are therefore likelier to survive and procreate, dispersing their advantageous genes. One such adaptation could be the maternal ability to influence M/F outcomes in pregnancy. In polygynous species, only the fittest males reproduce. For this reason, parental investment in a “good quality” son would, on average, yield greater numbers of descendants than an equivalent investment in a “good quality” daughter. It may therefore be advantageous for a mother to produce sons when she has sufficient resources to give them a better than average edge that will then give them a greater chance to reproduce, and daughters when she does not have. This is known as the Trivers–Willard hypothesis [7]. Recent studies have revealed that around 73% of natural singleton conceptions fail to survive beyond six weeks of gestation. Pregnancy is thus an opportunity for selection and/or culling, and significant wastage occurs before maternal or clinical recognition of pregnancy. It is also believed that multiple pregnancies may constitute over 12% of all natural conceptions but only approximately two percent reach term as live twin births, and 12% of these result in single births [8]. The sex ratio at conception in humans may be 0.545, with the highest sex ratio of foetal deaths in the second trimester. This data also suggests that late foetal deaths may be postponed to early infancy [9]. Male vulnerability is also manifest in premature births, as well as in term babies, with higher morbidity and mortality rates that persevere into early childhood. Women who fail to abort male foetuses in times of stress also reduce their own odds of survival due to the higher metabolic requirements necessitated by the gestation of a male baby to term. Conversely, a female who aborts a male baby under stressful circumstances fails to invest heavily in what would potentially result in a frail son, and makes herself available to potentially bear a daughter, or a robust son in future and less adverse times [10]. 6. Factors known to influence M/F The literature is replete with such factors [11], and the more important ones are listed hereunder. 6.1. Hormones While the physiological basis for the influences of external factors on M/F is not understood, alterations in parental sex hormone level/s and/ or differential gender-based survival modulated by stress during embryogenesis have been proposed as likely mediators. The hormonal theory is heavily subscribed to by William H. James, the foremost expert in the field. This theory states that higher levels of maternal gonadotrophins and progesterones lower M/F while elevated levels of testosterone and oestrogen increase M/F [12]. The luteal surge in the middle of the menstrual cycle has therefore been proposed to be the cause of
V. Grech / Early Human Development 90 (2014) 755–760
the excess conception of females noted in the fertile part of the cycle [13,14]. Older mothers tend to have a lower M/F and it has been speculated that this could be due to their overall higher levels of circulating gonadotrophins [15]. 6.2. Races Population differences have also been documented. The male excess has been historically shown to be significantly less in Black populations when compared to Caucasian populations [16] and significantly higher in Asian populations [17]. 6.3. Warfare Marked increases in M/F in the range of 1–2% have been noted in England, Wales and France during and after the First and Second World Wars [5]. This has been attributed to the coexistence of increased coital frequency and non-programmed copulation, with more conceptions occurring early or late in the menstrual cycle, increasing M/F [15].
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found in North America. The authors were unable to explain these findings, which do not support a latitude related effect [24]. 8. More recent work The next section expands on more recent work done by this author, and will be divided into individual countries, regions and continents. Each section will be self-contained. 8.1. Data sources Annual male and female live births were obtained directly from the WHO Health for All (HFA) database. 8.2. Statistics Excel was used for data entry, overall analysis and charting. The quadratic equations of Fleiss were used for the exact calculation of 95% confidence intervals for ratios [25]. Chi tests and chi tests for trends for male and female births were used throughout the study. A p value ≤ 0.05 was taken to represent a statistically significant result.
6.4. Stress 9. The United Kingdom Under situations of stress, it has been shown that M/F drops [18], and it has been proposed that this accedes to the Trivers–Willard hypothesis [7]. For example, M/F fell in New York City three months after the terrorist attacks of September 11, rather than seven or more months later as would be the case were male conceptions reduced by this event [19]. Similar findings were noted for the same time period following the abovementioned terrorist attack in California, suggesting that witnessing harm befalling on others induces biological responses that resemble those in the persons harmed [20]. 6.5. Environmental toxins and occupations A plethora of environmental toxins have been implicated as affecting M/F with parental exposure and these invariably reduce M/F [5,6]. Radiation is a unique toxin in that it increases M/F while decreasing the total number of births, presumably due to lethal mutations [21]. 7. Continent-wide secular trends and latitude gradients M/F over the second half of the 20th century (127,034,732 North American and 157,947,117 European live births) showed a highly significant overall decline in male births in both Europe and North America (p b 0.0001), particularly in Mexico (p b 0.0001). European countries were banded by latitude. Southern countries (latitude 35–40° N) included Bulgaria, Greece, Italy, Malta, Portugal, and Spain. Central Europe (40–55° N) included Austria, Belgium, Czech Republic, France, Germany, Hungary, Ireland, Luxembourg, Netherlands, Poland, Romania, Switzerland, and the United Kingdom. Nordic countries (N55° N) included Denmark, Finland, Iceland, Norway, and Sweden. In Europe, male births declined in North European countries (latitude N 40° N, p b 0.0001) while rising in Mediterranean countries (latitude 35–40° N, p b 0.0001) [22]. 7.1. Latitude gradients in M/F Analysis of European births showed a much higher ratio of male births in the south of Europe than in the north (p b 0.0001) [23]. The same technique was then applied to the North American continent which was divided by latitude into Canada (N50° N), the United States (30–50° N), and Mexico (b 30° N) [23]. The converse latitude gradient was found in the North American continent. In summary, in Europe, significantly more male babies were born in southern latitudes than in northern latitudes, whereas the reverse was
For the purposes of latitude gradients, northern regions (Scotland and Northern Ireland) were compared with southern regions (England and Wales and the Republic of Ireland). This study analysed 49,263,493 live births [26]. More males were born in cooler (more northern) latitudes (p = 0.02). An overall decreasing trend in M/F was noted but this was only significant for England and Wales (p = 0.02) and for the entire dataset (p = 0.04). For all regions, there was an overall rise in M/F up to the late 1970s, followed by a sharp decline, which slowly plateaued. The step-down for the period 1975–79 to 1980–84 was highly significant for both regions (p b 0.001). The secular trends may have been influenced by several factors which may have led to increasing maternal age, which is associated with a lower M/F. Legislation was enacted in the 1970s that encouraged women to work. This would tend to delay pregnancy. Legislation included the Equal Pay Act (1970) and the Sex Discrimination Act (1975). The Equal Opportunities Commission was set up in 1976 in order to oversee the abovementioned Acts. Other relevant legislation that may have increased maternal age included the Maternity Leave & Pay Regulations Act (1975) and the Employment Protection Act (1978) [27]. Furthermore, in 1974, contraception became available to all through the National Health Service, including unmarried and single mothers, further potentially increasing maternal age [28], all of which may have lowered M/F. 10. Germany Annual data on male and female live births for both Germanies were obtained from the Human Mortality Database (Max Plank Institute) for 1946–2009. There were 60,548,522 births available for analysis. M/F for both regions during this period showed a significant decline in both German Republics from 0.518 to 0.514–0.513 (p b 0.0001). This decline was significant for the period before reunification in 2009 (p b 0.0001) but not for the period after it [29]. M/F for West Germany was significantly less than that of East Germany (p = 0.001). No such difference was present after reunification (1992–2009). It has been proposed that contracting economies or stressed societies result in a decrease in M/F [18]. Before reunification, the population of the communist occupied East Germany faced economic privation and stress but this is not reflected in the results which appear to show that although M/F declined in both Germanies prior to reunification, M/F
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was overall lower in the Federal Republic than in the Democratic Republic [29]. 11. Yugoslavia The former Yugoslavia spanned 40°–47° N. Latitude trend testing was applied after the post-Yugoslavian states were ranked in the following order (north to south): Slovenia (45°–47°), Croatia (42°–47°), Serbia and Montenegro (41°–47°) and Macedonia (40°–43°), with unavoidable latitude overlap. Data for Bosnia and Herzegovina was unavailable. There were 22,020,729 live births available for analysis [30]. There was a decline in M/F to 1961 and a subsequent increase to 1969 (p b 0.0001). There was also a rise in M/F between 1985 and 1990 (p = 0.02). These countries also exhibit a latitude gradient after the breakup of Yugoslavia (for the period 1991–2003, p b 0.0001), with more males being born to the south of the region than to the north. The decline in M/F in ex-Yugoslavia coincides with the country's degenerating economy and the ensuing movement to initiate economic reforms in the early 1960s. The so-called “Third Five-Year Plan”, which commenced in 1961, was abandoned during the following year because of its failure [31]. In late 1962, a series of debates were conducted in order to identify and correct economic flaws, resulting in a new constitution in 1963 which introduced market socialism with decentralized decision making [31,32]. This coincides with the observed rise in M/F. Yugoslavia faced another grave economic crisis in 1982, and in 1983, the so-called “Long-Term Economic Stabilization Program” was adopted with the introduction of a free market economy and a reduction in austerity measures [33]. These events coincide with the observed rise in M/F. Despite the small dataset over a very small latitude spread, these findings support the European latitude gradient with more males born at southern and therefore warmer latitudes [30]. 12. Czechoslovakia and the post-Czechoslovakian states Czechoslovakia was a sovereign central European country which existed from 1918 to 1992, when it peacefully split into the Czech Republic and Slovakia. There were 13,123,538 live births available for analysis. There have been no significant increases or decreases in M/F over the period 1985–2009 for the Czech Republic or for Slovakia or for the amalgamation of the two. However, there was a declining trend in M/F for Czechoslovakia and its continuation as the amalgamated total of the two aforementioned states, for the period 1950–2009 (p b 0.0001) [34]. 13. Scandinavia For the purposes of latitude gradients, M/F for the more northern parts of the region (Iceland, Norway, Sweden and Finland) was compared with that of Denmark which is the southernmost country in this region. This study analysed 18,250,193 live births. A significant secular decreasing trend in M/F was found overall and for Norway only. There were no latitude gradients [35]. The Windscale fire of 1957 was the worst nuclear accident in the history of the United Kingdom. It was a scale five event on the International Nuclear Event Scale which ranges in level from one to seven. The reactor burned for three days, releasing substantial amounts of radionuclides into the surrounding area and in a north-easterly fallout pattern across the Nordic countries. There was a significant decline in M/F for Denmark and a significant rise in M/F for Norway (1953–7 vs 1963–67, p = 0.039) and Finland (1953–7 vs 1958–62, p = 0.047) in relation to Windscale.
For Norway, for the period 1953–1967, there was a significant rising trend in M/F (p = 0.03). The annual percentage change in live births, averaged over 5 year periods, was 0.172 (1953–57), − 0.2548 (1958–62) and 1.421 (1963–67). This was used to extrapolate male and female live births (expected values using linear regression) for 1958–67. Expected births (based on a percentage increment of 0.172/annum) and observed totals (actual) show that for the period 1958–62, there was a deficit of over 4000 births. For Finland, for the period 1953–1967, the rising trend in M/F was not significant but for the period 1958–62, there was a deficit of over 4000 births to those expected [36]. The Windscale accidents had significant repercussions on Scandinavia. 14. The former USSR, Europe and Chernobyl The 15 independent states that comprise this region are conventionally divided into the Russian Federation and four subregions: the Baltic States (Estonia, Latvia and Lithuania), Eastern Europe (Belarus, Moldova and Ukraine), the Southern Caucasus (Armenia, Azerbaijan and Georgia) and Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan). The Chernobyl incident in April 1986 was a scale seven event. The seven countries with the highest exposure levels (as measured using 137 Cs) were Belarus, Ukraine, Russia, Sweden, Finland, Austria and Norway. M/F was significantly lower in the three northern regions (Russian Federation, Baltic States and Central Asia) than the two southern regions (Southern Caucasus and Eastern Europe). This difference remained even when the data analysed was restricted to the era before the sharp rise in M/F in the South Caucasus, that is, for the period 1980– 1989 [37]. After this period, sex-selective technology has been widely used in this region, artificially skewing M/F. For all regions, the annual number of births increased for the period 1981–1986/7 except for the Russian Federation. After this period, annual births steadily declined for all regions. Except for the Baltic States, all regions showed a significant rise in M/F after 1986. Analysis of the countries most irradiated by the Chernobyl event showed significant rises in M/F after 1986 only in the three most exposed countries, namely Belarus, Ukraine and the Russian Federation. Correlation was carried out between M/F and annual births for the period 1986–95 for the countries most affected. Significance was only reached for the three most exposed countries: Ukraine (rho = −0.66, p = 0.038), Belarus (rho = −0.34, p = 0.33) and the Russian Federation (rho = −0.58, p = 0.08). Birth deficits were calculated by taking the expected annual birth rate as the annual average of the years prior to 1986 (1981–3 and 1985) except for Central Asia. This region exhibited a rising birth rate and for this reason, 1985 alone was chosen as the expected annual number of births. The birth deficit due to the Chernobyl event is estimated at 2,072,666, of which 1,087,924 are accounted for by Belarus and Ukraine alone. 15. The Middle East: Israel, Egypt and Kuwait The only viable datasets within the WHO database available for this region were those of Israel, Egypt and Kuwait. Israel displayed a decline in M/F from 1950 to 1989 (p = 0.001). There was one outlier value in 1981 where M/F dipped significantly. In 1981, Israeli forces destroyed a nuclear reactor that was still under construction near Baghdad. It is possible that the spectre of war provided sufficient stress to cause a dip in M/F in Israel [38]. Egypt also showed an overall (albeit less smooth) decline in M/F from 1955 to 2009 (p b 0.0001). There was a single outlier value in 1979 where M/F dipped significantly. In 1978, Egypt and Israel signed the Camp David Accords, which resulted in the Israel–Egypt Peace Treaty in the following year. It is possible that these events are somehow related to the Egyptian M/F dip in 1979 [38].
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Kuwait showed a decline in M/F for 1972–87 (p = 0.005). M/F then increased sharply from 1985–87 to 1993–95 (p = 0.0005) and declined again over 1993–2009 (p b 0.0001). The increase in M/F appears to coincide with the invasion of Iraq in 1990 and the subsequent Desert Storm operations in 1990–91 [38]. It is possible that this may somehow have influenced M/F, increasing it in the same way that the Great Wars increased M/F in Europe during periods of prolonged conflict [5].
theories of communist spy rings in Australia, which led to a defeat of the Labour Party. This defeat led to the “Great Split” of the Labour Party in 1955, with the Liberal/Country coalition remaining in power for the next seventeen years. One wonders whether this turmoil may have influenced M/F, which spiked significantly in 1956 [40].
16. South America
Asia is conventionally divided into the Low latitudes (0°–23° N), the Middle latitudes (24° N–40° N) and the High latitudes (N41° N). Data was available for the following countries:
For the purposes of latitude analysis of M/F, South America may be arbitrarily divided into two geographical areas: one straddling the equator and one well below the equator. The first region spans from Honduras at circa 10° N to 20° S, and a second area starts from 20° S and extends southward. Thus, only small parts of Bolivia and Brazil extend south across this divide and only small parts of Chile and Paraguay extend north across this arbitrary division. This study analysed 145,665,983 live births. An overall increasing trend in M/F was found in both regions (p b 0.0001). A comparison of the two areas also showed a latitude gradient, with more males being born in cooler (more southern, N20° S) latitudes (p b 0.0001). This gradient is in the same direction as previously found in North America and contrary to the European gradient [39].
18. Asia
• 0°–23° N: Singapore, Thailand, Philippines, Hong Kong and Sri Lanka. • 24°–40° N: South Korea and Japan. • 41°–82° N: The Russian Federation. There were 245,938,211 live births available for analysis. A M/F latitude gradient was found (p b 0.0001). More males were born in southern, warmer latitudes. An overall increasing trend in M/F was found in the summation for all of the countries in this region (p b 0.0001) [41]. This trend is in accordance with that found in Europe and contrary to that in the Americas.
19. Cuba 16.1. Chile There was a rise in M/F between the period 1967–1971 and the year 1972 (p b 0.0001) and a decline in M/F for 1972–1976 (p b 0.0001). Economic forces may have played a role. Inflation had peaked at 463% in 1973, exacerbated by civil strife and general workers' strikes. The regime was overthrown in September 1973. While harshly repressive, the Junta's new economic policies rapidly resulted in the drop of inflation rates to double and single digits. Economic stress may be hypothesised as an explanation for the low M/F evidenced in 1970–1, and the improved economic situation after the coup may also have resulted in the higher M/F after 1972 [39]. 16.2. Argentina Inspection of the Argentine dataset shows several high and significant outliers (1978, 1988 and 1994). The M/F rise in 1994 may have been caused by the government's change in policy which stabilised the economy up to 1996. No explanation was found for the first two outliers [39]. 16.3. Brazil M/F rose significantly after 1992. An elected president took power in 1989 after years of military rule. The new administration actively battled hyperinflation which overcame the country during 1980–94 (peaking at 25% per month) and promoted free trade and programmes of privatisation. This may explain the rise in M/F in the 1990s during the period of economic recovery [39]. 17. Australasia Data was available only for Australia (latitude 9° to 44° S) and New Zealand (latitude 29° to 53° S). There were 17,035,325 live births. No significant secular trends overall were found in either country or in their amalgamation. There was no latitude gradient, an unsurprising finding in view of the wide latitude overlap. There was a sharp rise in the Australian M/F in 1956 (p b 0.001). In Australia, the 1950s were characterised by four close run elections, in 1951, 1954, 1955 and 1958. The 1954 election was tainted by conspiracy
7,270,988 live births were available for analysis. M/F was relatively stable over the period 1960–1985 with some exceptions. There were sharp dips in M/F for the single years 1966, 1980 and 1985 (p b 0.01). All were associated with emigration-related events. Cuba has had a long-standing connection with Florida since this is the closest part of the United States (Cuba is 366 km away from Miami) and is therefore a logical refuge for Cubans who decide to migrate away from their country. A Memorandum of Understanding was in effect between Cuba and the US between December 1965 and early 1973 in order to facilitate transfer of refugees to the US (over a quarter of a million). Matters were expedited for refugees to the US by the Cuban Adjustment Act of 1966. These laws coincide with the 1966 M/F dip [42]. Under the Refugee Act of 1980, the US provided political asylum and imposed an annual quota of refugees from Cuba. These laws coincide with the 1980 M/F dip [42]. In 1984, the US and Cuba negotiated an agreement not only to resume normal migration, but also to repatriate individuals who were excludable by US law. The US granted Cuba an annual minimum of 20,000 legal immigrant visas. The M/F dip in 1985 follows this agreement [42]. These findings expose a temporal relationship between Cuban emigration and M/F, with M/F dipping in response to the possibility of leaving Cuba. No such relationship has ever been previously noted in the literature.
20. Conclusion A veritable plethora of factors influence M/F in order to finally reach a value that approximates 0.515. It is no wonder that “the persistent, exactly equal difference in this proportion at birth, and even its relatively small variations have provided food for thought for theologians, mathematicians, social scientists and biologists ever since the first calculations were made, even up to the present day” [43]. Indeed, the factors that influence M/F are dynamic areas of research with over a thousand published papers to date. Doubtless, many more factors that influence M/F will be found through the utilisation of ever larger and pooled datasets including the effects of radiation.
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Conflict of interest statement If accepted for publication, copyright will naturally devolve to the journal. There are no real or potential conflicts, financial or otherwise. There was no funding for this work. Acknowledgements I acknowledge Mie Inoue and Gauden Galea from the World Health Organisation. References [1] Graunt J. Natural and political observations made upon the bills of mortality. London: Martyn; 1662. [2] Campbell RB. John Graunt, John Arbuthnott, and the human sex ratio. Hum Biol 2001;73:605–10. [3] Edwards AW. Carl Düsing (1884) on the regulation of the sex-ratio. Theor Popul Biol 2000;58:255–7. [4] Fisher RA. The genetical theory of natural selection. London: Clarendon Press; 1930. [5] James WH. The human sex ratio. Part 1: a review of the literature. Hum Biol 1987; 59:721–52. [6] James WH. The human sex ratio. Part 2: a hypothesis and a program of research. Hum Biol 1987;59:873–900. [7] Trivers RL, Willard DE. Natural selection of parental ability to vary the sex ratio of offspring. Science 1973;179:90–2. [8] Boklage CE. Survival probability of human conceptions from fertilization to term. Int J Fertil 1990;35:75 [79–80, 81–94]. [9] McMillen MM. Differential mortality by sex in fetal and neonatal deaths. Science 1979;204:89–91. [10] Wells JC. Natural selection and sex differences in morbidity and mortality in early life. J Theor Biol 2000;202:65–76. [11] Grech V, Mamo J. The male to female ratio at birth. Xjenza 2014;2:87–101. [12] James WH. Hormonal control of sex ratio. J Theor Biol 1986;118:427–41. [13] Guerrero R. Association of the type and time of insemination within the menstrual cycle with the human sex ratio at birth. N Engl J Med 1974;291:1056–9. [14] James WH. Further evidence that mammalian sex ratios at birth are partially controlled by parental hormone levels around the time of conception. Hum Reprod 2004;19:1250–6. [15] James WH. Gonadotrophins and the human secondary sex ratio. Br Med J 1980; 281:711–2. [16] Ciocco A. Variation in the sex ratio at birth in the U.S. Hum Biol 1938;10:36–64. [17] Visaria PM. Sex ratio at birth in territories with a relatively complete registration. Eugen Q 1967;14:132–42. [18] Catalano RA. Sex ratios in the two Germanies: a test of the economic stress hypothesis. Hum Reprod 2003;18:1972–5. [19] Catalano R, Bruckner T, Marks AR, Eskenazi B. Exogenous shocks to the human sex ratio: the case of September 11, 2001 in New York City. Hum Reprod 2006;21:3127–31. [20] Catalano R, Bruckner T, Gould J, Eskenazi B, Anderson E. Sex ratios in California following the terrorist attacks of September 11, 2001. Hum Reprod 2005;20:1221–7.
[21] Scherb H, Voigt K. The human sex odds at birth after the atmospheric atomic bomb tests, after Chernobyl, and in the vicinity of nuclear facilities. Environ Sci Pollut Res Int 2011;18:697–707. [22] Grech V, Vassallo-Agius P, Savona-Ventura C. Secular trends in sex ratios at birth in North America and Europe over the second half of the 20th century. J Epidemiol Community Health 2003;57:612–5. [23] Grech V, Vassallo-Agius P, Savona-Ventura C. Declining male births with increasing geographical latitude in Europe. J Epidemiol Community Health 2000;54:244–6. [24] Grech V, Savona-Ventura C, Vassallo-Agius P. Research pointers: unexplained differences in sex ratios at birth in Europe and North America. BMJ 2002;324: 1010–1. [25] Fleiss JL. Statistical methods for rates and proportions. 2nd ed. New York: John Wiley and Sons; 1981 14–5. [26] Grech V. Sex ratios at birth in the British Isles over the past sixty years. Eur J Pediatr 2013;172:525–8. [27] Waldfogel J. The family gap for young women in the United States and Britain: can maternity leave make a difference? J Labor Econ 1998;16:505–45. [28] Family Planning Association. The Women's Timeline. Factsheet; 2011 1–6. [29] Grech V. Declining male births in Germany before and after reunification. Turk J Pediatr 2013;55:300–3. [30] Grech V. Secular trends and latitude gradients in the male–female ratio at birth in Yugoslavia and the ex-Yugoslavian states. Acta Medica (Hradec Kralove) 2013;56:47–51. [31] Horvat B. Yugoslav economic policy in the post-war period: problems, ideas, institutional developments. Am Econ Rev 1971;61:71–169. [32] Lampe JR. Yugoslavia as history: twice there was a country. Cambridge: Cambridge University Press; 2000. [33] Sachs JD. Economic transition and the exchange-rate regime. Am Econ Rev 1996;86: 147–52. [34] Grech V. Secular trends and latitude gradients in sex ratios at birth in Czechoslovakia and the post-Czechoslovakian states. Acta Medica (Hradec Kralove) 2012;55: 138–41. [35] Grech V. Sex ratios at birth in Scandinavia over the past sixty years. Scand J Public Health 2012;40:761–4. [36] Grech V. Births and male:female birth ratio in Scandinavia and the United Kingdom after the Windscale fire of October 1957. Int J Risk Saf Med 2014;26:45–53. [37] Grech V. The Chernobyl accident, the male to female ratio at birth and birth rates. Acta Medica (Hradec Kralove) 2014 [in press]. [38] Grech V. The effect of warfare on the secular trends in sex ratios at birth in Israel, Egypt, and Kuwait over the past 60 years. Libyan J Med 2014;9:23448. [39] Grech V. Secular trends in sex ratios at birth in South America over the second half of the 20(th) century. J Pediatr (Rio J) 2013;89:505–9. [40] Grech V. Secular trends and latitude gradients in sex ratios at birth in Australia and New Zealand (1950–2010) demonstrate uncharacteristic homogeneity. Malta Med J 2013;25:25–7. [41] Grech V. Secular trends and latitude gradients in sex ratio at birth in Asia during the past 60 years. Pediatr Int 2013;55:219–22. [42] Grech V. The influence of migration on secular trends in sex ratios at birth in Cuba in the past fifty years. West Indian Med J 2014 [in press]. [43] Brian E, Jaisson M. The descent of human sex ratio at birth: a dialogue between mathematics, biology and sociology. New York: Springer; 2007.
Contents lists available at ScienceDirect
Early Human Development journal homepage: www.elsevier.com/locate/earlhumdev
Secular trends in newborn sex ratios Victor Grech ⁎ Academic Department of Paediatrics, Medical School, Mater Dei Hospital, Malta
a r t i c l e
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Keywords: Birth rate/*trends Sex ratio Chernobyl nuclear accident Infant, newborn Radiation, ionizing Toxins, biological
a b s t r a c t A wide variety of factors have been shown to influence the male to female ratio at birth, which invariably displays a male excess. This paper will review and amplify recent work by the author, with specific references to individual countries, regions and entire continents in order to provide a global overview of this subject. It will be shown that stress, including stress related to political events, influences this ratio. Man-made radiation is also shown to have played a significant role in relation to the Windscale fire (1957) and Chernobyl (1986). © 2014 Elsevier Ireland Ltd. All rights reserved.
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Introduction . . . . . . . . . . . . . . . . . . . Definitions . . . . . . . . . . . . . . . . . . . Historical aspects . . . . . . . . . . . . . . . . Broad epidemiological aspects of human M/F . . . . M/F physiology . . . . . . . . . . . . . . . . . Factors known to influence M/F . . . . . . . . . . 6.1. Hormones . . . . . . . . . . . . . . . . 6.2. Races . . . . . . . . . . . . . . . . . . 6.3. Warfare . . . . . . . . . . . . . . . . . 6.4. Stress . . . . . . . . . . . . . . . . . . 6.5. Environmental toxins and occupations . . . Continent-wide secular trends and latitude gradients 7.1. Latitude gradients in M/F . . . . . . . . . More recent work . . . . . . . . . . . . . . . . 8.1. Data sources . . . . . . . . . . . . . . . 8.2. Statistics . . . . . . . . . . . . . . . . . The United Kingdom . . . . . . . . . . . . . . . Germany . . . . . . . . . . . . . . . . . . . . Yugoslavia . . . . . . . . . . . . . . . . . . . Czechoslovakia and the post-Czechoslovakian states . Scandinavia . . . . . . . . . . . . . . . . . . . The former USSR, Europe and Chernobyl . . . . . . The Middle East: Israel, Egypt and Kuwait . . . . . South America . . . . . . . . . . . . . . . . . . 16.1. Chile . . . . . . . . . . . . . . . . . . . 16.2. Argentina . . . . . . . . . . . . . . . . 16.3. Brazil . . . . . . . . . . . . . . . . . . Australasia . . . . . . . . . . . . . . . . . . . Asia . . . . . . . . . . . . . . . . . . . . . . Cuba . . . . . . . . . . . . . . . . . . . . . .
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⁎ Corresponding author at: Department of Paediatrics, Mater Dei Hospital, Malta. Tel.: +356 99495813. E-mail address: [email protected].
http://dx.doi.org/10.1016/j.earlhumdev.2014.08.018 0378-3782/© 2014 Elsevier Ireland Ltd. All rights reserved.
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1. Introduction The male to female ratio at birth invariably yields an excess of males. However, this ratio has been shown to be influenced by a very wide number of factors. This review will enlarge on recent work by the author and will not only provide a general history of and an introduction to this topic, but also provide a global overview. It will also be shown that stress related to political events has influenced this ratio, as has radiation from accidents including nuclear facilities. 2. Definitions The male to female ratio at birth (technically known as the secondary sex ratio) is commonly abbreviated as M/F, a potentially misleading term as this defines the ratio of male to total births (M/T). M/F will be used in this paper, denoting male live births divided by total live births. 3. Historical aspects In ancient times, it was widely believed that an infant's gender was determined by the degree of heat that a man's ejaculate was exposed to during insemination. It was only until much later that formal analysis of M/F was undertaken. Such a statistical study requires not only raw data but also statistical tools for calculations that provide probabilities of deviation from preset values. The collection of data from London in the 1600s allowed John Graunt (1620–74) to publish the first descriptive statistical analysis of M/F data [1]. Graunt's work included an analysis of annual variation of M/F in London and Romsey. He noted that male births exceeded female births and that this excess was greater in urban London than in rural Romsey. His findings were statistically non-significant but he noted secular variation in M/F which is significant with modern day testing. John Arbuthnott (1667–1735) was a mathematics teacher in London who went on to study medicine [2]. He demonstrated that M/F is significantly in excess of 0.5, the first use of inferential statistics [2]. Ronald Fisher (1890–1962) was an English statistician, evolutionary biologist, geneticist, and eugenicist who popularised the theories of Carl Düsing of Jena [3]. The Fisherian explanation for the skew in M/F is that were male births less common than female births, a male would have better mating prospects and would sire more offspring. Thus, parents genetically disposed to produce males would have more offspring and this tendency would spread within the community, increasing male births, such that this advantage disappears when M/F of 0.5 is reached. The converse would apply were there a dearth of females [4]. These studies comprised the first application of mathematical methods and models to evolutionary biology. 4. Broad epidemiological aspects of human M/F Random meiosis would lead to a mean (Mendelian) M/F of 0.5, with binomial variation around this value. However, this is based on the following assumptions: 1. Males produce equal numbers of X- and Y-bearing sperm in mammalian species. 2. X- and Y-bearing sperm stand equal chances of achieving conception. 3. Equal numbers of male and female zygotes are conceived.
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Thus, any M/F variation would be due to sex-selective foetal wastage. In humans, M/F exhibits a male excess and is expected to approximate 0.515 with a range of 0.505 to 0.520 [5]. The excess of male births may be nature's compensatory mechanism for increased postnatal male mortality. A veritable legion of factors has been proposed for this disparity potentially influencing it [5,6]. 5. M/F physiology Evolutionary theory proposes that mutations may produce individuals who are fitter in a given environment and who are therefore likelier to survive and procreate, dispersing their advantageous genes. One such adaptation could be the maternal ability to influence M/F outcomes in pregnancy. In polygynous species, only the fittest males reproduce. For this reason, parental investment in a “good quality” son would, on average, yield greater numbers of descendants than an equivalent investment in a “good quality” daughter. It may therefore be advantageous for a mother to produce sons when she has sufficient resources to give them a better than average edge that will then give them a greater chance to reproduce, and daughters when she does not have. This is known as the Trivers–Willard hypothesis [7]. Recent studies have revealed that around 73% of natural singleton conceptions fail to survive beyond six weeks of gestation. Pregnancy is thus an opportunity for selection and/or culling, and significant wastage occurs before maternal or clinical recognition of pregnancy. It is also believed that multiple pregnancies may constitute over 12% of all natural conceptions but only approximately two percent reach term as live twin births, and 12% of these result in single births [8]. The sex ratio at conception in humans may be 0.545, with the highest sex ratio of foetal deaths in the second trimester. This data also suggests that late foetal deaths may be postponed to early infancy [9]. Male vulnerability is also manifest in premature births, as well as in term babies, with higher morbidity and mortality rates that persevere into early childhood. Women who fail to abort male foetuses in times of stress also reduce their own odds of survival due to the higher metabolic requirements necessitated by the gestation of a male baby to term. Conversely, a female who aborts a male baby under stressful circumstances fails to invest heavily in what would potentially result in a frail son, and makes herself available to potentially bear a daughter, or a robust son in future and less adverse times [10]. 6. Factors known to influence M/F The literature is replete with such factors [11], and the more important ones are listed hereunder. 6.1. Hormones While the physiological basis for the influences of external factors on M/F is not understood, alterations in parental sex hormone level/s and/ or differential gender-based survival modulated by stress during embryogenesis have been proposed as likely mediators. The hormonal theory is heavily subscribed to by William H. James, the foremost expert in the field. This theory states that higher levels of maternal gonadotrophins and progesterones lower M/F while elevated levels of testosterone and oestrogen increase M/F [12]. The luteal surge in the middle of the menstrual cycle has therefore been proposed to be the cause of
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the excess conception of females noted in the fertile part of the cycle [13,14]. Older mothers tend to have a lower M/F and it has been speculated that this could be due to their overall higher levels of circulating gonadotrophins [15]. 6.2. Races Population differences have also been documented. The male excess has been historically shown to be significantly less in Black populations when compared to Caucasian populations [16] and significantly higher in Asian populations [17]. 6.3. Warfare Marked increases in M/F in the range of 1–2% have been noted in England, Wales and France during and after the First and Second World Wars [5]. This has been attributed to the coexistence of increased coital frequency and non-programmed copulation, with more conceptions occurring early or late in the menstrual cycle, increasing M/F [15].
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found in North America. The authors were unable to explain these findings, which do not support a latitude related effect [24]. 8. More recent work The next section expands on more recent work done by this author, and will be divided into individual countries, regions and continents. Each section will be self-contained. 8.1. Data sources Annual male and female live births were obtained directly from the WHO Health for All (HFA) database. 8.2. Statistics Excel was used for data entry, overall analysis and charting. The quadratic equations of Fleiss were used for the exact calculation of 95% confidence intervals for ratios [25]. Chi tests and chi tests for trends for male and female births were used throughout the study. A p value ≤ 0.05 was taken to represent a statistically significant result.
6.4. Stress 9. The United Kingdom Under situations of stress, it has been shown that M/F drops [18], and it has been proposed that this accedes to the Trivers–Willard hypothesis [7]. For example, M/F fell in New York City three months after the terrorist attacks of September 11, rather than seven or more months later as would be the case were male conceptions reduced by this event [19]. Similar findings were noted for the same time period following the abovementioned terrorist attack in California, suggesting that witnessing harm befalling on others induces biological responses that resemble those in the persons harmed [20]. 6.5. Environmental toxins and occupations A plethora of environmental toxins have been implicated as affecting M/F with parental exposure and these invariably reduce M/F [5,6]. Radiation is a unique toxin in that it increases M/F while decreasing the total number of births, presumably due to lethal mutations [21]. 7. Continent-wide secular trends and latitude gradients M/F over the second half of the 20th century (127,034,732 North American and 157,947,117 European live births) showed a highly significant overall decline in male births in both Europe and North America (p b 0.0001), particularly in Mexico (p b 0.0001). European countries were banded by latitude. Southern countries (latitude 35–40° N) included Bulgaria, Greece, Italy, Malta, Portugal, and Spain. Central Europe (40–55° N) included Austria, Belgium, Czech Republic, France, Germany, Hungary, Ireland, Luxembourg, Netherlands, Poland, Romania, Switzerland, and the United Kingdom. Nordic countries (N55° N) included Denmark, Finland, Iceland, Norway, and Sweden. In Europe, male births declined in North European countries (latitude N 40° N, p b 0.0001) while rising in Mediterranean countries (latitude 35–40° N, p b 0.0001) [22]. 7.1. Latitude gradients in M/F Analysis of European births showed a much higher ratio of male births in the south of Europe than in the north (p b 0.0001) [23]. The same technique was then applied to the North American continent which was divided by latitude into Canada (N50° N), the United States (30–50° N), and Mexico (b 30° N) [23]. The converse latitude gradient was found in the North American continent. In summary, in Europe, significantly more male babies were born in southern latitudes than in northern latitudes, whereas the reverse was
For the purposes of latitude gradients, northern regions (Scotland and Northern Ireland) were compared with southern regions (England and Wales and the Republic of Ireland). This study analysed 49,263,493 live births [26]. More males were born in cooler (more northern) latitudes (p = 0.02). An overall decreasing trend in M/F was noted but this was only significant for England and Wales (p = 0.02) and for the entire dataset (p = 0.04). For all regions, there was an overall rise in M/F up to the late 1970s, followed by a sharp decline, which slowly plateaued. The step-down for the period 1975–79 to 1980–84 was highly significant for both regions (p b 0.001). The secular trends may have been influenced by several factors which may have led to increasing maternal age, which is associated with a lower M/F. Legislation was enacted in the 1970s that encouraged women to work. This would tend to delay pregnancy. Legislation included the Equal Pay Act (1970) and the Sex Discrimination Act (1975). The Equal Opportunities Commission was set up in 1976 in order to oversee the abovementioned Acts. Other relevant legislation that may have increased maternal age included the Maternity Leave & Pay Regulations Act (1975) and the Employment Protection Act (1978) [27]. Furthermore, in 1974, contraception became available to all through the National Health Service, including unmarried and single mothers, further potentially increasing maternal age [28], all of which may have lowered M/F. 10. Germany Annual data on male and female live births for both Germanies were obtained from the Human Mortality Database (Max Plank Institute) for 1946–2009. There were 60,548,522 births available for analysis. M/F for both regions during this period showed a significant decline in both German Republics from 0.518 to 0.514–0.513 (p b 0.0001). This decline was significant for the period before reunification in 2009 (p b 0.0001) but not for the period after it [29]. M/F for West Germany was significantly less than that of East Germany (p = 0.001). No such difference was present after reunification (1992–2009). It has been proposed that contracting economies or stressed societies result in a decrease in M/F [18]. Before reunification, the population of the communist occupied East Germany faced economic privation and stress but this is not reflected in the results which appear to show that although M/F declined in both Germanies prior to reunification, M/F
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was overall lower in the Federal Republic than in the Democratic Republic [29]. 11. Yugoslavia The former Yugoslavia spanned 40°–47° N. Latitude trend testing was applied after the post-Yugoslavian states were ranked in the following order (north to south): Slovenia (45°–47°), Croatia (42°–47°), Serbia and Montenegro (41°–47°) and Macedonia (40°–43°), with unavoidable latitude overlap. Data for Bosnia and Herzegovina was unavailable. There were 22,020,729 live births available for analysis [30]. There was a decline in M/F to 1961 and a subsequent increase to 1969 (p b 0.0001). There was also a rise in M/F between 1985 and 1990 (p = 0.02). These countries also exhibit a latitude gradient after the breakup of Yugoslavia (for the period 1991–2003, p b 0.0001), with more males being born to the south of the region than to the north. The decline in M/F in ex-Yugoslavia coincides with the country's degenerating economy and the ensuing movement to initiate economic reforms in the early 1960s. The so-called “Third Five-Year Plan”, which commenced in 1961, was abandoned during the following year because of its failure [31]. In late 1962, a series of debates were conducted in order to identify and correct economic flaws, resulting in a new constitution in 1963 which introduced market socialism with decentralized decision making [31,32]. This coincides with the observed rise in M/F. Yugoslavia faced another grave economic crisis in 1982, and in 1983, the so-called “Long-Term Economic Stabilization Program” was adopted with the introduction of a free market economy and a reduction in austerity measures [33]. These events coincide with the observed rise in M/F. Despite the small dataset over a very small latitude spread, these findings support the European latitude gradient with more males born at southern and therefore warmer latitudes [30]. 12. Czechoslovakia and the post-Czechoslovakian states Czechoslovakia was a sovereign central European country which existed from 1918 to 1992, when it peacefully split into the Czech Republic and Slovakia. There were 13,123,538 live births available for analysis. There have been no significant increases or decreases in M/F over the period 1985–2009 for the Czech Republic or for Slovakia or for the amalgamation of the two. However, there was a declining trend in M/F for Czechoslovakia and its continuation as the amalgamated total of the two aforementioned states, for the period 1950–2009 (p b 0.0001) [34]. 13. Scandinavia For the purposes of latitude gradients, M/F for the more northern parts of the region (Iceland, Norway, Sweden and Finland) was compared with that of Denmark which is the southernmost country in this region. This study analysed 18,250,193 live births. A significant secular decreasing trend in M/F was found overall and for Norway only. There were no latitude gradients [35]. The Windscale fire of 1957 was the worst nuclear accident in the history of the United Kingdom. It was a scale five event on the International Nuclear Event Scale which ranges in level from one to seven. The reactor burned for three days, releasing substantial amounts of radionuclides into the surrounding area and in a north-easterly fallout pattern across the Nordic countries. There was a significant decline in M/F for Denmark and a significant rise in M/F for Norway (1953–7 vs 1963–67, p = 0.039) and Finland (1953–7 vs 1958–62, p = 0.047) in relation to Windscale.
For Norway, for the period 1953–1967, there was a significant rising trend in M/F (p = 0.03). The annual percentage change in live births, averaged over 5 year periods, was 0.172 (1953–57), − 0.2548 (1958–62) and 1.421 (1963–67). This was used to extrapolate male and female live births (expected values using linear regression) for 1958–67. Expected births (based on a percentage increment of 0.172/annum) and observed totals (actual) show that for the period 1958–62, there was a deficit of over 4000 births. For Finland, for the period 1953–1967, the rising trend in M/F was not significant but for the period 1958–62, there was a deficit of over 4000 births to those expected [36]. The Windscale accidents had significant repercussions on Scandinavia. 14. The former USSR, Europe and Chernobyl The 15 independent states that comprise this region are conventionally divided into the Russian Federation and four subregions: the Baltic States (Estonia, Latvia and Lithuania), Eastern Europe (Belarus, Moldova and Ukraine), the Southern Caucasus (Armenia, Azerbaijan and Georgia) and Central Asia (Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan and Uzbekistan). The Chernobyl incident in April 1986 was a scale seven event. The seven countries with the highest exposure levels (as measured using 137 Cs) were Belarus, Ukraine, Russia, Sweden, Finland, Austria and Norway. M/F was significantly lower in the three northern regions (Russian Federation, Baltic States and Central Asia) than the two southern regions (Southern Caucasus and Eastern Europe). This difference remained even when the data analysed was restricted to the era before the sharp rise in M/F in the South Caucasus, that is, for the period 1980– 1989 [37]. After this period, sex-selective technology has been widely used in this region, artificially skewing M/F. For all regions, the annual number of births increased for the period 1981–1986/7 except for the Russian Federation. After this period, annual births steadily declined for all regions. Except for the Baltic States, all regions showed a significant rise in M/F after 1986. Analysis of the countries most irradiated by the Chernobyl event showed significant rises in M/F after 1986 only in the three most exposed countries, namely Belarus, Ukraine and the Russian Federation. Correlation was carried out between M/F and annual births for the period 1986–95 for the countries most affected. Significance was only reached for the three most exposed countries: Ukraine (rho = −0.66, p = 0.038), Belarus (rho = −0.34, p = 0.33) and the Russian Federation (rho = −0.58, p = 0.08). Birth deficits were calculated by taking the expected annual birth rate as the annual average of the years prior to 1986 (1981–3 and 1985) except for Central Asia. This region exhibited a rising birth rate and for this reason, 1985 alone was chosen as the expected annual number of births. The birth deficit due to the Chernobyl event is estimated at 2,072,666, of which 1,087,924 are accounted for by Belarus and Ukraine alone. 15. The Middle East: Israel, Egypt and Kuwait The only viable datasets within the WHO database available for this region were those of Israel, Egypt and Kuwait. Israel displayed a decline in M/F from 1950 to 1989 (p = 0.001). There was one outlier value in 1981 where M/F dipped significantly. In 1981, Israeli forces destroyed a nuclear reactor that was still under construction near Baghdad. It is possible that the spectre of war provided sufficient stress to cause a dip in M/F in Israel [38]. Egypt also showed an overall (albeit less smooth) decline in M/F from 1955 to 2009 (p b 0.0001). There was a single outlier value in 1979 where M/F dipped significantly. In 1978, Egypt and Israel signed the Camp David Accords, which resulted in the Israel–Egypt Peace Treaty in the following year. It is possible that these events are somehow related to the Egyptian M/F dip in 1979 [38].
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Kuwait showed a decline in M/F for 1972–87 (p = 0.005). M/F then increased sharply from 1985–87 to 1993–95 (p = 0.0005) and declined again over 1993–2009 (p b 0.0001). The increase in M/F appears to coincide with the invasion of Iraq in 1990 and the subsequent Desert Storm operations in 1990–91 [38]. It is possible that this may somehow have influenced M/F, increasing it in the same way that the Great Wars increased M/F in Europe during periods of prolonged conflict [5].
theories of communist spy rings in Australia, which led to a defeat of the Labour Party. This defeat led to the “Great Split” of the Labour Party in 1955, with the Liberal/Country coalition remaining in power for the next seventeen years. One wonders whether this turmoil may have influenced M/F, which spiked significantly in 1956 [40].
16. South America
Asia is conventionally divided into the Low latitudes (0°–23° N), the Middle latitudes (24° N–40° N) and the High latitudes (N41° N). Data was available for the following countries:
For the purposes of latitude analysis of M/F, South America may be arbitrarily divided into two geographical areas: one straddling the equator and one well below the equator. The first region spans from Honduras at circa 10° N to 20° S, and a second area starts from 20° S and extends southward. Thus, only small parts of Bolivia and Brazil extend south across this divide and only small parts of Chile and Paraguay extend north across this arbitrary division. This study analysed 145,665,983 live births. An overall increasing trend in M/F was found in both regions (p b 0.0001). A comparison of the two areas also showed a latitude gradient, with more males being born in cooler (more southern, N20° S) latitudes (p b 0.0001). This gradient is in the same direction as previously found in North America and contrary to the European gradient [39].
18. Asia
• 0°–23° N: Singapore, Thailand, Philippines, Hong Kong and Sri Lanka. • 24°–40° N: South Korea and Japan. • 41°–82° N: The Russian Federation. There were 245,938,211 live births available for analysis. A M/F latitude gradient was found (p b 0.0001). More males were born in southern, warmer latitudes. An overall increasing trend in M/F was found in the summation for all of the countries in this region (p b 0.0001) [41]. This trend is in accordance with that found in Europe and contrary to that in the Americas.
19. Cuba 16.1. Chile There was a rise in M/F between the period 1967–1971 and the year 1972 (p b 0.0001) and a decline in M/F for 1972–1976 (p b 0.0001). Economic forces may have played a role. Inflation had peaked at 463% in 1973, exacerbated by civil strife and general workers' strikes. The regime was overthrown in September 1973. While harshly repressive, the Junta's new economic policies rapidly resulted in the drop of inflation rates to double and single digits. Economic stress may be hypothesised as an explanation for the low M/F evidenced in 1970–1, and the improved economic situation after the coup may also have resulted in the higher M/F after 1972 [39]. 16.2. Argentina Inspection of the Argentine dataset shows several high and significant outliers (1978, 1988 and 1994). The M/F rise in 1994 may have been caused by the government's change in policy which stabilised the economy up to 1996. No explanation was found for the first two outliers [39]. 16.3. Brazil M/F rose significantly after 1992. An elected president took power in 1989 after years of military rule. The new administration actively battled hyperinflation which overcame the country during 1980–94 (peaking at 25% per month) and promoted free trade and programmes of privatisation. This may explain the rise in M/F in the 1990s during the period of economic recovery [39]. 17. Australasia Data was available only for Australia (latitude 9° to 44° S) and New Zealand (latitude 29° to 53° S). There were 17,035,325 live births. No significant secular trends overall were found in either country or in their amalgamation. There was no latitude gradient, an unsurprising finding in view of the wide latitude overlap. There was a sharp rise in the Australian M/F in 1956 (p b 0.001). In Australia, the 1950s were characterised by four close run elections, in 1951, 1954, 1955 and 1958. The 1954 election was tainted by conspiracy
7,270,988 live births were available for analysis. M/F was relatively stable over the period 1960–1985 with some exceptions. There were sharp dips in M/F for the single years 1966, 1980 and 1985 (p b 0.01). All were associated with emigration-related events. Cuba has had a long-standing connection with Florida since this is the closest part of the United States (Cuba is 366 km away from Miami) and is therefore a logical refuge for Cubans who decide to migrate away from their country. A Memorandum of Understanding was in effect between Cuba and the US between December 1965 and early 1973 in order to facilitate transfer of refugees to the US (over a quarter of a million). Matters were expedited for refugees to the US by the Cuban Adjustment Act of 1966. These laws coincide with the 1966 M/F dip [42]. Under the Refugee Act of 1980, the US provided political asylum and imposed an annual quota of refugees from Cuba. These laws coincide with the 1980 M/F dip [42]. In 1984, the US and Cuba negotiated an agreement not only to resume normal migration, but also to repatriate individuals who were excludable by US law. The US granted Cuba an annual minimum of 20,000 legal immigrant visas. The M/F dip in 1985 follows this agreement [42]. These findings expose a temporal relationship between Cuban emigration and M/F, with M/F dipping in response to the possibility of leaving Cuba. No such relationship has ever been previously noted in the literature.
20. Conclusion A veritable plethora of factors influence M/F in order to finally reach a value that approximates 0.515. It is no wonder that “the persistent, exactly equal difference in this proportion at birth, and even its relatively small variations have provided food for thought for theologians, mathematicians, social scientists and biologists ever since the first calculations were made, even up to the present day” [43]. Indeed, the factors that influence M/F are dynamic areas of research with over a thousand published papers to date. Doubtless, many more factors that influence M/F will be found through the utilisation of ever larger and pooled datasets including the effects of radiation.
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