Brit.J.
Psycliiat. (@977), 131, 339-44
Season of Birth in the Population of Norway, with Particular Reference to the September Birth Maximum By ØRNULV ØDEGARD The monthly number of births in Norway fluctuates between a maximum in January-May and a minimum in October-December. This cyclic seasonality is assumed to be of biological origin, but in directly it is influenced by social factors and consequently tends to vary a great deal. There is a secondary birth maximum in September, corresponding to a peak of conceptions during the traditional festivities around Christmas and New Year. It is shown that this maximum is much less variable within the country studied, though it probably varies from one country to another in relation to the impact of the mid-winter traditions. The January-May birth maximum is known to be higher in schizo. phrenia than in the general population, while no such difference is observed in Norway for the September birth peak. It is felt that there is a fundamental difference between the two birth maxima and a corre' sponding difference between schizophrenic patients and the general population. Minimal paranatal brain damage of seasonal origin is suggested as an explanation.
because
It is a well established fact that the number of births varies from one month to another in a regular pattern, and that this seasonality is more marked in psychiatric patients (parti culaly in schizophrenia) than in the general population (Barry at al, 1964; Dalén at al, 1974;
Hare
et al, 1974).
In
Norway
of its obvious
mid-winter
New Year.
maximum
in the month
of September
which
has
a sharper appearance and is strictly localized within one month, contrasting sharply with the adjoining months of August and October. This September birth peak has been observed in Sweden mention
The
Ødegârd
by Dalén, but many authors fail to it, and in fact are bound to miss it
maximum
mental
compare
months.
birth maximum
(@@) has shown that the birth rate fluctuates
in an approximately sinusoidal curve between a maximum in January—May and a minimum in October—December (Fig @).This cyclic curve follows the seasons of the year in a way which suggests a predominantly biological background. The Norwegian material shows a second birth
they
individual
the
rather
than
September
deserves a closer study because
association
festivities
special is
quarters
Actually,
character confirmed
disorder.
with
during
When
the traditional
Christmas
of the by
the
its
September relations
monthly
pattern of the general population was compared with that of 62,190
and to
birth
of Norway psychiatric
patients born between 1870 and 1939, it proved possible to show that the schizophrenics had higher monthly fluctuations than either the other diagnostic groups or the general popula tion. This is in agreement with the findings of most other authors (Mednick, 1970; Samaroff and
Zak,
1972;
Parker
at al, 1976).
But the
present author has pointed out that it is not true of the September
to be practically
339
birth peak, which was found
the same in all diagnostic
SEASON
340
OF BIRTH
IN THE
groups (including schizophrenia) as in the general population. Fig i illustrates the monthly distribution of marriages and births in the populatioii of Norway. So as @ogive an impression of the time
trend,
investigation,
the
first
25 years
namely
of our
1891—1915,
period
are
of
con
trasted with the last 20 (1951—1970).
The most striking feature of Fig i is the difference between marriages and births, the monthly fluctuations being from three to four times as high in the former. Marriage comes first in the causal chain; it is influenced by social and economic factors which tend to be variable. Births are more influenced by biological factors and follow the principle of homeostasis, which leads to more moderate
variations.
Apart from this difference in amplitude, there is a fairly close association between the monthly pattern of marriages and that of births occurring nine months later, as will readily be seen from Fig i and Table I. For each of the eleven five year periods between 1891 and 1945 the correla tion was found to be around 0@50. From 1946 on it increased, and in 1966—70it reached the high level of o ‘¿ This 92. increase is probably due to the increasing proportion of first-born (from
POPULATION
OF NORWAY
20 to 40 per cent
the decreasing
association
of all births),
a consequence
birth rate. For the first-born
between
month
of marriage
of
the
and
month of birth is close, while for all later births it is at best doubtful.* This is the reason why the 195 1—70curve has higher fluctuations and more precisely defined maxima and minima than the 1891—1915curve. Around 1916 there was an interesting change in the location of the monthly maxima. The June marriage maximum shifted to July, most likely because the institution of summer vacation was extended to an increasing number of social groups, and because this vacation was more and more concentrated in July. This shows that even the supposedly biological seasonality is under the influence of social factors. The October—November marriage maximum shifted to December for similar (i.e. social) reasons. Before 1916 Norway was predomi nantly agricultural, and the completion of the harvest was decisive for the planning of such events as weddings. After 1916 non-agricultural population groups gained in importance, and •¿ It would in fact be preferable to leave out the first born, or to study the season of birth by parity,but unfortunately no such statistical information is available.
0 0. U) 0@ 0@.
I I I I I Oct. Dec. Feb. Apr. June Aug. Oct. Marriages 1891-1915:——————Births 1891-1915:—————— 1951-1970: 1951-1970: Fia i.—The monthly distribution of marriages and births per ioo in Norway. for two periods: 1891—1915and 195'—7o.
Means
BY ØRNULV
TAai2I
Correlation coe,fficients for the monthly number of marriages
and the numberof birthsoccurring9 monthslater
instance, cal.ulated to I 13- I per cent because mean number of births per day in September the was 185 as against 163 in August and 164 in October
1891—I9oo 1906—10
Table
O@45 0501931—35..O@471941—45 1916—25.. ..0-51 ..
1946—50
..
1951—55
..
1956—65 1966—70..
o'86 o-88 092
..051
(185/163.5
=
“¿ 3.'))
II shows that the September
peak
remains
107
and
very much
the same
during
birth the
period of investigation, with variations between
0@76
..
34'
ØDEGARD
the traditional season of Christmas and New Year festivities came to be the popular time for weddings. There resulted the sharply defined December marriage peak and a corre sponding September birth maximum. In Table II the monthly fluctuations are studied in some detail, with particular attention to the time trend. The amplitudes of the two birth maxima are presented for periods of five years as expressed by the proportion of each maximum to the adjoining minima. (The September maximum for 1891—95 is, for
113
per
cent.
The
January—May
maximum, on the other hand, varies between io8 5 in 1906—10and 125 @5in 1945—50.The variations in the January—May maximum are clearly inversely related to the birth rate: the birth maximum started to increase in 1920, and at the same time the birth rate started its really steep decline. Until 1945 the two were inversely related, and particularly impressive is the steep rise of the birth rate during the period of World War II and the corresponding decrease of the birth maximum. During the post-war period the trend was reversed: the birth rate increased, mainly because of the considerable number of delayed marriages and delayed childbirths. But at the same time the January May birth maximum also increased sharply.
T&ai.x II Marriages and births in Xorway. The time trend by periods of fwe years For thedefinition of‘¿ maximum', seetext MarriagesBirthsIllegitimate birthsMarr.
18g6—190o
..
1901—05
..
ig@i—45 1946—50
@@@i—55
1956—60
,96i—6s
marr.
marr.
6-8
i8i
207
170
193
I ,oooSummermax.Winter max.Birth
max.1891-95
1905—10 1911—15 1916—20 1921—25 1926—30 i@@i—35 1936—40
rate per
6@z
i68
rate
per
birth
birth max.Rate
137.3
110.0
II2@5 1101
i,oooJan.-Maymax.Sept.
131.0
109-8 1085
..
6@o 6-3
..
7@0
175
195 261
6-i
i8i i8i
197 215 24!
182
268
971 77.0 63-2 62@I
120'4 1200
110-5
173
232
75-2
111-9
109-4
88@6
1255 1205
..
.. .. .. .. .. .. .. .. ..
1966-70
..
1971—75..
..6-@.
6@ 6-@ 86 8-o 90
8-o 6-8 6@6
76175
162 163
223 222 250 251
245198
192
230 240
260
238
2261378
I2I@5 113'I
Io8@2
9O@4 90.9 89-3
III-! 111-2 118-2
fl9.7
ii8@i
1142 92-31109 11401131
IO9@2
112-0 112-I 1105
@9.5
II0@4
iio-8 iio@6
107-07-12
per i,ooo
birth
birth
7.44
io6@8
6'72 7-00
118-9 1115
112-4
birthsJan.-Maymax.Sept.
705
6-94
1091
I07@6
6.44
114.4 126.4 I20@4 128-8 II4@3
5.03
128-5
706
7.17 7.17
6-78 @-6o 3-56 3@85 560
iio@6 1o9'9 1o9.5 1045
98.o
II2@6
114.4 1117 1112 112-I
1o9-6 113.7
ii@@8 105-9
ioi-6 114.7
io@-i
907107-6 107-5117.9 100.1
SEASON OF BIRTH IN THE POPULATION OF NORWAY
342 From
1950 on there is again a slight tendency
towards a negative correlation. During these considerable changes the Sep.. tember birth peak remained unchanged, cvi. dently because it was relatively independent of the birth rate as well as of such upheavals a.s the war. Now it becomes understandable why the September birth peak is the same in schizo phrenics as in the general population. The relatively superficial traditions which lie behind this birth peak are not related to any predisposi.i tion towards
@
schizophrenia.
The January—May
maximum, on the other hand, is related to socio@ biological factors of a more basic significance for brain function and mental health. The mid-winter traditions vary from one country
to another,
although
they
are older
and more universal than Christendom. Inter national comparisons are, therefore, of great importance. So far it is known that the Septem ber birth peak is the same in Sweden as in Norway (Dalén, 1974). In Great Britain it seems to be less in evidence, and Hare et al (i@7@) do not discuss the problem explicitly. From their figures the following September peaks can be calculated: Schizophrenia Other psychoses Not psychotic
I()@3.9per cent
xoo@8
,,
98@7
In native schizophrenics in South Africa the findings were conificting (Dalén, 1974). Here, as well as in Australia (Parker et al, 1976), the situation is complicated because in the southern hemisphere the cyclic birth maximum is in August—October and so will cover up any possible September maximum. The most useful data are probably the Japanese presented by Shimura et al (@77). For 7,78! schizophrenic patients from Tokyo there was a September peak of 105' I per cent (calculated by the present author from a table given by Miura). The monthly distribution of illegitimate births is of interest
because
the influence
of marriage
is eliminated and any birth maximum will correspond to a maximum of conceptions. Besides this, there is the possibility that con ceptions out of wedlock are less carefully
planned, for instance with regard to of ‘¿ winter babies'. During our investigation there were in Norway 293,772 illegitimate births, but considerable number study of the monthly
avoidance period of a total of even this
was barely sufficient for a pattern during periods of
five years. There are; therefore, irregularities due to smailnumbers, and besides this there are problems in connection with the registration of illegitimate births. Over a period of 8o years there are likely to have been changes in legisla tion and in administrative practice. Up to 1931—35 around 7 per cent
of the
births were illegitimate. Then a rapid decrease set in, and a minimum of @6per cent was reached
in 1951—60. This
decrease
rupted by a peak of illegitimacy
was inter
in 1943—45,
undoubtedly in connection with wartime condi tions and the German occupation. Since 1965
there has been a remarkable
increase from 4 to
10 per cent in ten years. During economic situation has been
marriage and childbirth, must
therefore
be
a
this period the favourable to
and the explanation
fundamental
change
in
(Table
II).
attitudes towards sex and marriage. The two birth maxima are largely the same in
illegitimate
as in legitimate
births
While legitimate births are most frequent in April, the illegitimate births have their peak in May, corresponding to August rather than
July conceptions. the July summer
illicit connections. During
most
Most probably, relations to vacations are less close for
of the five-year
periods,
both
birth maxima are slightly higher for illegitimate births, but the figures are irregular. After 1950 this trend is clearly reversed, and the monthly fluctuations are higher for legitimate births.
The monthly fluctuations represent a pattern of regularity which seems to be in part lost in the illegitimate births, but this is a reJatively
recent trend which did not exist before 1950. It may have something to do with the fact that births out of wedlock are more frequent and acceptable
than they were previously.
It is possible that planned
conceptions
tribute to the monthly pattern wedlock, for instance as a planned
of births avoidance
con
in of
‘¿ winter babies', a planning which is to some extent lost in illegitimate births.
BY ØRNULVØDEGARD 1896-1900
t906-10
1916-20
1926-30
1936-40
E x
343 1946-50
1956.60
1966-70
C
a, E
‘¿ U
E
0
.C
00
Q
a,
0.
a,
.C
00
:January-May birth maximum —¿ ———-:September birth maximum —¿
:Birth
rate
Fio 2.—I.Amplitude of the January-May birth maximum per ioo of the corresponding October December minimum; 2. September birth maximum per ‘¿ of 00 the August+ October minimum; 3. Birth rate per i,ooo women 15 to 44 years of age. The curve shows the time trend 1891—1970.
0 0
a
U) .C .0 a,
‘¿ U
E 0)
a,
Legitimate: lUegitperlOO:— IUegitimate:—————— FIG 3.—The amplitude of the April and September birth maxima
legitimate
and illegitimate
births.
Also number
of illegitimate
per ioo of the corresponding minima
for
births per 100 total births. The curve shows
the time trend 1891—1970.
Minimal paranatal brain damage is supposed to be more frequent in illegitimate births. If such brain damage is a contributory cause of
the increased seasonality in schizophrenics, then one would expect higher monthly fluctuations in illegitimate births. The evidence in favour of
SEASON OF BIRTH
344
IN THE POPULATION
thishypothesisis not convincing in our data,
but then the relations between illegitimacy and a predisposition towards schizophrenia are far from close. Conclusions The cyclic January—May maximum is basic ally of biological origin but with secondary influence from such social factors as marriage rate and birth rate, and consequently with a rather variable amplitude. The September birth peak is due to one special social factor, namely a conception maximum festivities.
during From
the traditional mid-winter a biological point of view,
these traditions are superficial, but nevertheless they are powerful and resistant to change, and during our 8o-year-long period of investigation the September peak has shown practically no variation. Schizophrenic patients have the same January—May birth maximum as the general population, only with a higher amplitude. This suggests
that
the
pronounced
maximum
of the corresponding pregnancies) coincide with a season which is known to have an increased
ørnulv
Ødeg@rd,
Oslo 3, Xorway
Professor
Emeritus,
University
risk of brain damage. There results an increased risk of schizophrenia in those who are genetically
predisposed. There is no such increased risk of non-schizophrenic psychoses, because brain damage is less important in the etiology of these disorders.
No association is observed between schizo phrenia and the September birth peak, because this maximum results from social factors (i.e. mid-winter traditions) which are not associated with either brain damage or schizophrenia. References BAaay, HERB. & BAJUW,HERB. JR (1964) Season of birth in schizophrenia. Archmes of General P@ychiat,y, II, 385—91. DAL*N,
PER
1974, HARE,
E.
(1974)
Gotab, H.,
Season
of Birth
in Schizophrenia.
Gsteborg
Kungalv.
Piuca,
Jom@
& SI@vrRB,
E.
(ig74)
Mental
disorder and season of birth: a national sample
compared with the general population.
of P@ychiatiy,124, 8i-6. MEDNICK, S.A.
(1970)
Breakdown
British Journal
in individuals
risk of schizophrenia: possible dispositional factors. Mental Hygiene, 54, 50-63.
in
schizophrenia results from the same causes as the somewhat lower maximum in the general population, i.e. predominantly biological causes. The tentative hypothesis is offered that these biological factors, which so far are not com pletely understood, lead to a seasonal pattern of April—August conceptions. The consequent January—May births (as well as the last weeks
OF NORWAY
ODEGARD,
0.
(,@7i)
Season
of birth
with high
perinatal
in the general
popula
tion and in patients with mental disorder in Norway.
British Journal of Psychiatry,125, 397-405. PARKER,
GORDON
&
NEILSON,
disorder and season of birth. chiahy, 129, 355, 361.
S@s&AItoFp, A. J.
serious
& ZAK, M.
mental
disorder.
grammeBulletin, 10, 423-5.
Mao@u@i
(1972)
(1976)
British Journal Neonatal
Mental
of Psy
factors
SrnMuit@, M., NAK@usuit@,I. & Miua@, T. (ig77) Season of birth of schizophrenics in Tokyo, Japan. Ada
psychiat., wand, 55, 225—32.
of Oslo,
(Received 26 Xovember 1976; revised 5 Janua@y 1977)
in
Xeuroscience Research Pro
Gaustad
Psychiatric
Hospital,
Box
z6 Gaustad,
Psycliiat. (@977), 131, 339-44
Season of Birth in the Population of Norway, with Particular Reference to the September Birth Maximum By ØRNULV ØDEGARD The monthly number of births in Norway fluctuates between a maximum in January-May and a minimum in October-December. This cyclic seasonality is assumed to be of biological origin, but in directly it is influenced by social factors and consequently tends to vary a great deal. There is a secondary birth maximum in September, corresponding to a peak of conceptions during the traditional festivities around Christmas and New Year. It is shown that this maximum is much less variable within the country studied, though it probably varies from one country to another in relation to the impact of the mid-winter traditions. The January-May birth maximum is known to be higher in schizo. phrenia than in the general population, while no such difference is observed in Norway for the September birth peak. It is felt that there is a fundamental difference between the two birth maxima and a corre' sponding difference between schizophrenic patients and the general population. Minimal paranatal brain damage of seasonal origin is suggested as an explanation.
because
It is a well established fact that the number of births varies from one month to another in a regular pattern, and that this seasonality is more marked in psychiatric patients (parti culaly in schizophrenia) than in the general population (Barry at al, 1964; Dalén at al, 1974;
Hare
et al, 1974).
In
Norway
of its obvious
mid-winter
New Year.
maximum
in the month
of September
which
has
a sharper appearance and is strictly localized within one month, contrasting sharply with the adjoining months of August and October. This September birth peak has been observed in Sweden mention
The
Ødegârd
by Dalén, but many authors fail to it, and in fact are bound to miss it
maximum
mental
compare
months.
birth maximum
(@@) has shown that the birth rate fluctuates
in an approximately sinusoidal curve between a maximum in January—May and a minimum in October—December (Fig @).This cyclic curve follows the seasons of the year in a way which suggests a predominantly biological background. The Norwegian material shows a second birth
they
individual
the
rather
than
September
deserves a closer study because
association
festivities
special is
quarters
Actually,
character confirmed
disorder.
with
during
When
the traditional
Christmas
of the by
the
its
September relations
monthly
pattern of the general population was compared with that of 62,190
and to
birth
of Norway psychiatric
patients born between 1870 and 1939, it proved possible to show that the schizophrenics had higher monthly fluctuations than either the other diagnostic groups or the general popula tion. This is in agreement with the findings of most other authors (Mednick, 1970; Samaroff and
Zak,
1972;
Parker
at al, 1976).
But the
present author has pointed out that it is not true of the September
to be practically
339
birth peak, which was found
the same in all diagnostic
SEASON
340
OF BIRTH
IN THE
groups (including schizophrenia) as in the general population. Fig i illustrates the monthly distribution of marriages and births in the populatioii of Norway. So as @ogive an impression of the time
trend,
investigation,
the
first
25 years
namely
of our
1891—1915,
period
are
of
con
trasted with the last 20 (1951—1970).
The most striking feature of Fig i is the difference between marriages and births, the monthly fluctuations being from three to four times as high in the former. Marriage comes first in the causal chain; it is influenced by social and economic factors which tend to be variable. Births are more influenced by biological factors and follow the principle of homeostasis, which leads to more moderate
variations.
Apart from this difference in amplitude, there is a fairly close association between the monthly pattern of marriages and that of births occurring nine months later, as will readily be seen from Fig i and Table I. For each of the eleven five year periods between 1891 and 1945 the correla tion was found to be around 0@50. From 1946 on it increased, and in 1966—70it reached the high level of o ‘¿ This 92. increase is probably due to the increasing proportion of first-born (from
POPULATION
OF NORWAY
20 to 40 per cent
the decreasing
association
of all births),
a consequence
birth rate. For the first-born
between
month
of marriage
of
the
and
month of birth is close, while for all later births it is at best doubtful.* This is the reason why the 195 1—70curve has higher fluctuations and more precisely defined maxima and minima than the 1891—1915curve. Around 1916 there was an interesting change in the location of the monthly maxima. The June marriage maximum shifted to July, most likely because the institution of summer vacation was extended to an increasing number of social groups, and because this vacation was more and more concentrated in July. This shows that even the supposedly biological seasonality is under the influence of social factors. The October—November marriage maximum shifted to December for similar (i.e. social) reasons. Before 1916 Norway was predomi nantly agricultural, and the completion of the harvest was decisive for the planning of such events as weddings. After 1916 non-agricultural population groups gained in importance, and •¿ It would in fact be preferable to leave out the first born, or to study the season of birth by parity,but unfortunately no such statistical information is available.
0 0. U) 0@ 0@.
I I I I I Oct. Dec. Feb. Apr. June Aug. Oct. Marriages 1891-1915:——————Births 1891-1915:—————— 1951-1970: 1951-1970: Fia i.—The monthly distribution of marriages and births per ioo in Norway. for two periods: 1891—1915and 195'—7o.
Means
BY ØRNULV
TAai2I
Correlation coe,fficients for the monthly number of marriages
and the numberof birthsoccurring9 monthslater
instance, cal.ulated to I 13- I per cent because mean number of births per day in September the was 185 as against 163 in August and 164 in October
1891—I9oo 1906—10
Table
O@45 0501931—35..O@471941—45 1916—25.. ..0-51 ..
1946—50
..
1951—55
..
1956—65 1966—70..
o'86 o-88 092
..051
(185/163.5
=
“¿ 3.'))
II shows that the September
peak
remains
107
and
very much
the same
during
birth the
period of investigation, with variations between
0@76
..
34'
ØDEGARD
the traditional season of Christmas and New Year festivities came to be the popular time for weddings. There resulted the sharply defined December marriage peak and a corre sponding September birth maximum. In Table II the monthly fluctuations are studied in some detail, with particular attention to the time trend. The amplitudes of the two birth maxima are presented for periods of five years as expressed by the proportion of each maximum to the adjoining minima. (The September maximum for 1891—95 is, for
113
per
cent.
The
January—May
maximum, on the other hand, varies between io8 5 in 1906—10and 125 @5in 1945—50.The variations in the January—May maximum are clearly inversely related to the birth rate: the birth maximum started to increase in 1920, and at the same time the birth rate started its really steep decline. Until 1945 the two were inversely related, and particularly impressive is the steep rise of the birth rate during the period of World War II and the corresponding decrease of the birth maximum. During the post-war period the trend was reversed: the birth rate increased, mainly because of the considerable number of delayed marriages and delayed childbirths. But at the same time the January May birth maximum also increased sharply.
T&ai.x II Marriages and births in Xorway. The time trend by periods of fwe years For thedefinition of‘¿ maximum', seetext MarriagesBirthsIllegitimate birthsMarr.
18g6—190o
..
1901—05
..
ig@i—45 1946—50
@@@i—55
1956—60
,96i—6s
marr.
marr.
6-8
i8i
207
170
193
I ,oooSummermax.Winter max.Birth
max.1891-95
1905—10 1911—15 1916—20 1921—25 1926—30 i@@i—35 1936—40
rate per
6@z
i68
rate
per
birth
birth max.Rate
137.3
110.0
II2@5 1101
i,oooJan.-Maymax.Sept.
131.0
109-8 1085
..
6@o 6-3
..
7@0
175
195 261
6-i
i8i i8i
197 215 24!
182
268
971 77.0 63-2 62@I
120'4 1200
110-5
173
232
75-2
111-9
109-4
88@6
1255 1205
..
.. .. .. .. .. .. .. .. ..
1966-70
..
1971—75..
..6-@.
6@ 6-@ 86 8-o 90
8-o 6-8 6@6
76175
162 163
223 222 250 251
245198
192
230 240
260
238
2261378
I2I@5 113'I
Io8@2
9O@4 90.9 89-3
III-! 111-2 118-2
fl9.7
ii8@i
1142 92-31109 11401131
IO9@2
112-0 112-I 1105
@9.5
II0@4
iio-8 iio@6
107-07-12
per i,ooo
birth
birth
7.44
io6@8
6'72 7-00
118-9 1115
112-4
birthsJan.-Maymax.Sept.
705
6-94
1091
I07@6
6.44
114.4 126.4 I20@4 128-8 II4@3
5.03
128-5
706
7.17 7.17
6-78 @-6o 3-56 3@85 560
iio@6 1o9'9 1o9.5 1045
98.o
II2@6
114.4 1117 1112 112-I
1o9-6 113.7
ii@@8 105-9
ioi-6 114.7
io@-i
907107-6 107-5117.9 100.1
SEASON OF BIRTH IN THE POPULATION OF NORWAY
342 From
1950 on there is again a slight tendency
towards a negative correlation. During these considerable changes the Sep.. tember birth peak remained unchanged, cvi. dently because it was relatively independent of the birth rate as well as of such upheavals a.s the war. Now it becomes understandable why the September birth peak is the same in schizo phrenics as in the general population. The relatively superficial traditions which lie behind this birth peak are not related to any predisposi.i tion towards
@
schizophrenia.
The January—May
maximum, on the other hand, is related to socio@ biological factors of a more basic significance for brain function and mental health. The mid-winter traditions vary from one country
to another,
although
they
are older
and more universal than Christendom. Inter national comparisons are, therefore, of great importance. So far it is known that the Septem ber birth peak is the same in Sweden as in Norway (Dalén, 1974). In Great Britain it seems to be less in evidence, and Hare et al (i@7@) do not discuss the problem explicitly. From their figures the following September peaks can be calculated: Schizophrenia Other psychoses Not psychotic
I()@3.9per cent
xoo@8
,,
98@7
In native schizophrenics in South Africa the findings were conificting (Dalén, 1974). Here, as well as in Australia (Parker et al, 1976), the situation is complicated because in the southern hemisphere the cyclic birth maximum is in August—October and so will cover up any possible September maximum. The most useful data are probably the Japanese presented by Shimura et al (@77). For 7,78! schizophrenic patients from Tokyo there was a September peak of 105' I per cent (calculated by the present author from a table given by Miura). The monthly distribution of illegitimate births is of interest
because
the influence
of marriage
is eliminated and any birth maximum will correspond to a maximum of conceptions. Besides this, there is the possibility that con ceptions out of wedlock are less carefully
planned, for instance with regard to of ‘¿ winter babies'. During our investigation there were in Norway 293,772 illegitimate births, but considerable number study of the monthly
avoidance period of a total of even this
was barely sufficient for a pattern during periods of
five years. There are; therefore, irregularities due to smailnumbers, and besides this there are problems in connection with the registration of illegitimate births. Over a period of 8o years there are likely to have been changes in legisla tion and in administrative practice. Up to 1931—35 around 7 per cent
of the
births were illegitimate. Then a rapid decrease set in, and a minimum of @6per cent was reached
in 1951—60. This
decrease
rupted by a peak of illegitimacy
was inter
in 1943—45,
undoubtedly in connection with wartime condi tions and the German occupation. Since 1965
there has been a remarkable
increase from 4 to
10 per cent in ten years. During economic situation has been
marriage and childbirth, must
therefore
be
a
this period the favourable to
and the explanation
fundamental
change
in
(Table
II).
attitudes towards sex and marriage. The two birth maxima are largely the same in
illegitimate
as in legitimate
births
While legitimate births are most frequent in April, the illegitimate births have their peak in May, corresponding to August rather than
July conceptions. the July summer
illicit connections. During
most
Most probably, relations to vacations are less close for
of the five-year
periods,
both
birth maxima are slightly higher for illegitimate births, but the figures are irregular. After 1950 this trend is clearly reversed, and the monthly fluctuations are higher for legitimate births.
The monthly fluctuations represent a pattern of regularity which seems to be in part lost in the illegitimate births, but this is a reJatively
recent trend which did not exist before 1950. It may have something to do with the fact that births out of wedlock are more frequent and acceptable
than they were previously.
It is possible that planned
conceptions
tribute to the monthly pattern wedlock, for instance as a planned
of births avoidance
con
in of
‘¿ winter babies', a planning which is to some extent lost in illegitimate births.
BY ØRNULVØDEGARD 1896-1900
t906-10
1916-20
1926-30
1936-40
E x
343 1946-50
1956.60
1966-70
C
a, E
‘¿ U
E
0
.C
00
Q
a,
0.
a,
.C
00
:January-May birth maximum —¿ ———-:September birth maximum —¿
:Birth
rate
Fio 2.—I.Amplitude of the January-May birth maximum per ioo of the corresponding October December minimum; 2. September birth maximum per ‘¿ of 00 the August+ October minimum; 3. Birth rate per i,ooo women 15 to 44 years of age. The curve shows the time trend 1891—1970.
0 0
a
U) .C .0 a,
‘¿ U
E 0)
a,
Legitimate: lUegitperlOO:— IUegitimate:—————— FIG 3.—The amplitude of the April and September birth maxima
legitimate
and illegitimate
births.
Also number
of illegitimate
per ioo of the corresponding minima
for
births per 100 total births. The curve shows
the time trend 1891—1970.
Minimal paranatal brain damage is supposed to be more frequent in illegitimate births. If such brain damage is a contributory cause of
the increased seasonality in schizophrenics, then one would expect higher monthly fluctuations in illegitimate births. The evidence in favour of
SEASON OF BIRTH
344
IN THE POPULATION
thishypothesisis not convincing in our data,
but then the relations between illegitimacy and a predisposition towards schizophrenia are far from close. Conclusions The cyclic January—May maximum is basic ally of biological origin but with secondary influence from such social factors as marriage rate and birth rate, and consequently with a rather variable amplitude. The September birth peak is due to one special social factor, namely a conception maximum festivities.
during From
the traditional mid-winter a biological point of view,
these traditions are superficial, but nevertheless they are powerful and resistant to change, and during our 8o-year-long period of investigation the September peak has shown practically no variation. Schizophrenic patients have the same January—May birth maximum as the general population, only with a higher amplitude. This suggests
that
the
pronounced
maximum
of the corresponding pregnancies) coincide with a season which is known to have an increased
ørnulv
Ødeg@rd,
Oslo 3, Xorway
Professor
Emeritus,
University
risk of brain damage. There results an increased risk of schizophrenia in those who are genetically
predisposed. There is no such increased risk of non-schizophrenic psychoses, because brain damage is less important in the etiology of these disorders.
No association is observed between schizo phrenia and the September birth peak, because this maximum results from social factors (i.e. mid-winter traditions) which are not associated with either brain damage or schizophrenia. References BAaay, HERB. & BAJUW,HERB. JR (1964) Season of birth in schizophrenia. Archmes of General P@ychiat,y, II, 385—91. DAL*N,
PER
1974, HARE,
E.
(1974)
Gotab, H.,
Season
of Birth
in Schizophrenia.
Gsteborg
Kungalv.
Piuca,
Jom@
& SI@vrRB,
E.
(ig74)
Mental
disorder and season of birth: a national sample
compared with the general population.
of P@ychiatiy,124, 8i-6. MEDNICK, S.A.
(1970)
Breakdown
British Journal
in individuals
risk of schizophrenia: possible dispositional factors. Mental Hygiene, 54, 50-63.
in
schizophrenia results from the same causes as the somewhat lower maximum in the general population, i.e. predominantly biological causes. The tentative hypothesis is offered that these biological factors, which so far are not com pletely understood, lead to a seasonal pattern of April—August conceptions. The consequent January—May births (as well as the last weeks
OF NORWAY
ODEGARD,
0.
(,@7i)
Season
of birth
with high
perinatal
in the general
popula
tion and in patients with mental disorder in Norway.
British Journal of Psychiatry,125, 397-405. PARKER,
GORDON
&
NEILSON,
disorder and season of birth. chiahy, 129, 355, 361.
S@s&AItoFp, A. J.
serious
& ZAK, M.
mental
disorder.
grammeBulletin, 10, 423-5.
Mao@u@i
(1972)
(1976)
British Journal Neonatal
Mental
of Psy
factors
SrnMuit@, M., NAK@usuit@,I. & Miua@, T. (ig77) Season of birth of schizophrenics in Tokyo, Japan. Ada
psychiat., wand, 55, 225—32.
of Oslo,
(Received 26 Xovember 1976; revised 5 Janua@y 1977)
in
Xeuroscience Research Pro
Gaustad
Psychiatric
Hospital,
Box
z6 Gaustad,
