J. biosoc. Sci. (1990) 22, 113-119
SEASONAL VARIATION IN HUMAN BIRTHS WILLIAM H. JAMES MRC Mammalian Development Unit, University College London Summary. During the first half of this century, the seasonal pattern of births in European countries showed a major peak in the spring and a minor peak in the autumn. In contrast, the pattern in the US was of a minor peak in spring and a major peak in autumn. Over the last 20 years, the pattern in England and Wales has changed to resemble the US pattern, and the same seems to be true of several other European countries. A hypothesis is offered to account for the difference between the European and the US patterns and for the change from one to the other in some countries. The magnitude of seasonality correlates positively with latitude: it is suggested that this is partially consequent on variation in luminosity. Introduction
There is a substantial literature on the seasonality of human births. Cowgill (1966a,b, 1969) dealt with such phenomena worldwide, and Rosenberg (1966) considered in particular the seasonal variation in births in the United States. Some of the empirical findings in thisfieldillustrate the problems to be dealt with here, and the following is a summary of what was known in the late 1960s. The countries in Europe show a major peak of births in spring and a minor peak of births in autumn. Those European countries which celebrate Christmas have a minor peak of births in September; those which celebrate the New Year (or Christmas according to the Julian Calendar) have a minor peak of births in October. The histograms for New Zealand, South Africa and temperate South America resemble that of England and Wales, displaced by 6 months. In contrast, the major peak of births in the United States is in the autumn and corresponds to conceptions in November-December. Moving south in the US, this peak is accentuated, presumably because of climatic inhibition of summer conceptions. There is a similar variation in degree of seasonality with latitude in Australia (Mathers & Harris, 1983) but in recent decades in the US, the extreme seasonality in the southern states has moderated (Querec & Spratley, 1978), apparently following the use of air conditioning (Seiver, 1985). In earlier years in this century, Canada's pattern was similar to that in Europe, but over the years, the pattern began to become more like that of the US; this process has never been completed, Canada having had spring and autumn peaks of births of roughly equal height in the years 1960-84 (Ashley, 1988). Conclusions to the contrary (Halli, 1989) seem to be based on insufficient material. In developed societies, seasonality of births is scarcely affected by seasonality of 113
114
W.H.James
Table 1. Monthly birth indexes* and range, and latitude: selected countries Month Latitude Austria 1986 Belgium 1985 Denmark 1986 Eire 1985 England & Wales 1966 1976 1986 Finland 1976-80 Hungary 1986 Italy 1981 Jugoslavia 1973 1978 1983 Norway 1966-70 1976-80 1983 Spain 1980 Sweden 1971-75 1986 Switzerland 1971 1978 1985 United States 1980-84 Canada 1980-84
(°N)
J
A
S
48 51 56 53
101 96 87 102
103 98 102 103
100 101 101 104
103 105 111 103
99 104 105 107
99 100 103 98
103 107 106 102
101 99 100 97
105 103 108 104
97 100 96 100
93 93 90 91
96 95 91 91
12 14 24 16
53 62 47 44
98 98 97 95 100 97
103 101 98 103 101 96
108 105 101 111 102 99
103 102 100 110 102 103
103 103 102 105 97 105
101 102 102 103 103 105
100 102 103 99 108 107
98 99 102 98 104 101
101 104 105 100 102 107
96 98 99 93 95 97
93 94 97 91 92 93
95 93 94 92 95 91
11 20 16 16
45
108 103 104 106 104 99 98 98 104 103 94 80 108 102 101 103 103 99 100 101 105 99 93 86 101 98 101 103 102 101 102 104 106 100 93 90
16
61 41
97 97 94 101
94 89 92 95
21 11
60
98 106 112 117 109 102 98 95 98 92 88 87 99 105 111 117 106 104 101 100 104 89 84 83
34
47
100 105 111 110 106 106 102 94 97 92 90 87 99 104 108 109 106 100 94 98 93 93 90 91 100 101 107 105 103 100 104 98 102 98 93 89
18
97 93
F M A M J
101 103 102 99
106 110 110 100
115 114 111 102
106 106 103 104
J
101 103 105 101
97 99 102 103
96 97 99 100
O
103 100 102 104
N
96 93 91 99
D
89 89 90 93
Range
98 97 97 96 100 105 105 107 100 97 98 98 103 104 103 103 102 100 105 98 95 94
* Numbers of births per day, as a % of the overall annual number per day.
marriage. This is because the seasonal pattern of illegitimate births is similar to that of legitimate ones; and the seasonal pattern of first births does not differ appreciably from that of other births. Cowgill (1969) noted that in the years following World War 2, Puerto Rico, which had formerly had a seasonal birth pattern of the European type, developed a pattern of the US type. Similar phenomena have been observed in respect of Japan (Shimura, Richter & Miura, 1981) and England and Wales (Lyster, 1985), while in the southern hemisphere, the Australian major seasonal birth peak in September in the early 1960s was replaced by a peak in February and March in the late 1970s (Mathers & Harris, 1983). It seems worth examining some recent data on births in selected European
Seasonally in human births
115
countries to see whether their patterns of seasonality show any changes since they were described by earlier writers. Table 1 gives data assembled from volumes of vital statistics held in the OPCS library. They show first that changes towards a New World pattern of birth seasonality have occurred in a number of European countries. In others the seasonal birth patterns may be in transition or (as apparently in Canada) they may have stabilized at shapes intermediate between the Old and New World patterns. Secondly, the amplitude of seasonality (as measured by the range of monthly birth indexes) is clearly greater in the Scandinavian countries than in other European countries and in Canada. The correlation between latitude and range of birth indexes for the thirteen European countries in Table 1 may be assessed from a Spearman's p of +0-6 (/ J
SEASONAL VARIATION IN HUMAN BIRTHS WILLIAM H. JAMES MRC Mammalian Development Unit, University College London Summary. During the first half of this century, the seasonal pattern of births in European countries showed a major peak in the spring and a minor peak in the autumn. In contrast, the pattern in the US was of a minor peak in spring and a major peak in autumn. Over the last 20 years, the pattern in England and Wales has changed to resemble the US pattern, and the same seems to be true of several other European countries. A hypothesis is offered to account for the difference between the European and the US patterns and for the change from one to the other in some countries. The magnitude of seasonality correlates positively with latitude: it is suggested that this is partially consequent on variation in luminosity. Introduction
There is a substantial literature on the seasonality of human births. Cowgill (1966a,b, 1969) dealt with such phenomena worldwide, and Rosenberg (1966) considered in particular the seasonal variation in births in the United States. Some of the empirical findings in thisfieldillustrate the problems to be dealt with here, and the following is a summary of what was known in the late 1960s. The countries in Europe show a major peak of births in spring and a minor peak of births in autumn. Those European countries which celebrate Christmas have a minor peak of births in September; those which celebrate the New Year (or Christmas according to the Julian Calendar) have a minor peak of births in October. The histograms for New Zealand, South Africa and temperate South America resemble that of England and Wales, displaced by 6 months. In contrast, the major peak of births in the United States is in the autumn and corresponds to conceptions in November-December. Moving south in the US, this peak is accentuated, presumably because of climatic inhibition of summer conceptions. There is a similar variation in degree of seasonality with latitude in Australia (Mathers & Harris, 1983) but in recent decades in the US, the extreme seasonality in the southern states has moderated (Querec & Spratley, 1978), apparently following the use of air conditioning (Seiver, 1985). In earlier years in this century, Canada's pattern was similar to that in Europe, but over the years, the pattern began to become more like that of the US; this process has never been completed, Canada having had spring and autumn peaks of births of roughly equal height in the years 1960-84 (Ashley, 1988). Conclusions to the contrary (Halli, 1989) seem to be based on insufficient material. In developed societies, seasonality of births is scarcely affected by seasonality of 113
114
W.H.James
Table 1. Monthly birth indexes* and range, and latitude: selected countries Month Latitude Austria 1986 Belgium 1985 Denmark 1986 Eire 1985 England & Wales 1966 1976 1986 Finland 1976-80 Hungary 1986 Italy 1981 Jugoslavia 1973 1978 1983 Norway 1966-70 1976-80 1983 Spain 1980 Sweden 1971-75 1986 Switzerland 1971 1978 1985 United States 1980-84 Canada 1980-84
(°N)
J
A
S
48 51 56 53
101 96 87 102
103 98 102 103
100 101 101 104
103 105 111 103
99 104 105 107
99 100 103 98
103 107 106 102
101 99 100 97
105 103 108 104
97 100 96 100
93 93 90 91
96 95 91 91
12 14 24 16
53 62 47 44
98 98 97 95 100 97
103 101 98 103 101 96
108 105 101 111 102 99
103 102 100 110 102 103
103 103 102 105 97 105
101 102 102 103 103 105
100 102 103 99 108 107
98 99 102 98 104 101
101 104 105 100 102 107
96 98 99 93 95 97
93 94 97 91 92 93
95 93 94 92 95 91
11 20 16 16
45
108 103 104 106 104 99 98 98 104 103 94 80 108 102 101 103 103 99 100 101 105 99 93 86 101 98 101 103 102 101 102 104 106 100 93 90
16
61 41
97 97 94 101
94 89 92 95
21 11
60
98 106 112 117 109 102 98 95 98 92 88 87 99 105 111 117 106 104 101 100 104 89 84 83
34
47
100 105 111 110 106 106 102 94 97 92 90 87 99 104 108 109 106 100 94 98 93 93 90 91 100 101 107 105 103 100 104 98 102 98 93 89
18
97 93
F M A M J
101 103 102 99
106 110 110 100
115 114 111 102
106 106 103 104
J
101 103 105 101
97 99 102 103
96 97 99 100
O
103 100 102 104
N
96 93 91 99
D
89 89 90 93
Range
98 97 97 96 100 105 105 107 100 97 98 98 103 104 103 103 102 100 105 98 95 94
* Numbers of births per day, as a % of the overall annual number per day.
marriage. This is because the seasonal pattern of illegitimate births is similar to that of legitimate ones; and the seasonal pattern of first births does not differ appreciably from that of other births. Cowgill (1969) noted that in the years following World War 2, Puerto Rico, which had formerly had a seasonal birth pattern of the European type, developed a pattern of the US type. Similar phenomena have been observed in respect of Japan (Shimura, Richter & Miura, 1981) and England and Wales (Lyster, 1985), while in the southern hemisphere, the Australian major seasonal birth peak in September in the early 1960s was replaced by a peak in February and March in the late 1970s (Mathers & Harris, 1983). It seems worth examining some recent data on births in selected European
Seasonally in human births
115
countries to see whether their patterns of seasonality show any changes since they were described by earlier writers. Table 1 gives data assembled from volumes of vital statistics held in the OPCS library. They show first that changes towards a New World pattern of birth seasonality have occurred in a number of European countries. In others the seasonal birth patterns may be in transition or (as apparently in Canada) they may have stabilized at shapes intermediate between the Old and New World patterns. Secondly, the amplitude of seasonality (as measured by the range of monthly birth indexes) is clearly greater in the Scandinavian countries than in other European countries and in Canada. The correlation between latitude and range of birth indexes for the thirteen European countries in Table 1 may be assessed from a Spearman's p of +0-6 (/ J
