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  • Title: The Season-of-Birth Paradox.
    Author: Rodgers JL, Udry JR.
    Journal: Soc Biol; 1988; 35(3-4):171-85. PubMed ID: 3241987.
    Abstract:
    There is a seasonal pattern to births in the US: they peak in August-September, peak less strongly in December-February, and show a deep valley in April. What makes this pattern paradoxical is that, as was shown by a survey of 235 undergraduates, the preferred birth months are April-May, and the least desired months are July-August and December-January. 3 hypotheses have been put forward to explain this paradox. The Bad Data Hypothesis holds that college students are not a proper model for the decision making patterns of actual married couples. The Biological Domination Hypothesis holds that birth patterns are not under volitional control, but are determined by hormones, the pineal gland, weather patterns and fetal mortality patterns. The Misinformed Reproducer Hypothesis holds that couples underestimate the lag time between discontinuing contraception and becoming pregnant. To test this hypothesis, married women, aged 15-44, whose contraceptive and pregnancy histories were known, were selected from the National Survey of Family Growth Cycles for births in 1973-75 and 1979-81. Of these, 1271 women stopped contracepting in order to get pregnant and remembered when they had stopped contracepting. The months of stopping contraception were statistically averaged over the 6 years, and in each year there was a valley in February, March or April; and in 5 of the 6 years there was a peak in June. An 11-month dummy variable regression model was used to test the reliability of these patterns for statistical significance. The analysis showed that couples stopped using contraceptives on the assumption that pregnancy would ensue almost immediately. 10 months from February-April are December-February, and 10 months from June is April. If a 5-month pregnancy lag were added to the 9-month gestation, then births would peak in May-August and the valley would be in January-March. This pattern is still 2 months off from the actual birth distribution; however, the retrospective data probably underestimate the real pregnancy lag.
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