Abstract
Background and objectivesThe sex ratio of human birth cohorts predicts the health and longevity of their members. Most literature invokes natural selection in support of the argument that heritable tendencies to produce male or female offspring induce oscillation in the sex ratio and its sequelae. Tests of the argument remain exceedingly rare because they require vital statistics describing many generations of a population both unaffected by migration and exposed to an exogenous stressor virulent enough to change the sex ratio at birth. We contribute to the literature by using time-series modeling to detect oscillation in the best data currently available for such a test.MethodologyWe apply rigorous time-series methods to data describing Sweden from 1751 through 1830, a period when the population not only aged in place without migration, but also exhibited the effects of an Icelandic volcanic eruption including a historically low secondary sex ratio. That very low sex ratio should have induced oscillation if heritable mechanisms appear in humans.ResultsWe detected oscillation in the ratio but not that predicted by heritable tendencies to produce males or females. We found peak-to-trough oscillation at 14 rather than the approximately 32 years expected from the heritable tendencies argument.Conclusions and implicationsOur findings suggest that mechanisms other than perturbation of heritable tendencies to produce males or females induce oscillation in the human secondary sex ratio. These other mechanisms may include reproductive suppression and selection in utero.LAY SUMMARYThe male to female ratio in human birth cohorts predicts longevity but its variation over time remains unexplained. We test the long-held theory that the ratio oscillates due to heritable tendencies to produce males or females. We find oscillation, but it appears due to social processes rather than heritable mechanisms.
Highlights
Consistent with evolutionary theory [1], the health of human populations reportedly affects the ratio of males to females in birth cohorts [2]
Use that year to anchor our test of hypothesis two—that an extreme sex ratio associated with an exogenous shock should be followed by an opposite, though damped, outlier at a lag equal to that of the oscillation, if any, detected in the test of hypothesis one
The pattern of coefficients shows the signature of damping oscillation at 14 years rather than at 32–35 years, the age at which Swedish women and men reached peak fertility during this period, as predicted by Hamilton’s summary
Summary
Consistent with evolutionary theory [1], the health of human populations reportedly affects the ratio of males to females in birth cohorts (i.e. the secondary sex ratio) [2]. Males in higher sex ratio cohorts show greater mortality before adulthood [9] as well as diminished overall lifespan [3] In light of these reports, understanding temporal variation in the sex ratio at birth would seem an important topic for research at the intersection of evolutionary theory, medicine and public health. Conclusions and implications: Our findings suggest that mechanisms other than perturbation of heritable tendencies to produce males or females induce oscillation in the human secondary sex ratio. These other mechanisms may include reproductive suppression and selection in utero. But it appears due to social processes rather than heritable mechanisms
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