The sex ratio and age-specific male mortality: evidence for culling in utero.

While adverse conditions early in life reportedly predispose individuals to increased mortality in adulthood, controversy remains as to whether exogenous insults in utero, especially among male fetuses, induce similar cohort "damage" in populations. A rival theory postulates that exogenous stressors in gestation may "cull" frail male members of the cohort before birth, leaving a smaller but hardier cohort with improved survival. Recent tests, which use the sex ratio (i.e., the odds of a male live birth) as a gauge of insults inflicted upon cohorts in gestation, support the culled cohort argument. These tests, however, examined only aggregate male lifespan, thereby obscuring potential heterogeneity of both damaged and culled cohorts at specific ages over the life course. Using time-series methods, we explore associations between the sex ratio and cohort male mortality in infancy (before age 1), childhood (1-4 years), youth (5-19 years), adulthood (20-54 years), and old-age (55-79 years). We examine males born in Sweden (1751-1913), Denmark (1835-1913), and England and Wales (1841-1912). Our findings generally support culled cohorts in that male mortality across all ages fell below its expected value among cohorts in which the sex ratio dropped below its expected level. These findings suggest that exogenous shocks to gestation, as measured by a lower than expected sex ratio, may cull males in utero, leaving behind a less frail cohort over the entire life course. Copyright 2007 Wiley-Liss, Inc.