Using a fire-horse cohort to estimate mortality associated with dual inheritance stress

Dual Inheritance theory posits that persons reaching reproductive age must allocate their time, energy, and resources, at least temporarily, to either producing offspring or attaining social status. Ill health (including mortality) is hypothesized to arise from the stress induced by striving to reconcile these competing imperatives. Using data from the Human Mortality Database, we test predictions made by dual inheritance theory for population health. Our test exploits the 1966 episode of the Japanese superstition that stigmatizes women born in "Fire-Horse" years. This superstition reduced not only births by 25% in 1966 but also the marriage prospects of women born in that year. We applied Box-Jenkins time-series modeling to data describing mortality in 50 Japanese birth cohorts (i.e., 1945 through 1994) to test the hypothesis that sex specific death rates at ages 20-24 and 25-29 for both sexes of the 1966 cohort would exceed those expected from rates exhibited by cohorts born before and after. Contrary to the dual inheritance hypothesis, the 1966 cohort exhibited lower than expected mortality at ages 20-24 and 25-29 for both sexes. Our findings suggest that the socioeconomic advantages of being born into a small cohort may buffer the pathogenic effects of dual inheritance stress. These findings should improve forecasts of the health effects of strategies societies can choose from to influence, and adapt to, the ongoing decline in fertility in much of the developed world.