Does parental age difference affect offspring count in humans? Comment on Fieder and Huber.

In a recent paper, Fieder and Huber (2007) attempt to estimate the “fitness effects” of parental age differences, to test the hypothesis that age preferences are evolutionarily acquired. The authors use modern data from the Swedish Total Population Registry. In their specifications, Fieder and Huber (2007) use offspring count as their dependent variable and parental age difference as an independent variable. They report that in men, fitness is maximized when marrying a wife 4 years younger, and in women, it is maximized when marrying a man 6 years older. However, the authors do not control for women’s reproductive value in their regressions. Replicating their findings using a more appropriate dataset of pre-modern Swedish individuals, we show that when controlling for women’s reproductive value, the estimated effect of parental age differences on reproductive success disappears.