Smart Sperm Theory

This latest contribution to cognitive epidemiology by Arden et. al finds a correlation between intellignece as measured by “g” and sperm quality and brings up many sticky issues.

Cognitive epidemiology investigates correlations between health outcomes and intelligence and has found them for the most important health outcome mortality . Linda Gottfredson speculated that differences in IQ are the ultimate cause of the observed differences in health outcome by subjects in different socio economic stratum.

The problem is determining causality, is it poor implies stupid and fat that together imply early death or stupid implies fat and poor that both imply early death? Studies look at the relationship between IQ and specific events like accidents or heart attacks and then try to control for other factors. These epidemiological versions of The Bell Curve analyses suffer from the usual problems of untangling correlation from causality. This smart sperm paper is unique because the authors are not looking for causation, only correlation.

Arden et. al. reanalyzed some crusty data from a Vietnam Vet study done in 1985. They had intelligence and sperm data on 425 veterans and a found a small but significant correlation (0.14 – 0.19) between sperm motility, concentration and count and intelligence as measured by a “g” factor derived from a principle component analysis of four IQ test given around the same time as the semen collection. Further analyses controlled for a few variables known to be related to sperm quality (Body mass index, smoking history, age and abstinence) and”g” was still a significant predictor of sperm quality. So the correlation between “g” and sperm quality may not be mediated by the factors. That is, stupid does not imply fat which implies lazy sperm. In spite of assessment of homo and heteroscedasticity the analysis appears a bit perfunctory to me since there is no accounting for measurement error and other factors such as race or SES.

Regardless, the results lead to an explosive stream of evolutionary speculations from the authors about a general fitness factor: the “f” factor. If intelligence does not cause good sperm then their association may have a direct genetic basis like the correlation of intelligence and height. Unlike, height though, this association is probably not the direct result of assortive mating. Instead, it may be an indirect result of assortive mating through detrimental “mutation” load. Smarties have lower overall mutation loads and preferentially mate with other people with lower loads of mutations. Obviously this needs replication, but the Mensa guys should start chatting up the ladies with this news about mutation loads.