Does paternal investment increase child IQ?

The latest issue of Evolution and Human Behavior has an article on paternal investment and IQ. Using a longitudinal dataset of children born in Britain in 1958, Nettle (2008) found a significant positive correlation between a child’s IQ at age 11 and the father’s degree of family involvement. The less a father cared for his offspring, the less intelligent they were. Nettle concluded that paternal investment affects childhood IQ.

Now, correlation is not causation. It can be shown, for instance, that Presbyterian ministers in Boston have earnings that significantly correlate over time with the price of rum in Havana. But that doesn’t mean they’ve been dabbling in the rum trade. It simply means there’s a common causal factor, in this case the North American business cycle.

Similarly, a common cause may explain the correlation between low investment by fathers and low intelligence in their children. Deadbeat dads tend to be more present-oriented and probably less intelligent. Since intelligence has a large heritable component, their children would be less intelligent on average.

In all fairness, the Nettle study did control for social class, which in turn partially controls for time preference (i.e., whether the fathers were present-oriented or future-oriented). Specifically, the fathers were coded in terms of five occupational categories: professional, managerial and technical, skilled, partly skilled, and unskilled. I doubt, though, that this factor would have accounted for most variability in time preference. Even within the British working class, there is considerable variation, notably by religion and ethnicity, in the respective weighting that people give to present impulses versus future obligations.

Reference

Nettle, D. (2008). Why do some dads get more involved than others? Evidence from a large British cohort. Evolution and Human Behavior, 29, 416-423.