Brief: Intelligence and age at first birth

I've seen many pictures and tables showing that smarter people get married later and have fewer children. But what about when generation length -- how long do people wait until they actually have their first kid? I'm sure those data are out there, but I don't recall them off the top of my head, so they can't be too well known.

The General Social Survey asks what age you were when your first child was born, and also gives them a makeshift IQ test. Here is the relation between them:


Generation length increases as you look at smarter people, but it only becomes pronounced in the average-and-above range (6 to 10 words correct). Among those below-average in intelligence, there is no strong tendency toward longer or shorter generation length. So not only do below-average people reproduce more in terms of number of children, but they also reproduce faster. Over the course of a century, below-average people with a generation length of about 22.5 years will have had 4.4 generations, while the smartest people with a generation length of about 27 years will have had only 3.7.

To put this in an evolutionary context, think of a new selection pressure on both populations that was strong enough to cause a new mutant to reach fixation within 50 generations (a pretty strong pressure). It would take the below-average population 1125 years to get there, 225 years before the smartest group would get there (1350 years in total). These are not necessarily the *same* pressure for each group -- it doesn't matter what direction they are pushing adaptation in, just that the two pressures are pushing equally strongly on their respective populations. This huge split between smart and dull is pretty new, and who knows how long it will last, but over the next millennium we may see the duller people adapting at more impressive rates on a genetic level. Hopefully the smarties will figure out non-genetic ways to adapt just as impressively to their environment.

GSS variables used: wordsum, agekdbrn