Brief: Season of birth and signaling anger

The various questions in the General Social Survey that probe people's personalities -- for example, whether you consider yourself outgoing -- don't show strong differences in the responses by birth month. But what if we look at questions that focus more on people's actual behavior? I found one question that asks how much the person agrees with the statement, "When I'm angry, I let people know." Here is the percent who either strongly agree or agree by their birth month:


A majority in all groups signal their anger to others, but there is a very clear seasonal pattern, with this tendency increasing during the fall, winter, and spring birth groups, then falling during the summer group. Moreover, the change is not trivial -- 11.2 percentage points separate the low in August from the peak in April.

This could just be a reflection of genetics rather than environment, whereby anger-signaling mothers pass on their variants for the trait, but where they're more likely to conceive in the later part of summer. Still, I favor an environmental story where babies born into more stressful environments assume unconsciously that life is going to be tough, and so adopt a more defensive "don't mess with me" posture. Most such stress today will not come from parental strife or lack of basic nutrition but from the one source that we still don't have much control over -- pathogens that make us sick. That's why the flu season shows up pretty well in the graph.

There could also be a vitamin D story, where newborns produce less vitamin D during the less sunny months (sunlight hitting the skin is a necessary step), and that this somehow influences their personality or behavioral strategy. But without a clearer mechanism, I wouldn't put much emphasis on this -- why wouldn't the mother's vitamin D level matter, which would tend to go in the opposite direction? In any event, there's something neat to try to explain.

GSS variables used: showangr, zodiac

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