Monday, February 22, 2010

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

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