(By astrology, I mean the idea that personality traits are influenced by planetary motion. There are many forms of astrology, and I’m not talking about any particular form, rather about the more general claim. To my knowledge, no such claim has been empirically supported, but I don’t assume that we would have detected it even if it were empirically demonstrable. I don’t believe in astrology, and I’m not convinced that what follows is a good argument or is convincing, but it’s possible mechanism by which it could happen.)
Step one is to notice that holidays are family and cultural phenomena. Indeed, holidays are cultural events; part of what defines a commonality or overlap of culture between two people is the celebration of holidays together. And holidays tend to be a family affair, so that there are probably genetic patterns in holiday celebration (the gene pool of those who celebrate Diwali is very different from the gene pool of those who celebrate Yom Kippur).
Step two: Holidays make babies. People boink after they get together and have fun. I’m not sure that that’s true of all holidays, but I’d expect it to be true of many holidays. For example, there’s a big spike in the US 9ish months after Christmas/New Years.
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From 1 and 2, we can conclude that regularities in the prevalence of certain genes in the gene pool may cluster around certain times of year.
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Now, certain behaviors and personality predispositions are likely gene-modulated (see e.g. many mild mental disabilities or divergences, like autism, ADHD, schizophrenia, etc.)
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And many holidays are based on plantary patterns: solar patterns, i.e. time of year; and lunar patterns, e.g. certain Chinese and Jewish holidays)
Therefor, planetary motion could modulate behavioral predispositions indirectly, by causing holidays that cause sex that cause gene pool variations throughout the year.
Indeed, the fact that humans celebrate holidays may itself support this line of argument. After all, shouldn’t we assume that a behavioral trait that seems to arise naturally and universally is being selected for? Celebrating holidays may confer a selection benefit on human groups that participate (c.f. assortative mating in 13 and 17 year cicadas).
We might also expect variation over time, since the role of holidays in society and reproductive sex will vary over time, as well as the genetic uniformity of the people celebrating them. That would allow for astrological patterns that were detectable when initially described by a pre-modern society, but which were no longer detectable by the time we had the tools to quantify the relationship.
I’d be interested to know what people who do believe in astrology think about this. I think it’s biggest weakness is how neat it feels, and how little susceptible it is to evidence. There are also issues with possible effect sizes and noise that might make an observable effect impossible to see. But it’s the kind of argument I could buy.