I finally found an article that doesn't just assume that each time you have a baby there's a 50/50 (or technically 51/49) chance that you'll have the same gender again. This is a tougher search problem than it sounds, because so many science explainers take the easy way out and refer to pregnancy as a coin flip. Obviously the previous coin flips don't inform later ones. But how do you know birth is a coin flip? I suggest that the immune system + reproductive system + parental choice + environmental factors = something more complicated than a coin flip, something with memory potential. SO ... I finally found this article, which references an earlier one done by actual statisticians:
http://www.in-gender.com/XYU/Odds/Gender_Odds.aspx
It turns out there may be a slightly increased chance (up 2 to 6%) of having a fourth boy if you already have three. Maybe not the same for girls, interestingly. I'd guess if we have another child, there's a ~55% chance of having a boy, based just on this survey of actual family birth orders. And that sounds about right: nothing too different from the "coin flip" explanation, but something revealing a system that's more complex and interesting. Personally, I wonder if my NKG2D or MICA immunoproteins might not be involved?
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This is cool. Having married into a family with eight kids, of whom only the third is a girl (and cousins, uncles etc. are also overwhelmingly male--well, obviously the uncles are, but there aren't a lot of aunts), I'm convinced that there has to be something more to those assertive Y chromosomes than coincidence--although somehow I ended up with a girl the first time around.
Anyway, congratulations! Boy, girl, they're all good.
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