Saturday, December 10, 2005

Not Exactly Fifty Fifty

Here’s what I’m being told: Although it’s true that approximately 50% of babies born are girls, and 50% are boys, this doesn’t hold true within a family. Meaning, that apparently there are familial dispositions towards producing boys or girls. In other words if a couple’s first child is a boy, the likelihood of the next child being a boy is more than 50%. And vice versa.

This explains why there are more families with say, four boys and no girls or four girls and no boys, than would be statistically expected. The chance of having four children of the same sex, if there is no bias, is two to the power of three, which is one in eight. But when you look at families with four children, the chance of all the siblings being of the same sex is more than one in eight. You’ll ask me what the true chance is. I don’t know. But apparently it’s higher.

Which reminds me that many years ago there was a lot of buzz in Israel about army pilots having mostly daughters. It was deeply investigated. Was there something in the G forces exerted while flying fighter planes that killed off Y chromosome sperm? I don’t know if this question was ever resolved. It doesn’t have much to do with the subject at hand, because in this fighter pilot example you have a whole population (not just one family) where the distribution of sexes is not fifty fifty, but still, it’s interesting…


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