Is there any evidence that a family is more likely to have several children of the same gender? Anecdotally it seems that my friends whose first child was a boy went on to have other boys. Similarly those who had a girl (me included) went on to have other girls. Obviously I have friends with the full set of children, as it were, but they seem to be outnumbered slightly by the repeaters.
I recognise that this may well be confirmation bias on my part, but I wonder if anyone knows of any research done in this area? Is it supportable that some families (essentially some women) ‘have sons’ and some ‘have daughters’? Of course if gender selection were only down to probability, that would be impossible, but since there are all sorts of other factors at work, maybe it’s not.
I have idly looked for this sort of research through Google Scholar in the past, but it’s not my field and I don’t have easy access to a real library these days, so - anyone know?
If a couple has two kids, there’s a 50% chance that both kids will be the same gender. If a couple had three kids, there’s a 25% chance they’ll have three boys or three girls.
I’m one of four boys. There was a 1 in 16 (about 7%) chance of that happening, just as there was a 1 in 16 chance my parents would have four girls.
Not exactly long odds. Given all the families you’ve known, it’s not surprising you’ve met many whose kids are all-male or all-female.
Actually, it’s much more likely that some *men *‘have sons’ and some ‘have daughters’, not some women. On a chromosomal level, women have no input into the child’s sex: they’re contributing an X sex chromosome either way. It’s the man who contributes either an X chromosome, making a girl, or a Y, making a boy. So if a man’s X-chromosomed sperm are stronger than his Y-chromosomed ones for some reason, or vice versa, that would make him more likely to have children of one sex or the other.
Obviously it’s far from that simple - it’s possible that the environment in a woman’s cervix, or something, could favour one set of sperm over the other - but at the most obvious level, the guy is the one whose chromosomes make that call.
It’s seldom that everything occurs that matters only at the most obvious level. Vaginal, cervical, and uterine environments are now thought to contribute to success and failure in the ‘sperm wars’, even down to a single individual’s sperm.
This calculation assumes that the gender of each child is independent of the others. There is research which indicates that this is not the case, although it’s relatively close to true.
Right - that’s basically what I said, only I said it in less detail. My point was that the OP’s implication - that it’s the woman who determines the child’s sex - doesn’t fit with some basic facts.
Reminds me of the family who lived behind us when I was a kid. The father was big on passing down the family name, so they kept hoping the next kid was a boy. At last count, they had seven girls. And they lived in a small 2-bedroom, 1-bath house.
panache45’s example brings in another factor: the so-called “stopping rule”, or conscious family-planning decision to stop or continue having additional children depending on whether a particular gender distribution has been attained. As this article notes,
If the woman’s body determines which sperm wins, then in no real sense is sex determined by the man. Indeed, the effects of the woman’s body would seem to be the only way in which sex could be “determined” in any meaningful sense. If I toss a million pennies in the air and the one that lands in my coffee cup is face-up, did I determine whether I saw Lincoln’s beard?
If a couple has a child of gender X, it’s more likely that the father has genetic predisposition to fathering children of gender X. Therefore it is relatively more likely that the next child will also be of gender X.
I would guess that there is also some input from the mother’s side but the same reasoning applies.
And of course this assumes that the mother and father don’t use some kind of artificial sex selection technique. I recall reading somewhere that among Indian-American families with 2 daughters, the chances that the third child will be a son are a lot higher than 50%.
No one’s saying the woman’s body determines which sperm wins. There’s some evidence that the woman’s body contributes to determining which sperm wins, which is an entirely different thing.
And see Brazil84’s post for evidence that the woman’s body isn’t in fact the only way in which sex is determined.
Thank you all, and especially brazil84, that’s exactly the sort of thing I was looking for. And yes I was assuming there wasn’t a straight equal probability.
And I did totally know about the father and the chromosomes, honest guvnor. I plead half-awakeness this morning.
More than 50% of pregnancies terminate with early miscarriage that generally the woman is not even aware of, and some women may preferentially miscarry male children more often than female, or the reverse, for some reason (I don’t know if this is the case, just suggesting a reason why the above is not necessarily correct).
