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  • Most parents-to-be assumed that the sex of their child comes down to a flip of the chromosomal coin, with an equal chance at having a boy or girl.

  • But in reality, the odds aren't even.

  • For every 100 girls born, the world gains about 106 baby boys.

  • Currently that skewed sex ratio comes out to roughly ten million more baby boys than girls born worldwide each year.

  • There are some countries where human meddling stacks the odds even higher in favor of boys.

  • But that doesn't explain the fixed odds everywhere else.

  • The intrinsic boy-girl ratio is rigged by mother nature.

  • In fact, it's even more rigged than birth rates show.

  • Human conception results in about 150 male zygotes for every 100 females.

  • But there's a rather tragic reason for this big biological boy bias early on.

  • Male fetuses are much more likely to be miscarried or stillborn than female fetuses.

  • And boys that do make it out of the womb suffer more fatal diseases, take more mortal risks and fall prey to more violence than girls.

  • So by the time kids grow up and reach baby-making age, the ratio of males to females is just about one to one.

  • But the likelihood of a boy even making it to birth is also influenced by his mom's living conditions during pregnancy.

  • For example, when a massive famine struck China in the 1960s, the relative likelihood of having a son suddenly dropped until the famine ended.

  • And male Americans born to billionaires seem to have higher than average odds of fathering sons.

  • Somehow, female biology suppresses boys' survival in the womb during tough times, and boost it when times are good.

  • We've seen same pattern in other mammals too.

  • When resources are scarce, mothers give birth to fewer males than normal.

  • When resources are plentiful, they bear more.

  • The best explanation we have for this has to do with sex -- the other kind

  • In biological terms, the whole goal of copulation is to reproduce to pass on your genes to someone who will someday pass them on again.

  • Female offspring are almost guaranteed to reproduce, famine or no famine because male mammals are pretty much always willing to mate.

  • Males on the other hand have to compete for mating privileges.

  • A well-nurtured hunk has a good chance of mating with lots of females.

  • While a male weakened by famine might not score at all.

  • Some male offspring are a bigger risk in general. At all stages they're more likely to die.

  • And even if they live they might not reproduce.

  • But when times are good, boys' potential to father lots and lots of babies make them a biological risk worth taking.

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Most parents-to-be assumed that the sex of their child comes down to a flip of the chromosomal coin, with an equal chance at having a boy or girl.

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