Subtitles section Play video Print subtitles 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. Thanks to everyone who support us on Subbable. Without you we literally couldn't continue to make these videos. If you aren't yet a supporter of MinuteEarth and want to become one, head over to subbable.com/minuteearth to make a one-time or monthly pledged support and thanks for watching.