https://www.wired.com/story/first-big-survey-of-births-finds-millions-of-missing-women/

Every time a man ejaculates, he produces somewhere between 40 million and 1.2 billion sperm. About half of those tiny swimmers carry an X chromosome and about half carry a Y. So you’d think that the odds that one or the other would be the first to fertilize an egg would be about the same as a coin flip. And yet, for as long as people have been keeping records, nature shows a different, dependable pattern: For every 100 babies born biologically female, 105 come out biologically male. Scientists have speculated this mysteriously male-biased sex ratio is evolution’s way of evening things out, since females consistently outlive their XY-counterparts—for every man that reaches the age of 100, four women have also joined the Century Club.

This biological maxim has been so drilled into the heads of demographers—the researchers responsible for keeping tabs on how many people there are on the planet—that most don’t think twice before plugging it into any projections they’re making about how populations will change in the future. But a massive effort to catalog the sex ratios at birth, for the first time, for every country, shows that’s not such a smart strategy after all.