Although sperm have a 50–50 chance of having either X or Y chromosomes, the natural occurrence of births reveal 4.4% more males, according to this study published by the European Society for Human Reproduction and Embryology.
But why is that?
It has long been theorized that it is because males are more prone to “injuries, accidents, and war,” according to this article by the UT Southerwestern Medical Center.
But this write-up by the San Jose State University really clears things up. X chromosomes are literally heavier than Y chromosomes because they have more genetic code, which makes X-chromosome sperm harder to swim upstream.
In fact, X-chromosome sperm have 4.4% more DNA material than Y-chromosome sperm. That’s exactly the same % of more male births and female.