Men are taller than women, by an average of about 5 inches. But why? It’s not a genetic inevitability — there are many species in which females outclass males.A new study published Monday in the Proceedings of the National Academy of Sciences has found a partial explanation. It involves a gene called SHOX, which is known to be associated with height.SHOX: present in both sexes…SHOX is present on both the X chromosome (females have two X chromosomes) and the Y chromosome (males have one X and one Y). If it were to explain the height difference between males and females, SHOX would need to have a different effect on each chromosome.To investigate this hypothesis, researchers asked if an extra Y chromosome boosted a person’s height more than an extra X chromosome.There are rare conditions in which people are born with an extra X or an extra Y, or have a missing X or Y. To find people with these conditions, researchers plumbed data from three biobanks, one from Britain, and two from the US.Looking at nearly a million individuals’ data, the researchers were able to find 1,225 people with either missing or extra X or Y chromosomes. Some of these conditions, like in people with one X and no Y, were known to be associated with health issues — as well as, in this case, short stature.And, researchers found, an extra Y did provide more height than an extra X. Their hypothesis was thus borne out.Story continues below this ad… But more active in malesThe biochemistry of the SHOX gene may be the reason.Matthew Oetjens, a genetics researcher at Geisinger College of Health Sciences in Danville, Pennsylvania, and senior author of the study, explained.The placement of the SHOX gene is near the end of the sex chromosomes. In females, most genes on one of the two Xs are silenced, or inactive. But one region where the genes remain active is at the very tip of the X. The SHOX gene is close enough to the tip that it is not quite silenced. In men, the X, with its SHOX, is fully active. So is the Y.This means that a woman, with her two X chromosomes, will have a slightly lower dose of the SHOX gene than a man, with an X and a Y. As a result, males get a slightly bigger SHOX gene effect.Story continues below this adThat, the researchers calculated, accounts for nearly a quarter of the average difference in height between men and women. Oetjens said that other features of male sex hormones account for most of the rest of the difference, and other genetic factors are thought to play a role.The work is “definitely cool,” said Eric Schadt, a professor in the department of genetics and genomic science at Mount Sinai School of Medicine in New York City. He said: “It is a great use of these biobanks to uncover what is still somewhat of a mystery… Even though the effect is modest, it does explain 20% or so of the height difference.”