Men are usually about five inches taller than women, but scientists have long wondered why. Now, a new study points to a possible reason: A gene called SHOX.

The study — published May 19 in the Proceedings of the National Academy of Sciences — was drawn from genetic data from roughly 1 million people, The New York Times reported. Researchers looked at information from three large biobanks in the U.S. and the U.K.

They focused on a small group of people with either an extra or missing X or Y chromosome. 

These rare conditions gave researchers a chance to examine how extra copies of the SHOX gene affect height. 

They found that people with an extra Y chromosome were taller than those with an extra X chromosome. That suggests the SHOX gene, which appears on both the X and Y chromosomes, has a stronger effect when it’s on the Y, according to a report from The Times.

Here’s why: In women, who have two X chromosomes, one is mostly inactive and the SHOX gene is only partly active. In men, who have one X and one Y, both versions of the SHOX gene are fully active. 

In other words, the gene gives men get a bigger boost, researchers say.

That slightly larger gene effect explains about one-quarter of the average height difference between men and women, study senior author Matthew Oetjens, told The Times. He's a genetics researcher at Geisinger College of Health Sciences in Danville, Pa.

The rest of the height difference likely comes from other genes and from male sex hormones, Oetjens added.

Eric Schadt, a genetics expert at Mount Sinai in New York City, called the findings "definitely cool."

“It is a great use of these biobanks to uncover what is still somewhat of a mystery,” he said. “Even though the effect is modest, it does explain 20 percent or so of the height difference.”