Women, in other words, are more sensitive to pain than men. They report feeling it more in just about every way: more intensely, more often, for a longer time. They grapple with more headaches, more painful gut conditions, more pain in their backs and pelvises and bones and, research suggests, virtually every other part of their bodies. Of the hundreds of millions of chronic pain patients around the world, they comprise roughly 70 percent.

And underlying those striking disparities, studies are finding, is a still more extensive web of differences connected to both gender and biological sex that help shape how pain manifests, and how badly it hurts.

Distinct types of cells appear to be involved in processing pain in each sex. Sex hormones have been shown to exacerbate or dampen it. Disparate stress levels, gender roles and even the ways men and women tend to think about their own pain all seem to influence how hard it hits. The list goes on — and likely keeps going on far beyond what research has so far uncovered.

Though studies have long pointed toward sex and gender differences in pain, until recently most researchers devoted little attention to them, if not dismissing them entirely. 

That has at last started to change in the past decade, as new requirements from health agencies have driven a surge of new findings on the subject and, for the first time, brought it out of the narrow corner of the field to which it historically was relegated. 

But researchers may still just be scratching the surface of the disparities.

"We're not even remotely close to answering the question of how much is sex and how much is gender," says Jeffrey Mogil, a psychology professor at McGill University. "We only know a fraction of the biological sex differences. I mean, they're only starting to emerge now. And pain and gender is almost completely unstudied. There's only a handful of papers that have ever been done."