Narrator: This is Science Today. Dr. Jon Levine of the University of California, San Francisco compared men and women who came out of surgery. He gave them both a pain medicine called a kappa-opioid, which relieved the women's pain, but not the men's -- the first known gender difference in a pain reliever.

Levine: What kind of biological differences that underlie this I think is a whole set of questions that have just opened up.

Narrator: Levine thinks evolution may have something to do with it.

Levine: From an evolutionary point of view, there are lots of differences between men and women. The aspects of childbearing -- female sex hormones, male sex hormones affect cells in the nervous system that may play a role in reproductive function. And they may well have evolved in relationship to that.

Narrator: Although Levine is the first to find the difference, it may have shown up in earlier studies without scientists realizing it.

Levine: We've certainly received phone calls from investigators around the United States and around the world who've gone back and looked at some of the data that they've collected and found similar effects -- just they had never compared their male patients to their female patients.

Narrator: For Science Today, I'm Steve Tokar.