Men and women's brains respond to pain differently, researchers have found.
Scans showed parts of women's brains linked to emotion were stimulated when they felt pain.
But the researchers, from the University of California Los Angeles (UCLA) found that, in men's brains analytical areas showed greater activity.
They say the study of people with irritable bowel syndrome (IBS) could help the development of targeted treatments for the illness.
IBS causes discomfort in the abdomen and diarrhoea or constipation.
The UCLA researchers carried out positron emission tomography (PET) brain scans of 26 women and 24 men, while they experienced a small amount of pain.
Although there were some areas of the brain which were stimulated in both men and women, differences were seen between the sexes.
The female brain showed greater activity in limbic regions, which are emotion-based centres. In men, the cognitive regions, or analytical centres, showed greater activity.
Dr Emeran Mayer, director of the Center for Neurovisceral Sciences & Women's Health who led the research, said: "We are finding more scientific differences between men and women as we improve research methods and broaden study populations.
"This growing base of research will help us develop more effective treatments based on a new criteria: gender."
Dr Bruce Naliboff, clinical professor of psychiatry and biobehavioral sciences, at UCLA, added: "The reason for the two different brain responses may date back to primitive days, when the roles of men and women were more distinct."
He suggested the gender differences may have evolved as part of a more general difference in stress responses between men and women.
Men's cognitive areas may be more highly triggered because they would have had to make "fight-or-flight" decisions, while women would have reacted to threats by thinking about how they could protect their children.
The study may help develop treatments that could be targeted at men or women with IBS.
One drug currently available, called Lotronex, affects the limbic system and has worked more successfully in women than men.
The research is published in the journal Gastroenterology.