Oxytocin is a hormone that is released during breastfeeding, orgasms and close contact. It lowers blood pressure, gives a feeling of calmness and is a natural pain killer for both sexes.
In a study of 76 adults in committed relationships, the better the relationship the higher their levels of oxytocin were. Loving interactions between the couples also revealed a difference between men and women; all of the women had a much larger increase in oxytocin during loving interactions. The study was carried out by psychologist Karen Grewen of the University of North Carolina-Chapel Hill.
"It makes sense in evolution. Yes, you needed the testosterone — someone to go out and kill the animals. But you needed a healthy person to stay and care for the kids too" says Grewen.
Women also have higher levels of oxytocin in general and have a much higher spike in oxytocin levels during times of stress. Testosterone seems to hinder the bonding effect that oxytocin has, but men do become calmer, healthier and sometimes sleepy when their levels of the hormone rise.
Studies show women get more stressed out over marital issues "so this perhaps shows the positive side, that they're buffered more by happy contacts," says Brian Baker of The University of Toronto. Baker is a psychiatrist who studies how men's family lives affect their heart disease prognosis. Men with heart disease do much better when their marriages are happier he says. "There are definite gender differences, but gender doesn't tell the whole story."