Girl talk apparently is good for health, according to a new study. When women feel emotionally close to a girlfriend, levels of the hormone progesterone increase, helping to boost mood and alleviate stress.

The study, by researchers at the University of Michigan, examined progesterone levels in the saliva of 160 female college students.

The researchers measured progesterone levels and levels of the stress hormone cortisol and obtained information about menstrual cycles and any use of hormonal medication.

The women were randomly assigned to partners and asked to perform a task designed to elicit feelings of emotional closeness or a task that was emotionally neutral. After 20-minute talks, the women played a computerized cooperative game with their partners, then had their progesterone and cortisol levels checked again.

The progesterone levels in the women who participated in the emotionally close task remained the same or increased, while those in the emotionally neutral group tended to decline. In the study, progesterone was used as a marker for oxytocin, a hormone linked to relationship trust and bonding. Oxytocin itself can be measured only through spinal fluid or brain scans.

A week later, the participants played a computerized card game with their partners again, had their saliva tested and were asked how willing they would be to risk their lives for their partners. The study found that increased progesterone levels predicted a willingness to say they would risk their lives for their partners.

The study supports a concept in evolution that is gaining momentum — that the hormonal basis of social bonds enables people to suppress self-interest when necessary to promote the well-being of another person.

The research also helps explain why social contact appears to lead to improved health.

“Most of the hormones involved in bonding and helping behavior lead to reductions in stress and anxiety in both humans and other animals,” the lead author of the paper, Stephanie Brown, said in a news release. “Now we see that higher levels of progesterone may be part of the underlying physiological basis for these effects.”

The study is published in the June issue of the journal Hormones and Behavior.