At the core of every relationship lies chemistry.
In the case of friendship, it's all about hormones.
Researchers at the University of Michigan have found chemical evidence to support an evolutionary theory of altruism that suggests hormones enable us to suppress our own well-being in order to help others.
The researchers examined 160 coeds at the University of Michigan.
Baseline levels of progesterone were tested in each of the students, and information was gathered about the timing of menstrual cycles and whether the women were taking birth control.Then the women were randomly assigned to a partner, and asked to participate in a task designed to be emotionally neutral or one that should elicit feelings of closeness.
After completing the assigned tasks, the women then played a computerized card game with their partner and had their progesterone and cortisol levels tested.
Levels of progesterone in women who'd participated in emotionally neutral tasks generally declined, while the hormone increased in women who engaged in the closeness exercises.
Cortisol levels didn't change in the same way.
The women returned a week later and played the computerized card game again with their partner.
Again, their hormone levels were tested, and researchers correlated those levels to the participants' response to a series of questions -- including one that asked how likely it was that they'd risk their life for their new partner.
"During the first phase of the study, we found no evidence of a relationship between progesterone and willingness to sacrifice," said Stephanie Brown, lead author of the article in a press statement. "But one week later, increased progesterone predicted an increased willingness to say you would risk your life to help your partner."
The study is published in the journal Hormones and Behavior.