Feeling emotionally close to a friend boosts levels of the hormone progesterone, promoting well-being and reducing anxiety and stress, according to a new study.

`This study establishes progesterone as a likely part of the neuro-endocrine (brain-hormone) basis of social bonding in humans," said University of Michigan (U-M) researcher and study co-author Stephanie Brown.

Brown and colleagues examined the link between interpersonal closeness and salivary progesterone in 160 female students, said a U-M release.

A sex hormone that fluctuates with the menstrual cycle, progesterone is also present in low levels in post-menopausal women and in men.

Earlier research has shown that higher levels of progesterone increases the desire to bond with others, but the current study is the first to show that bonding with others increases levels of progesterone.

The study also links these increases to a greater willingness to help other people, even at our own expense.

`It's important to find the links between biological mechanisms and human social behaviour. These links may help us understand why people in close relationships are happier, healthier, and live longer,` said Brown.

These findings were published in the current issue of the journal Hormones and Behaviour.