Across species and throughout human cultures, females have banded together for protection and mutual support.
They have groomed each other, tended each other's young, nursed each other in illness and engaged in the kind of aimless sociability that has generally mystified male anthropologists.
It may help explain one of medical science's most enduring mysteries: why women, on average, have lower rates of heart disease and longer life expectancies than men.
"Women are much more social in the way they cope with stress," says Shelley E. Taylor, author of "The Tending Instinct" and a social neuroscientist at the University of California, Los Angeles.
"Men are more likely to deal with stress with a 'fight or flight' reaction --- with aggression or withdrawal."
But aggression and withdrawal take a physiological toll, and friendship brings comfort that mitigates the ill effects of stress, Taylor says.
To be sure, friendships --- the feeling of being connected to a supportive network --- profoundly affect the health of both genders, according to researchers.
Men and women who report loneliness die earlier, get sick more often and weather transitions with greater physical wear and tear than those who say they have a support network of friends or family.
Researchers attribute the difference to women's greater reliance on friendships outside of marriage.
Increasingly, researchers think that the hormone oxytocin is, for women especially, the elixir of friendship --- and, by extension, of health.
Present in men and women, oxytocin levels spike in females following childbirth and when nursing.
Stacy Anderson, a 36-year-old Culver City, Calif., mother of two young children, recognizes oxytocin's effects.
For women, there is some evidence that a male partner, in times of stress, can make things worse.