W omen are keepers of each other's secrets, boosters of one another's wavering confidence, co-conspirators in life's adventures. Through laughter, tears and an inexhaustible river of talk, they keep each other well, and make each other better. 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. But the power of girlfriends is beginning to yield its secrets to science. For women, friendship not only rules, it...