There are a lot of ways to study a painting, and one of the best is to get to know the painter. The splash or splatter of color makes a lot more sense when you understand the rage or whimsy or heart behind it. The songwriter, similarly, can lay bare the song, the poet the poem, the builder the building.

So what explains the complex bit of artistry that is the human personality? We may not be born as tabulae rasae. Any parent can tell you that each child comes from the womb with an individual temperament that seems preloaded at the factory. But from the moment of birth, a lot of things set to work on that temperament--moderating it, challenging it, annealing it, wounding it. What we're left with after 10 or 20 or 50 years is quite different from what we started out with.

For a long time, researchers have tried to nail down just what shapes us--or what, at least, shapes us most. And over the years, they've had a lot of eureka moments. First it was our parents, particularly our mothers. Then it was our genes. Next it was our peers, who show up last but hold great sway. And all those ideas were good ones--but only as far as they went.

The fact is once investigators had strip-mined all the data from those theories, they still came away with as many questions as answers. Somewhere, there was a sort of temperamental dark matter exerting an invisible gravitational pull of its own. More and more, scientists are concluding that this unexplained force is our siblings.

From the time they are born, our brothers and sisters are our collaborators and co-conspirators, our role models and cautionary tales. They are our scolds, protectors, goads, tormentors, playmates, counselors, sources of envy, objects of pride. They teach us how to resolve conflicts and how not to; how to conduct friendships and when to walk away from them. Sisters teach brothers about the mysteries of girls; brothers teach sisters about the puzzle of boys. Our spouses arrive comparatively late in our lives; our parents eventually leave us. Our siblings may be the only people we'll ever know who truly qualify as partners for life. "Siblings," says family sociologist Katherine Conger of the University of California, Davis, "are with us for the whole journey."

Within the scientific community, siblings have not been wholly ignored, but research has been limited mostly to discussions of birth order. Older sibs were said to be strivers; younger ones rebels; middle kids the lost souls. The stereotypes were broad, if not entirely untrue, and there the discussion mostly ended.

But all that's changing. At research centers in the U.S., Canada, Europe and elsewhere, investigators are launching a wealth of new studies into the sibling dynamic, looking at ways brothers and sisters steer one another into--or away from--risky behavior; how they form a protective buffer against family upheaval; how they educate one another about the opposite sex; how all siblings compete for family recognition and come to terms--or blows--over such impossibly charged issues as parental favoritism.