Forget "What's your sign?" -- "What's your birth order?" may yield more revealing information. "Birth order is only one of a multitude of things that affect personality. It just happens to be one of the best predictors of behavior," says Frank Sulloway, Ph.D., a research scholar at MIT in Cambridge, whose provocative new book, Born to Rebel: Birth Order, Family Dynamics, and Creative Lives (Pantheon, 1996), is based on thousands of family histories. Here, a quick take on what's behind each birth-order niche.

Clues to the character of. . . .Firstborns:
WHAT YOUR BEST FRIEND SAYS ABOUT YOU: You're capable, loyal and responsible, a true friend to the end.

WHAT YOUR EX SAYS ABOUT YOU: You're possessive and totally focused on doing things your way.

THE BIRTH-ORDER FACTOR: High achievers, you firstborns stayed in Mom and Dad's good graces -- while using your superior size to boss younger sibs around. You may have slid into the role of surrogate parent, which gave you a respect for authority and a sense of responsibility.

Clues to the character of. . . .Middles:
WHAT YOUR BEST FRIEND SAYS ABOUT YOU: You're easy to get along with, open-minded and adventurous.

WHAT YOUR EX SAYS ABOUT YOU: You're inconsistent, even wishy-washy. You'd rather tell a lie than start an argument.

THE BIRTH-ORDER FACTOR: You realized you couldn't outmuscle the oldest kid, or simply act cute, like the family baby. What you were -- and still are -- good at: mediating between warring factions.

Clues to the character of. . . .Laterborns:
WHAT YOUR BEST FRIEND SAYS ABOUT YOU: You love discovering new things, and you're happy to take your friends along with you. You're very affectionate.

WHAT YOUR EX SAYS ABOUT YOU: You don't take responsibility for what you do, and you have to be the center of attention.

THE BIRTH-ORDER FACTOR: You resisted copying your older sibs -- who wants to feel inadequate? Instead, you carved out your own space by trying new things: maybe playing a musical instrument, or developing a distinctive sense of humor. Since your parents already had their hands full by the time you arrived, you grew up less invested in authority: the perfect setup for rebellion and risk-taking.

Clues to the character of. . . .Onlies:
WHAT YOUR BEST FRIEND SAYS ABOUT YOU: You were hard to get to know, but it was well worth the effort. You're conscientious, determined to do your personal best.

WHAT YOUR EX SAYS ABOUT YOU: You've got to stop judging everyone else by the high standards you impose on yourself.

THE BIRTH-ORDER FACTOR: As the oldest and the youngest, you've got it all. But because you identify closely with your parents, you often have more in common with firstborns than rebellious laterborns.

-- adapted from Mademoiselle