The attention-seeking baby

The baby of the family tends to receive the least discipline, the fewest responsibilities and the biggest audience.

While last-borns are rarely consulted for their opinions or listened to for their views, they soon learn that being funny and adorable goes a long way toward gaining attention and approval.

It's no wonder that many comedians are last-borns. Jim Carrey, Drew Carey, Ellen DeGeneres, Tina Fey, Martin Short, Robin Williams, Steve Carell, Billy Crystal and Cathy Jones are all the babies of their families. Super-zany satirist Stephen Colbert is a super-baby-the youngest of 11.

Onlies

Only children, who have no siblings to play off, can't be easily pigeonholed. Many, like first-borns, are high achievers, but those who can't or won't excel may rebel and follow their own path.

There's some evidence that onlies are more trusting than first-borns but more goal-oriented than later-borns. (Famous onlies: Lucy Maud Montgomery, Barbra Streisand, Frank Sinatra, John Lennon and Elvis.)

Twins

As for twins, "all bets are off," says Sulloway. Sometimes the first-born twin, even if older by only two minutes, takes on first-born traits, but much depends on how they're treated. Environmentalist David Suzuki is a twin who was born moments before his sister, but Japanese tradition holds that the second-born twin is considered the elder, having graciously moved aside to allow the younger sibling to enter the world first. Interestingly, Suzuki, as an accomplished scientist and attention-loving broadcaster, exhibits traits of all birth orders.

The effect of age gap on birth order

There's a slew of variables that can skew birth order effects, say the experts. One is gender. If a boy is born after one or more girls (or a girl after one or more boys), that child could be raised as a "functional first-born" - the first son or daughter in the family.

If the spacing between siblings is five years or more, the effects of birth order fade. If some children are closer in age than others - for instance, if six siblings are ages 14, 12, 11, five, three and two - it can create separate clusters, with two first-borns, two middles and two babies.

Divorce, disability or death of any family member can interfere with the expected effects. Blended families can alter the birth order of very young children, but the older the kids are, the more ingrained their pecking order in the family and the more resistant to change.

Today's smaller families of one or two children mean a surfeit of first-borns and onlies, and a deficit of middles. A boy and girl - a true first-born and a functional first-born - may exhibit similar characteristics. If both children are the same gender but several years apart, the younger one could display the same first-born traits as the elder. "By having smaller families, we're running the risk of having more perfectionistic, driven children," Leman says. We also run the risk of losing birth order theory altogether, as all the experts consulted for this story are later-borns. But every one of them emphasizes that birth order is only an influence, not a destiny.

One final note: Leman says that no matter what your birth order, you probably think your siblings got a better deal. If you're a baby, you think your eldest sibling got the first and best of everything; if you're a first-born, you're envious that the middle one had the freedom to choose an independent path; and everyone gripes that the baby always got away with murder.