What Science Really Says About Birth Order

© Blend Images / Mike Kemp / Getty How the number of siblings you have can affect your whole life

Is biology destiny?

That is, does your place in your family and your number of siblings determine who you will turn out to be? There's a lot of hand-wavey pseudo-science out there that suggests you can extrapolate everything from your IQ to your future spouse's hair color based on whether you're the eldest, middle or youngest child, all of which is nonsense. But strangely enough, there are some predictions science can make about you based purely on where you fall in the birth order. (Only predictions, though — these are general trends, not iron rules.)

Some of these predictions are based on the differing social structure a baby is greeted with when being born into a family with zero, one, two or five pre-existing kids. Some of it is pure chemistry that happens during a fetus's incubation in a previously inhabited uterus. And some of it is a mixture of both. So how might your birth order be affecting you?

Certain kinds of intelligence, at least — the kind that registers on IQ tests, where firstborn children enjoy a minuscule advantage (about 3 points, if you were curious). Firstborns get their parents' undivided attention, and possibly a big stack of expectations to go along with it. Plus, the amount of adult language they're exposed to isn't diluted by a bunch of preschooler babbling: "I wanna go toylut!" "I has cookie pweez?" They also get to act as teacher (and tyrant) to their younger siblings, which can help them practice the concepts they've already learned — some research suggests that it's probably family size, not birth order, that has the greatest effect.

As for baby No. 2, or 3, or more: By the time they arrive, Mom and Dad don't have time to drill them on their letters of the alphabet and multiplication tables. It's enough to stop them from eating paste, and that'll have to do. (Later born kids may, however, do better when it comes to other types of intelligence, as you'll see soon.)

Are you a man with a lot of older brothers? Statistically, you're a little more likely to beattracted to men than the general population. This appears to hold up even for boys who didn't grow up around their older brothers; however, adoptive older brothers don't do the trick. The hypothesis is that the repeated exposure makes Mom's body start to develop an immune response to something produced by her male fetuses, but no one really knows why this happens, and there doesn't appear to be any equivalent effect on the sexuality of girls with a lot of older sisters.

In general, firstborns tend to be reliable, risk-averse perfectionists; middle children are most commonly resilient born mediators; and youngest kids are the family clowns. But why?

It has a lot to do with what the family already looks like when the newest kid arrives on the scene. Eldest children have their parents' undivided attention for a spell -—but they're also probably the ones subjected to the strictest rules and tightest control, before parents loosen up and realize the occasional scraped knee or food eaten off the floor is rarely fatal: a good recipe for creating a people-pleasing, cautious adult. Middle kids have to negotiate the tricky business of never being an only child, and not being the doted-on youngest for very long; hence their ability to grow up rolling with the punches. And those who grow up as the youngest tend to be quite independent thanks to growing up with the fewest rules and restrictions, which makes them the ones who are confident enough to seek the spotlight — and the role as "the baby" may just help them get away with a little bit more, too.

Being a younger child is correlated to being involved in more accidents, while being the firstborn is correlated with having a higher premenopausal risk of breast cancer as well as a greater chance of being overweight. It's not 100 percent clear why these statistical differences exist (differences in the amount of estrogen in the womb between pregnancies? different lifestyles thanks to different socialization?) or how meaningful they really are; notice the key words above are "correlated with," not "causes."

Because as with the previous categories, these are predictions, not prophecies. Birth order personality types are based on the most common themes in families in our culture — they're not guarantees and they're not universally applicable. The actual statistical differences related to birth order are generally so small as to be almost totally irrelevant. And besides, some of the biggest effects of birth order on personality may be in how we perceive ourselves. So... who do you want to be?