We can all rest easy now. Kate Middleton is out of the hospital, back in fashionable clothes and ready to comfortably incubate an heir to the British throne.
I've heard a lot of sympathy for the pregnant duchess, who will now have to contend with strangers dissecting her diet, monitoring her weight and wondering if she'll buy cloth diapers. But this week, I felt more sorry for Prince Harry.
Thanks to nifty new rules that allow a firstborn girl to be queen, this baby will inevitably bump Harry down the line of succession. If Kate has twins, the one who comes out first will, by a matter of luck and minutes, be the heir. Perhaps the secondborn won't miss being a figurehead monarch; it doesn't seem like a joyride of a job. Still, the frenzy over the royal fetus shows how much weight we still place on the order of things.
Once, birth order had a measurable effect on people's lives, determining who inherited the estate and who was shuffled toward the priesthood (and heaven forbid you were born a girl). Even after laws of primogeniture largely disappeared, birth order has been part of the ongoing quest to explain each other's behavior, to fit complex human dynamics into a storyline that feels right: that older children, accustomed to rapt attention from their parents, are likely to be conservative; that younger kids, habitually ignored, turn more mischievous and creative.
For psychologists, birth order seems an ideal way to measure nature against nurture, said Joshua Hartshorne, a postdoctoral fellow at the Massachusetts Institute of Technology, who grew interested in the subject when he realized that he, his wife and most of their friends were all firstborn.
The British psychologist and eugenicist Sir Francis Galton wrote a book on the subject in the 1880s, intrigued that the scientists of his day were largely firstborn sons. The book "Born to Rebel," published in 1997, declares that birth order affected human history: Thomas Jefferson, Karl Marx and Fidel Castro were all rebellious younger brothers.
In a study published in 2009, Mr. Hartshorne found that birth order has a small but measurable effect on whom we choose as spouses and friends -- perhaps because people bond over shared experiences growing up. He also learned that if you put a birth-order-related questionnaire on the Internet, more firstborns tend to answer. But when he reviewed the broader studies about birth order and personality, Mr. Hartshorne found many built-in flaws, and scant evidence that birth order affects people's relationship to the outside world.
Some failed to account for the fact that richer people tend to have fewer kids, so that being a middle child makes you statistically more likely to be poor. An influential study from Norway, which found higher IQs among older siblings, didn't correct for the fact that IQ tests have grown harder over time.
"It's been very, very hard to show that, in fact, birth order matters," Mr. Hartshorne told me.
And yet, just as we cling to superstition, we find birth-order theories hard to resist. The royals seem a perfect example: William is the upstanding older child, who found a responsible wife. Prince Harry, the younger son, cavorts nude, and in a Nazi suit.
That's what celebrities exist for, after all: to test our theories of psychology and determinism, give us stories to compare with our own lives. (Is Lindsay Lohan a bad seed or a child star fulfilling her destiny?) For Americans, the royals are particularly fun: a parable of social mobility, played out by wealthy people in a fishbowl, living by rules that we think don't apply to us.
We're all enraptured by the notion of a commoner, now poised to give birth to a future king or queen. But we forget that Kate's life has followed the same track that often governs people's futures in America. She was born to a family that could afford to send her to posh private school and then St. Andrews -- where she, the eldest child, had the chance to meet a firstborn prince.
Unless you live by royal rules of succession, nothing is set in stone. But luck and money still can make a life.opinion_commentary
Joanna Weiss is a columnist for the Boston Globe (firstname.lastname@example.org).