Opposites attract, but not in marriages or other long-term relationships. As I describe in BLONDES, we value in others what we value in ourselves. So many of us prefer those with similar background, religion, ethnicity, education, values, and so on. Women often prefer potential spouses who look like themselves or family members. We even prefer those whose body language and verbal intonation resembles our own.

The marriage-minded seek the like-minded.

So is it any wonder that researchers have now found evidence that birth order — one of the major forces that influences who we are — also influences our choice in marriage?

In a recent study, Harvard researcher Joshua Hartshone and his colleagues found evidence that people are more likely to form long-term romantic relationships with someone of the same birth order rank. Only children are likelier to marry other onlies, firstborns are likelier to marry firstborns, middleborns are likelier to marry middleborns, and lastborns are likelier to marry lastborns.

The study, based on the responses of over 2,500 visitors to the website www.coglanglab.org, found that the birth-order effects are independent of family size and unlikely to be a product of class or ethnicity. “If spouses correlate on personality,” Hartshone writes, “and personality correlates with birth order, spouses should correlate on birth order.”

Incidentally, the researchers found that best friends also tend to share the same birth order.

My parents are both firstborns. Many of my closest friends are onlies and firstborns who married other onlies and firstborns respectively. Me, I’m a firstborn who married a middleborn.