Mate Choice and Friendship in Twins: Evidence for Genetic Similarity

How alike are you and your husband or wife - or, you and your best friend? Probably more alike than you realize. A study of twins shows that people's spouses and best friends are much more similar to them than was previously recognized - about as close as brothers and sisters. The research also suggested that the preference for partners who are similar to us is partly due to our genes.

The research was conducted by J. Philippe Rushton and Trudy Ann Bons of the University of Western Ontario. Their findings are reported in "Mate Choice and Friendship in Twins: Evidence for Genetic Similarity," published in the July 2005 issue of Psychological Science, a journal of the American Psychological Society.

Several hundred pairs of identical and fraternal twins, their spouses, and their best friends were sent a 130-item questionnaire that measured social background, personality, and attitudes. Twins turned out to be as similar to their spouses and friends as they were to their fraternal twins, though not as similar as they were to identical twins. The spouses of identical twins (who share 100 percent of their genes) were also more similar to each other than were the spouses of fraternal twins (who only share 50 percent of their genes); the same was true of twins' best friends.

The researchers compared the genetic and environmental effects on partner choice among identical twins, and found that their preference for spouses and friends similar to themselves was about 34 percent due to participants' shared genes, 12 percent due to their common environment, and 54 percent due to factors in their unique (nonshared) environment - meaning that there is a sizeable genetic component to our tendency to seek out people like ourselves.

The researchers found that the greatest self-similarity between people and their partners and friends was on the more heritable items in the questionnaire. For example, "enjoying reading" had a relatively high level of 41 percent heritability, while preferring to "travel the world alone" was only 24 percent heritable. "From arrays of possible alternatives, people seek those compatible with their genotypes," the authors wrote. "People prefer their own kind - extraverts favor extraverts; traditionalists, traditionalists."

Unconsciously favoring heritable criteria in choosing our mates supports evolutionary psychology's theory that social preferences follow lines of genetic similarity. "If you like, become friends with, come to the aid of, and mate with those people who are genetically most similar to yourself, you are simply trying to ensure that your own segment of the gene pool will be safely maintained and eventually transmitted to future generations," the authors wrote.

Rushton noted that the results joined a host of recent research showing that both genes and upbringing influence human behavior. "It is especially interesting to see that this applies to our preferences for mates and friendships," he said. "Choosing a life partner is one of the most important decisions that we make."

However, Rushton cautioned against over-interpretation of the results. "We found that more than half of the variance in this study was due to unique environmental effects such as being in the right place at the right time." He added, "Similarity is only one of many factors in choosing a partner."