In this and previous studies, Rushton found that we tend to select spouses on the basis of inheritable characteristics, even if we don't know those features are inheritable. For example, he says, the middle-finger length is primarily inheritable, whereas the upper-arm circumference is less so. And spouses who have participated in his studies tended to marry someone with a similarity in the length of the middle finger, but not in the upper-arm circumference.
The role of genetics is not limited to physical characteristics. To a lesser extent, genes also play a role in the formation of personality, and even personal likes and dislikes. The enjoyment of reading, for example, is believed to be more inheritable than having many hobbies.
But why would we prefer someone who is more genetically similar, especially if we don't even know the genetic nature of some of our peculiarities? Most likely, the researchers say, it's because of a subconscious desire to perpetuate our own genes.
"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," they write.
In the end, by the way, that strategy seems to pay good dividends. Some studies show that spouses who share inheritable traits seem to be the most satisfied with their marriage.
But this research is full of twists and turns. Apparently we don't really want to marry ourselves. One study looked at personal odors and olfactory preferences, which are inherited from the father. Women, according to that study, prefer the scents of men with genes somewhat similar to their own, but not those who are nearly identical or those who are very different.
"The optimal amount of similarity is not 100 percent," the researchers say.
Several other studies show that we're probably going to like someone better if he or she looks like us. In one study, men and women were shown pictures of themselves morphed into an image of someone of the opposite sex. They overwhelmingly preferred the faces of themselves over others.
And that's not just because of familiarity, the researchers say. It didn't work if the morphed image was of a movie star or some other celebrity. The participants liked those who looked like themselves more than they liked the luminaries.
In their latest study, Rushton and Bons sent questionnaires to 1,529 participants, including identical and fraternal twins, spouses and friends. Ages ranged from 18 to 75 years, with a mean of 32 years.
The questionnaires assessed physical characteristics, demographic background, such as educational level, social attitudes, and personality types. Measures were taken to keep the participants from collaborating on the answers. Each questionnaire was returned in a separate, self-addressed envelope.
The results, according to the researchers, show that genes do indeed play a significant role in who we pick to be our friends and mates, but not the dominant role. Our "unique environment," as they put it, may be the most important factor.