The reason our friends seem a bit kooky, and our mates may seem strange compared to ourselves, is that opposites attract. Right?
Nope. A large body of research suggests that we pick our friends, as well as our mates, because underneath it all they are very much like us.
So if our friends are kooky, and our mates a bit strange, chances are we are too.
And the latest study in this ongoing research takes it a little further. We can blame it at least partly on our genes. People tend to like others who have the same inheritable traits, so we often choose friends and mates who are genetically similar to ourselves.
"People prefer their own kind," says J. Philippe Rushton, a psychologist at the University of Western Ontario. "Extroverts favor extroverts; traditionalists, traditionalists."
That may not jibe with your own experience, but Rushton notes that genes are not the only players. We're not a bunch of robots that are being led around by genes that even pick our friends. Other factors, according to the researchers, play a significant role.
Rushton says our friends and our mates may also be a product of the "unique environmental effects such as being in the right place at the right time." You can't link up with an ideal mate if the two of you never meet.
Our genes, Rushton says, probably account for about a third of the reason why we pick someone else as a friend or a mate.
"But that's still pretty strong," he says. "Let's say it's a strong whisper from the genes."
This is a very different line of research from work by Rushton that has been very controversial. He claims that his research shows distinct differences between the races in intelligence (based on brain size), fertility, personality, and dozens of other variables. According to Rushton, Asians rank at the top in intelligence, with Caucasians in the middle and Africans at the bottom. Many other researchers strongly disagree.
Rushton, who has been researching the subject of friendship for 20 years, says clear patterns emerged from a study of hundreds of identical and fraternal twins, as well as their spouses and friends. It's no surprise that identical twins, who share 100 percent of the same genes, picked friends and mates who were very similar to those picked by their twins.
But here's the twist. Fraternal twins, who share only 50 percent of their genes, picked friends and spouses who are so much like themselves they could be their brothers and sisters. And, Rushton says, so do the rest of us.
"It's almost as similar as siblings," he says. "Not quite, but almost."
That "was not previously recognized," Rushton and a colleague, Trudy Ann Bons, report in the July issue of Psychological Science.
The researchers looked at a wide range of variables, including characteristics that are mostly inheritable, and those that are less so, to determine the role genes play in our social behavior. They found that "people are genetically inclined to choose as social partners those who resemble themselves on a genetic level."
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.
"Similarity, of course, is only one of many criteria people use in choosing social partners," the researchers conclude. "Physical appearance, status, control of resources, reciprocity, location and family situation all provide constraints and exert influence as well."
But that old claim that opposites attract still persists, despite decades of research showing that "it just ain't so," Rushton says.
"Research has been clear, over and over again, that you can be opposite on one or two dimensions, but overall it's similarity that rules," he says.
The husband may love steak, and the wife may hate it. But too much of that can kill a relationship.
"Just think if you were married to somebody and you had the exact opposite political attitude," he says. "You wouldn't be able to greet each other over the breakfast table. It would just be awful."
Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.