While you may be familiar with the old saying, “opposites attract,” in reality, what the heart wants is someone who resembles its owner – and that resemblance increases the longer two lovebirds stay together.

University of Michigan psychologist Robert Zajonc conducted an experiment to test this phenomenon. He analyzed photographs of couples taken when they were newlyweds and photographs of the same couples taken 25 years later.

The results showed that the couples had grown to look more like each other over time. And, the happier that the couple said they were, the more likely they were to have increased in their physical similarity.

Zajonc suggested that older couples looked more alike because people in close contact mimic each other’s facial expressions. In other words, if your partner has a good sense of humor and laughs a lot, he or she will probably develop laugh lines around their mouth — and so will you.

Other evidence has also shown that men and women may be initially attracted to partners with similar personalities. In 2006, scientists at the University of Liverpool asked participants in a study to view individual photos of men and women and judge their personalities.

The participants did not know who in the photos was married to whom, but the couples that had been together the longest were judged to have more similar personalities. The researchers concluded that, “possessing personality traits that are attractive may be causal in making a face attractive.”

It turns out we may even be hard-wired to fall in love with people sporting DNA that is similar to our own in some ways. In a study of twins, University of Western Ontario scientists found that not only did the study participants tend to pick partners with similar genes; the spouses of the identical twins were also more alike than the spouses of non-identical twins.

Couples may start to look like each other because in some ways they already are like each other.

This article was provided by Life's Little Mysteries, a sister site to LiveScience.