And, a new study suggests, the longer we are with someone, the more similarities in appearance grow.

Researchers set out to investigate why couples often tend to resemble one another. They asked 11 male and 11 female participants to judge the age, attractiveness and personality traits of 160 real-life married couples. Photographs of husbands and wives were viewed separately, so the participants didn't know who was married to whom.

The test participants rated men and woman who were actual couples as looking alike and having similar personalities. Also, the longer the couples had been together, the greater the perceived similarities.

The researchers speculate that the sharing of experiences might affect how couples look.

A biological reason

The idea that there is a connection between appearance and personality might seem odd at first, but there could be biological reasons for a link, said study member Tony Little from the University of Liverpool in England.

"Testosterone is linked to masculine face shapes and it also affects behavior," Little told LiveScience. "Also, the face displays our emotions and over time emotional expressions may become written in the face."

For example, someone who smiles a lot may develop lines and muscles that are suggestive of someone who is happy.