Forget about opposites attracting. We like people who look like us, because they tend to have personalities similar to our own.
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 women 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.
Other studies have shown that partners who are genetically similar to each other tend to have happier marriages. Similarities in personalities and physical features might be one way to gauge genetic similarity.
Smiles and eyes
The new study, to be detailed in the March issue of the journal Personality and Individual Differences, indicates that people hone in on a variety of different features when using facial appearance to make decisions about someone's personality, and that the particular cues focused on change from face to face.
Vital to the decision, however, are eyes and smiles.
"Smiles are important social cues that may tell us whether or not someone is friendly, and eyes are also a traditional focus of attention," Little said.
Overall face shape can be important too. For example, the combination of masculine features, large chins and dominant brow ridges can create an impression of disagreeableness and being unwilling to cooperate, Little said.
The researchers are currently looking for people who are both single and in relationships to take part in an online study to investigate whether an individual's physical and personality traits influence their personal preferences.
More information can be found at www.alittlelab.com.
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