Have you ever looked at a couple and thought that one half was significantly more attractive than the other? You’re not alone.

Romantic partners with differing attractiveness levels stand out. Why? Because most people select partners who are in roughly the same “league.”

This stems from a general tendency for people to partner up based on similarity, a phenomenon social scientists have dubbed assortative mating. This preference for similarity is pervasive and extends to everything from our beliefs and values to our overall level of physical attractiveness.

Seeing two people who are dissimilar in an obvious way is therefore a violation of our expectations. And when this happens, we’re often quick to judge.

A recent study published in the Journal of Social and Personal Relationships found that people were more prejudiced against couples in which the partners had dissimilar body types—specifically, when one partner weighed a lot more than the other—compared to those who were matched for body type. In fact, people went as far as to say that mixed-weight couples should be less public about their relationships and keep their PDA to a minimum!

So, there’s clearly a lot of social pressure to avoid dating “out of your league.”

Not everyone succumbs to this pressure, though, which raises the interesting question of why—why do some people wind up with partners who are far more (or less) attractive than they are?

Another study, published recently in the journal Psychological Science sheds some light on the answer and suggests that it all comes down to how long we know someone before we start dating.

In this study, researchers looked at 167 dating and married couples who, on average, were about 32 years old and had been in their relationships for approximately nine years. Participants were asked how long they knew each other before they got together romantically and, further, whether they were platonic friends before becoming lovers.

For all couples, a team of research assistants rated the physical attractiveness of each partner separately. This information was then used to create an index of how similar the partners were to each other in terms of their attractiveness.

The longer two people knew each other before they started dating, the less likely they were to match in terms of their attractiveness.

What the researchers found was that the longer two people knew each other before they started dating, the less likely they were to match in terms of their attractiveness. Any tendency for people to partner up based on similarity in attractiveness disappeared among those who had known each other for nine months or longer.

People who said they had a platonic friendship before their romance began were also less likely to be matched for attractiveness.

On a side note, you might be wondering whether those who were more similar to their partners in terms of attractiveness were more satisfied with their relationships, especially given the powerful stigma that exists against mixed-attractiveness couples. This wasn’t the case.

Whether partners were mixed or matched for attractiveness didn’t matter, which means that going for a partner with similar looks doesn’t necessarily guarantee a happier relationship.

So why were people more willing to date a dissimilar partner when they’d known that person longer or were friends first?

The authors of this study reason that when an individual jumps into a relationship quickly, they’re likely to end up with someone relatively similar due to competition in the “mating market.” For instance, if a man of slightly below average attractiveness tries to pick up a model at the bar, he probably won’t be very successful because he’s going to be competing for her attention with a lot of guys who are far more attractive than him.

Scenarios like this pressure us to look toward romantic prospects who are on the same level instead of shooting for the sexiest person around.

However, the longer you get to know someone, the more your perception of that person can change. With a little bit of time, uniquely desirable traits that might not be immediately apparent can really shine through, such as your sense of humor, honesty and dependability—all of which are traits that heterosexual women say they really want in a man.

In other words, time can “level the playing field,” so to speak, for people who aren’t equally attractive.

This suggests that being in the so-called “friend zone” is underrated. In fact, it just might be your best friend when it comes to starting a relationship.

Justin Lehmiller, PhD is a sex educator and researcher at Ball State University and author of the blog Sex and Psychology. Follow him on Twitter @JustinLehmiller.

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