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You've probably been there. You're sitting on the bus or the subway, and across from you are two people you're pretty sure are siblings because, y'know, they look really similar.

And then they start making out. So … not siblings. Hopefully.

As strange as it sounds, multiple studies have shown that we find people who look like us more attractive. And it might be because, like the rest of animal kingdom, we're just trying to pass along our genes. People often choose partners that are similar to them in everything from personality to age to education level.

Which isn't too surprising. These are all examples of assortative mating: our tendency to select our life partners with an eye for certain traits and characteristics. Liking someone with the same interests and experiences makes sense -- but research suggests we prefer people who look like us, too.

For example, one study published in 2013 in PLOS One showed that subjects found their partner's face more attractive after it was digitally mixed with their own. Twenty heterosexual couples were asked to look at photos of seven versions of their partner's face. Each version was blended with another face -- some male, some female, some average-looking, and some really attractive.

Most importantly, one of the versions mixed the partner's face with the subject's face, although the subject didn't know it. And as a control, one of the versions combined mirrored versions of the partner's face -- so, it was all the same person, but the image looked digitally altered just like the rest of them and didn't stand out. When the subjects were asked to rank the photos from most to least attractive, they liked the version that was blended with their own face the most.

They liked it even more than the control, which was made up of just their partner's face. It might seem weird to want to date someone who looks like you. Sure, we tend to prefer things that we've seen before, and there's nothing more familiar than the person in the mirror.

Even so, incest is kind of a bad thing. Just ask the royal families of Europe or the Targaryens. So you'd think we'd have evolved to not be attracted to people who look like us.

But some research suggests that we're attracted to people who look like us because we are actually looking for some genetic similarity in our partners. A 2014 study published in the Proceedings of the National Academy of Sciences compared the genetic data for 862 older, married couples, and found that the married couples were more genetically similar than random, non-married pairs. In fact, the more genetically similar two of the subjects were, the more likely they were to be married.

Now, that definitely doesn't mean having similar genes automatically means you're going to get married. But the fact that scientists saw that relationship is still pretty strange. There seems to be a happy medium when it comes to genetics, though.

You might find someone especially attractive when they have some genetic similarity, but not too much. In that study I mentioned earlier, where subjects rated different pictures of their partners, the researchers also did a second experiment. They digitally combined different amounts of the subject's face with their partner's face, and they found that adding too much or too little of the subject's face made that version of their partner seem less attractive.

It was that middle amount of similarity -- about 22% -- that made their partner appear more attractive than normal. Research in both animal and human populations has found that picking a partner with some genetic similarity is related to more stable partnerships, more children, and greater altruism among family members. Evolutionary theory says we're always trying to pass along our genes by raising our kids until they're old enough to start their own families — and a partner that shares more of your genes has extra motivation to do that, too.

Plus, partners with similar genetics often share other characteristics, like personality, which can make for a more stable relationship. So from an evolutionary standpoint, it makes some sense to select a partner who looks like you. But we've also learned that too much similarity can increase our odds of passing along dangerous recessive traits or other developmental problems.

Plus it's just plain creepy. So for a safe, genetically-similar, non-creepy middle ground, we find people who look like us. This doesn't mean you have to pick a partner who looks like you, obviously.

Those other traits we select for, like personality and educational level, are usually more important for stable relationships than whether or not you get confused for cousins all the time. Physical similarity just seems to be another factor we consider before deciding to swipe right on Tinder -- and it's probably not the most important one. So don't be too weirded out.

We'll let the Targaryens keep on keeping on, and the rest of us can get back to our normal lives. Thanks for watching this episode of SciShow Psych, brought to you by our patrons on Patreon! If you'd like to help us keep researching weird questions like this one, you can head over to patron.com/scishow.

And don't forget to go to youtube.com/scishowpsych and subscribe! [♪OUTRO].