A lot of online dating sites attempt to pair people up based on their degree of similarity. They’re operating under the assumption that people who are more similar to one another will have a better shot at relationship success. Intuitively, this makes sense, doesn’t it? If you have a lot in common, you’ll probably have more fun together.

There’s a lot of research that would appear to support this idea, too. More than a half-century ago, social psychologists documented that similarity was one of the key features in attraction: The people we like the most are the ones we think are most similar to us.

However, the story isn’t quite as simple as it was once thought to be. While similarity does indeed play a role in what initially attracts us to someone, similarity in and of itself does not promise a happy relationship.

In a new study published in the Journal of Research in Personality, researchers at Michigan State University looked at 2,578 couples—it began in 1968 and includes a nationally representative US sample of individuals and their families. For this study, researchers focused specifically on a subset of people in heterosexual relationships who were, on average, 51 years old and married for 22 years. They looked at relationship satisfaction and various measures of psychological well-being (including positive vs. negative mood and overall life satisfaction) and considered how they were related to partner similarity on the Big Five personality traits.

The Big Five traits include openness to experience (being open to trying new things), conscientiousness (being organized and detail oriented), extraversion (being outgoing and sociable), agreeableness (having a lot of care and concern for others), and neuroticism (not dealing well with stress and having more mood swings).

So what did they find? First, people who were more conscientious, extraverted, and agreeable—but less neurotic—tended to be happier overall and with their relationship. Likewise, people whose partners were more conscientious and less neurotic were happier as well.

In other words, when considered in isolation, people’s own (and their partner’s) personality traits mattered when it came to how they felt about their relationships and lives more broadly—but what about having similar personalities? Not so much. The effects of similarity were small and inconsistent. It’s also worth noting that they tended to be smaller than the effects of one’s own personality.

These findings echo other research on similarity and satisfaction in relationships. For example, an earlier study found that when predicting relationship and life satisfaction, what matters most are your own personality traits, followed by your partner’s, followed by similarity between you and your partner. Again, the similarity effects were very weak in this study.

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Likewise, a 2008 meta-analysis of more than 300 studies looking at similarity and attraction concluded that while similarity is indeed linked to initial attraction, its effect on attraction and satisfaction in ongoing relationships is “not detectable.”

There’s an important wrinkle to this story, though: While actual similarity doesn’t seem to be an important predictor of relationship success, perceived similarity does. In other words, when people think that their personality is similar to their partner's (regardless of whether that's actually true), they tend to be happier.

One other wrinkle: There’s also some research finding that personality dissimilarity (or what social psychologists refer to as “complementarity”) sometimes predicts relationship quality. Specifically, it can pertain when looking at traits related to dominance and submissiveness, where people tend to be happier when they match with a partner who possesses the opposing trait rather than the same trait. In other words, there’s some truth to the idea that “opposites attract.”

What all of this tells us is that there’s good reason to be skeptical of dating apps that use computer algorithms to find your “ideal” match based solely on how similar you are. The truth is that being objectively similar doesn’t seem to matter all that much and that having certain personality traits (like high conscientiousness and low neuroticism) seems to be more important.

Also, not only is the personality matching approach likely to be ineffective, but if these programs just try to put similar people together on all traits, including traits where complementarity is usually preferred (like dominance-submission), it might even be counterproductive. None of this is to say that online dating is hopeless or that it can’t ever work. Many people meet online and end up happily ever after.

However, if you decide to go that route, be wary of the sweeping claims that some online dating sites make about the importance of similarity and compatibility. Go in with the recognition that relationship science isn’t perfect and who we’re going to be happy with in the long run isn’t easily predictable.