Do birds of a feather flock together? Or do opposites attract? Folk wisdom abounds on the role of differences and similarities in relationships, and anyone can probably generate examples from their life to reinforce either of those aphorisms. Perhaps you know a couple who seem like two halves of the same person: finishing each other’s sentences, sharing the same hobbies, ordering the same dish at restaurants. You may also know a long-time pair who couldn’t be more different: she’s a quiet bookworm, he’s the life of the party; he’s up at dawn, she’s a night owl. There might even be a happy couple in your life who are strikingly similar in some ways and starkly different in others. Such examples show how difficult it is to pin down a universal law of attraction.
But what does science have to say about it? Decades of research have shown that personality relates to nearly every aspect of people’s lives, from how we navigate daily challenges and long-term goals to how well we connect with others. Certain personality traits – such as emotional stability and conscientiousness – consistently predict higher relationship satisfaction. However, the effect of personality similarity in relationships is less straightforward.
Research suggests that people do have a tendency to seek out partners who are similar to them in certain ways, a phenomenon known as assortative mating. The logic behind assortative mating is intuitive: similarity can foster understanding, reduce conflict and reinforce shared goals, making relationships more harmonious and stable. This phenomenon has been well documented in areas such as sociopolitical values (political conservatism, religiousness) and lifestyle preferences (alcohol consumption, physical exercise).
You might expect that romantic partners will tend to match on personality, too – and that a match is likely to make the relationship more successful. We recently set out to test whether these things are true.
Figuring this out is more complicated than it might seem, because personality is not just one thing. When researchers study personality, they often focus on traits: broad, relatively stable patterns of individual differences in the way people think, feel and behave. The Big Five is a well-known taxonomy of traits, consisting of openness, conscientiousness, extraversion, agreeableness, and neuroticism. (Roughly speaking, these traits describe a person’s general tendencies toward being curious, hard-working, gregarious, warm, and anxious.) These broad traits are useful for understanding all manner of behaviours and life outcomes – but they provide only a general sketch of what someone is like. The Big Five traits are actually bundles of much more specific traits. For instance, the broad trait extraversion includes narrower traits such as enthusiasm, sociability and assertiveness.
Another way to understand personality in greater depth is to go beyond traits entirely. Although traits are useful for describing someone, you probably would not limit a description of a romantic partner or close friend to just their traits. Rather, you would likely think about their goals, motivations or habits. You might consider how they have grown over time and how they create a coherent story of their life. This level of personality is referred to as ‘characteristic adaptations’ and has to do with what people are like in the context of their actual lives. These aspects of personality may be relevant to assortative mating and relationship satisfaction as well.
Being similar in characteristics tied to relationship functioning might be more important than similarity in general traits
In our research, we started with the seemingly simple question of whether romantic partners tend to have similar personalities. Most studies to date have found moderate personality similarity between partners, supporting the ‘birds of a feather’ idea. That said, some studies suggest a complementarity effect, in which partners tend to be dissimilar from each other, suggesting that ‘opposites attract’. In an effort to clear up whether partners tend to have similar personalities or not, our approach involved examining each of the multiple levels of personality that we’ve described. We also assessed personality based on more than one source – not just a person’s self-ratings on a personality questionnaire, but also how their partner rated them.
In one of our studies, we analysed previously collected data from two groups: couples who had been dating for less than 12 months, and established couples in the first two years of parenthood. In both groups, we found evidence of similarity between partners. Generally, there was much more evidence for similarity in their characteristic adaptations. These included relationship-specific characteristics like: responsiveness, or being seen as understanding and validating; go-to conflict resolution strategies, such as collaboration; and trust in one’s partner. The results suggest that being similar in characteristics like these, which are most directly tied to relationship functioning, might be more important than similarity in general traits when it comes to choosing a partner or maintaining a relationship past the earliest stage.
Not every aspect of personality suggested a tendency to choose similar partners, however. For example, the trait of emotional volatility (a facet of neuroticism) showed the rare pattern in which partners are negatively correlated with one another. This makes sense as a buffering effect, where one partner acts as a stabilising force for the other. If both partners get easily agitated or have frequent changes in mood, the chaotic environment that results might not be conducive to forming a long-term relationship.
Interestingly, partners tended to perceive each other’s personalities as more similar than they actually were (according to their own self-ratings). This was particularly strong with regard to positive or negative emotional traits. It suggests that the sense of similarity that many people experience in relationships is not solely due to selecting a like-minded partner, but also how partners come to see each other over time.
The intuitive view suggests that having similar personality traits should strengthen a couple by promoting compatibility and reducing friction. But our research challenges this idea. To get a clear view of this, we had to account for the effects of each partner’s personality traits. For example, if you have a high level of extraversion – and/or your partner is extraverted – that might contribute to your relationship satisfaction. That doesn’t mean that you’re happy in your relationship because you both have matching levels of extraversion; it could just be that each partner’s trait has a positive effect on its own. We found that, after controlling for effects like these, there was no consistent evidence that personality-matching in itself is actually associated with greater relationship satisfaction.
When partners felt joyful or excited at the same time, their relationship satisfaction tended to be higher
That being said, we can’t conclude that similarity has no benefits at all. Since all of our couples were already in an established relationship, rather than at the initial attraction stage, there might be some benefit of personality assortment for weathering the early phase.
What’s more, there are other ways similarity might affect relationship satisfaction. The findings described so far were based on traditional longitudinal designs, in which participants are followed over weeks, months or years. However, we can think of broad personality traits as an aggregation of much smaller moments and behaviours throughout our daily lives. We can try to understand personality and its implications through fluctuations in personality states. For example, you might feel particularly extraverted and assertive while leading a work meeting, but less so while you’re on the couch at the end of a long day.
In another study we conducted, established romantic couples provided data on their own and their partners’ personality states five times per day for a week. One intriguing finding was that co-fluctuations in emotions – or, how much partners’ emotional states moved in sync – were more strongly related to relationship satisfaction than similarities in general personality traits were. For example, when partners experienced positive emotions together (such as feeling joyful or excited at the same time), their relationship satisfaction tended to be higher. Think of the simple joy of laughing at the same joke or sharing excitement about good news: these small, shared emotional moments might matter more than broad patterns like whether you and your partner are similarly introverted or conscientious. In other words, perhaps it is less important how we are than how we move together through the world.
On the whole, our findings might offer some reassurance to couples who don’t feel especially similar to each other in terms of personality. If you and your partner differ in key traits, that doesn’t necessarily spell trouble for your relationship. Think instead about whether you share emotional moments, support each other’s goals and needs, and can rely on each other during tough times. And for couples who are quite similar, it’s possible that this helped bring you together during the early stages, but it might not be the main ingredient for long-term happiness together. In both cases, the quality of a couple’s emotional connection and ways of growing together can be just as, if not more, significant than how alike their personalities are.
