Among many monogamous species, from cockatiels to cichlid fish, studies have revealed a clear pattern: it helps to be more similar to your mate. When mating pairs are behaviourally similar, their reproductive success tends to be higher.
In human terms, this would imply it’s better to be similar to your partner. Indeed, for a long time psychologists and others have argued that similarity is probably beneficial – after all, then we will be more likely to enjoy the same pursuits, values and outlook on life.
But no matter how intuitive the idea seems, for decades nearly every study has failed to support it.
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Now, though, a team of psychologists at the University of Amsterdam think they know why. They’ve taken a far more sophisticated and nuanced look at the issue than in previous research. Their findings suggest that partner similarity does actually matter – especially for the trait of ‘agreeableness’.
Their study is not alone. Other recent research looking at factors besides personality has found similarity is important in other ways, like whether you’re a morning person and if you share political attitudes. But above all – and perhaps more important than how similar you are – is how much you each come to develop a sense of a shared identity.
In terms of how much similarity affects relationships, lead researcher Manon van Scheppingen and her colleagues explain that virtually all previous research has taken an all-or-nothing approach, without factoring in the more nuanced question of whether the effect depends on the specific traits you are talking about and the relative score each partner has on those traits.
To take one example, common sense would suggest that if both partners are highly conscientiousness, then similarity in that case might well be beneficial. But if one partner has very low conscientiousness, it may actually be better for the relationship if the other partner differs from them and has more conscientiousness – leading to a kind of beneficial compensation effect.
Van Scheppingen’s team analysed several years of personality, wellbeing, and relationship satisfaction data from thousands of long-term married couples in America, taking into account each partner’s relative scores on each of the five main personality traits.
Consistent with past research, they found that by far the most important thing for overall wellbeing was the straight-forward effect of each person’s personality. Generally speaking, individuals tended to be happier if they, and/or their partner, had more agreeable, more conscientious, less neurotic personalities (which is consistent with what we know about the links between these traits and happiness).
But crucially, and contrary to past research, this wasn’t the whole story.
Having the same level of extraversion as one’s partner was not ideal for overall wellbeing
It turned out that the comparative level of each partner’s traits also mattered in a modest but meaningful way. Most of the time a perfect match was not beneficial. For instance, having the same level of extraversion as one’s partner was not ideal for overall wellbeing (instead the optimal situation, at least for wellbeing, was for one person to be somewhat more extraverted than their partner). For people who had low levels of conscientiousness, similarity wasn’t optimal either (it was better to be with someone with a somewhat higher level of conscientiousness).
The standout exception, but only for women, was agreeableness: a trait associated with trusting others and having more empathy. Greater similarity to one’s partner was the ideal situation in terms of feeling more supported in the relationship. Less clear-cut, but also in favour of a similarity effect for both men and women, was a degree of similarity in openness (a trait associated with enjoying new experiences and appreciating art and culture).
Van Scheppingen and her team speculated that some similarity in openness might be beneficial because of this trait’s links with values and politics (greater openness is associated with holding more liberal attitudes, for example). Similarity therefore would lead to “less conflict between spouses’ views and actions, which could be linked to experiencing higher levels of relational well-being”, the researchers write.
One research team found that the couples who showed greater similarity in the trait of openness were more likely to stay together
This tentative finding of a similarity effect for openness chimes with another recent paper that looked whether there is any connection between how similar partners are and how long their relationship lasts. Arguably this is a more objective measure than people’s ratings of their wellbeing and feelings of support. Beatrice Rammstedt at the Gesis Leibniz Institute for the Social Sciences in Germany had nearly 5,000 German couples complete personality questionnaires and tracked them for five years. Her team found that the couples who showed greater similarity in the trait of openness were more likely to stay together.
These weren’t the only recent studies finding a benefit in similarity. Another recent paper also found that women benefit when they have similar levels of openness to their partners (the optimal situation was when both partners reported modest levels of openness). Yet another study found partner similarity was specifically helpful to anxiously attached individuals – people who worry about being abandoned.
There’s evidence that similarities beyond the main personality traits matter, too. For instance, a recent study of heterosexual couples by the University of Warsaw’s Paulina Jocz and her colleagues showed that women were happier in their relationship when they and their partner shared the same chronotype (that is, whether they were a morning or an evening person). They also found that both genders were more sexually satisfied if they had a shared preference for when in the day to have sex.
Another study has found that women were happier in their relationship when they shared political attitudes with their partners. And both men and women were happier if they and their partner put about the same amount of value on being free and independent-minded.
These studies focus on comparing partners’ similarity as objectively as possible. But of course our subjective perceptions and feelings about our partners are probably just as important – if not more so – to how we feel about our relationships. And in this regard, psychologists have also been examining the effect of feeling a sense of shared identity with our partners, or what Courtney Walsh and Lisa Neff at the University of Texas, Austin call “identity fusion”.
In their paper studying newlyweds, Walsh and Neff found that those individuals who felt their sense of identity was fused in a balanced way with their spouse’s also tended to be more confident in their relationship and to deal more constructively with any marital turbulence.
It would be interesting to know how perceptions of a shared identity might interact with partner similarity. After all, if you can manage to achieve a form of companionship where it feels as though you and your partner have become one, then it seems likely that questions of similarity and difference will become a side issue – because now their traits and values are yours, too.
In general? It is probably safe to conclude that partner similarity does matter to relationships. The specific implications depend on gender, the traits in question, and even one’s attachment style. There is no simple rule that applies to everyone, but it would be wrong to conclude that similarity is irrelevant.
Dr Christian Jarrett edits the British Psychological Society's Research Digest blog. His next book, Personology, will be published in 2019.
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