Similarity in long‑term romantic couples probably matters less than we think

People have long believed that couples who have a lot in common are more satisfied and stay together longer. But a new review of nearly 340 studies finds that this popular idea may not be as strong as many assume. The review, published in the Journal of Social and Personal Relationships, shows that while people often value feeling like their partner is similar to them, actual similarity across traits like personality, interests, or background has only a modest and inconsistent link with better relationship outcomes.

The idea that “birds of a feather flock together” is deeply rooted in Western ideas about romance and guides online dating platforms and everyday thinking about relationships. At the same time, some psychologists have argued that differences can balance couples out, making relationships richer and stronger. The available evidence has been scattered, making it hard to draw a firm conclusion about the role that similarity plays. The researchers wanted to unite these threads of evidence and understand how both actual and perceived similarity relate to long‑term relationship satisfaction and stability.

“I’ve always been interested in understanding what predicts compatibility between different people and whether similarity is important for this,” said study author Annika From, a PhD candidate at the University of Michigan. “However, despite the fact that people have strong beliefs about the importance of similarity (or the importance of having a dissimilar partner who balances you out) and that researchers have been studying this topic for decades, we still have little consensus as a field about whether similarity matters. This paper aimed to find a more conclusive answer.”

To do this, the researchers conducted what is called a scoping review, a method suited for making sense of a very large and diverse body of literature. They searched four academic databases for articles published in English that examined established couples, focusing only on studies that measured long‑term characteristics, such as personality or interests, and connected them to outcomes like satisfaction, stability, or divorce.

They started by identifying over 7,600 studies. After removing duplicates and screening for relevance, they narrowed the sample down to 339 articles published between 1937 and 2024. These studies came from across the globe, although more than half were conducted in the United States and the rest were primarily from other Western countries.

The review included couples from many different walks of life, though most were heterosexual and married. The researchers looked at studies that measured actual similarity, where both partners completed questionnaires about themselves, as well as perceived similarity, where one person rated how similar they felt to their partner.

The review captured six domains of similarity: beliefs and values, demographics and background, lifestyle and interests, personality, physical characteristics, and romantic habits. Most studies looked at only one of these areas, making direct comparisons challenging.

To understand how researchers quantified similarity, the review assessed a range of statistical methods, from basic matching approaches that grouped couples with comparable scores, to more advanced analyses that accounted for overlap between couples’ characteristics and general trends across people. The researchers also examined how statistical controls for individual tendencies or stereotype effects impacted the results. They found that studies using robust statistical methods tended to report weaker connections between actual similarity and relationship outcomes.

The results were largely mixed. In studies that calculated actual similarity, most found no strong link between being alike and having a higher-quality or more stable relationship. The review identified very few instances where actual similarity clearly predicted better outcomes across domains.

The strongest evidence was for similarity in demographics, such as race or education, but even this was far from universal. Meanwhile, perceived similarity appeared to be more important. Studies in this category were far more likely to find that seeing one’s partner as similar was associated with higher satisfaction and stability.

Closer inspection revealed that statistical methods shaped the results. Studies relying on basic matching approaches often found significant benefits for actual similarity, but those using methods that accounted for confounding variables tended to find weaker or no effects.

Studies that adjusted for a person’s general tendency to rate themselves highly or the general trend for people to pick others like themselves were less likely to find benefits from actual similarity. This suggests that much of the evidence for the benefits of actual similarity may be influenced by how the data were analyzed.

The review also tested whether certain characteristics acted as moderators, making the link between similarity and outcomes stronger or weaker. The researchers looked at gender, marital status, and location, finding little evidence that any of these factors reliably shaped the results. The biggest difference came from statistical method. Analyses using self-reported perceptions of similarity were much more likely to find a link between similarity and satisfaction, whereas more complex statistical methods rarely did.

The researchers conducted sensitivity analyses as well, estimating the smallest effect sizes that the studies in their review could reliably detect. They found that many studies, especially those with null results, were only powered to detect relatively large effects. This means that smaller benefits of similarity could have gone unnoticed in some studies, making it challenging to draw firm conclusions about the role of actual or perceived similarity.

This review highlights a significant paradox in how people understand long‑term relationships. Despite a strong belief in the benefits of being alike, the evidence does not clearly support actual similarity as a strong or universal factor in determining how well couples fare over time. The review suggests that perceptions of similarity matter more than measurable overlap, hinting at deeper questions about why this is the case.

“We do not see evidence that being similar to your romantic partner affects your relationship quality or whether or not you stay together with your partner,” From told PsyPost. “This is true across different domains of similarity, such as demographics, beliefs and values, and lifestyle. However, there is more evidence that perceived similarity (thinking you are similar to your partner) is positive for relationships.”

The review has some limitations. Its focus was primarily on studies conducted in English and published in peer‑reviewed journals, which may have overlooked relevant work conducted in other languages or settings. The review also concentrated on long‑term couples and trait‑like characteristics, making it unclear how these findings apply to early dating relationships or other forms of interpersonal connections, such as friendships or work collaborations.

“As this is a scoping review, we present a comprehensive overview of the research that has been done on this topic,” From explained. “However, there are many different ways that similarity has been studied, and some studies may be more representative and use better methods than others. Additionally, this work is limited in its ability to investigate potential moderators, so future work should look into whether there are more specific cases when similarity may matter.”

“Our main finding presents an interesting tension—that perceptions of similarity may be important, though there is little evidence that actual similarity is. I hope to explore why these perceptions matter, how they form, and if they can be changed in future work.”

The study, “Does similarity matter? A scoping review of perceived and actual similarity in romantic couples,” was authoerd by Annika From, Emily Diamond, Nazanin Kafaee, Miranda Reynaga, Robin S. Edelstein, and Amie M. Gordon.