Do Opposites Attract? Similarity vs Complementarity in Psychology

Two Competing Hypotheses

In romance and human relationships, "birds of a feather flock together" (the similarity hypothesis) and "opposites attract" (the complementarity hypothesis) are among the topics that have been debated for the longest time. Because everyday intuition can call up examples of both, many people probably find it hard to judge which one is correct.

However, accumulated psychological research points to a clear answer. To state the conclusion first, in long-term relationships, "feeling that you are similar" tends to provide support. "Opposites attract" is partly correct at the stage of initial attraction, but at the stages of relationship maintenance and satisfaction, the evidence supporting complementarity is limited.

Research Supporting the Similarity Hypothesis

Since Byrne's (1971) "similarity-attraction paradigm," hundreds of studies have reported positive correlations between similarity in attitudes / values / personality and interpersonal attraction.

A meta-analysis by Montoya, Horton & Kirchner (2008) integrated 460 effect sizes from 313 studies and confirmed strong positive associations: r = .47 between actual similarity and attraction, and r = .39 between perceived similarity (the degree to which partners feel they are alike) and attraction. What matters is the breakdown. The effect of actual similarity was strong in brief experimental settings but not significant in existing couple relationships, while perceived similarity consistently predicted attraction both in experimental settings and in existing relationships. In other words, in real couples, "feeling that you are similar" matters more than "being objectively similar."

Regarding personality traits, a meta-analysis by Malouff et al. (2010) examined the association between the level of one partner's Big Five traits and the other partner's relationship satisfaction. This is the perspective that not only "how similar the partners are" but also "what traits the partner has" itself bears on the quality of the relationship.

The following mechanisms have been proposed to explain why similarity is advantageous. (1) Self-verification: a similar partner gives you the sense that your values are affirmed. (2) Predictability: a similar partner's behavior is easier to predict, which creates a sense of security. (3) Shared activities: when interests and values are similar, there are many activities you can enjoy together. (4) Reduced conflict: when fundamental values match, there are fewer clashes in everyday decision-making.

Limited Validity of the Complementarity Hypothesis

The complementarity hypothesis that "opposites attract" has not been completely rejected. However, its range of application is very limited.

The complementarity hypothesis proposed by Winch (1958) assumed complementary relationships between specific interpersonal needs, such as "dominant people are attracted to submissive people" and "nurturing people are attracted to people who need care." However, later replication studies produced only limited results supporting this hypothesis.

Modern research shows that complementarity works mainly in the following situations. (1) The initial attraction stage: people feel novelty and admiration toward a partner who has traits they themselves lack. (2) Specific divisions of roles: cases where a functional division of labor works, such as one partner being extraverted and handling social life while the other is introverted and looks after the home. (3) Short-term relationships: in short-term relationships where stimulation and novelty are important, being different can be attractive.

In long-term relationships, however, the initial "novelty" fades, and everyday clashes of values come to the fore. "It's fun because we're opposites" can easily turn into "It's tiring because we're opposites" within a few months.

Optimal Compatibility - Similarity + Moderate Difference

Accumulated research does not call for "complete similarity." Read in practical terms, it points to "high similarity in core areas + tolerable differences."

Gonzaga, Carter & Buckwalter (2010) studied couples who met through an online matching service and married, examining spouses' personality similarity (assortative mating), its convergence over time, and how these relate to marital satisfaction (in the journal Personal Relationships). In addition, the finding of Montoya et al. (2008) that "in existing relationships, perceived similarity outweighs actual similarity" suggests that not everything needs to match, and that the key is whether the two partners can feel "similar" in the areas they consider core.

This also makes intuitive sense. Even if there were two completely identical people, they could not offer each other new perspectives. Moderate differences bring stimulation and opportunities for growth to a relationship. What matters is "where you are similar and where you differ."

Areas that should match: Values, life goals, conscientiousness, agreeableness, emotional stability

Areas where differences are acceptable: Hobbies, degree of extraversion, openness (attitude toward new experiences)

This site's compatibility assessment also looks at the degree of similarity across all traits, while placing particular weight on agreement in several traits.

Practical Advice

Here is concrete advice for applying scientific findings in daily life.

When choosing a partner: Place weight not only on early "butterflies" and "novelty" but also on how well your values and lifestyles match. "Can I naturally reach agreement with this person about everyday meals, how to spend days off, and how to spend money?" is an important indicator that predicts the quality of a long-term relationship.

Improving an existing relationship: If there are already large differences between you and your partner, it is important to "understand" them rather than "change" them. "Agreed-upon differences," in which you acknowledge that the differences exist and respect each other's areas, do not threaten the relationship. What threatens it is the pressure of "trying to make your partner fit you."

The importance of self-understanding: Before thinking about compatibility, it is important to first understand your own personality traits accurately. By knowing what you value and in what kind of environment you feel most comfortable, the characteristics of a compatible partner will naturally come into view.