Two Competing Hypotheses
In love and human relationships, "birds of a feather flock together (similarity hypothesis)" and "opposites attract (complementarity hypothesis)" are among the oldest debated topics. Since everyday intuition can summon examples of both, many people find it hard to judge which is correct.
However, the accumulation of psychological research provides a clear answer. Bottom line: in long-term relationships, similarity has an overwhelming advantage. While "opposites attract" is partially correct at the early attraction stage, similarity consistently dominates in relationship maintenance and satisfaction.
Research Supporting the Similarity Hypothesis
Since Byrne (1971)'s "similarity-attraction paradigm," hundreds of studies have reported positive correlations between attitude, value, and personality similarity and interpersonal attraction.
Montoya, Horton & Kirchner (2008)'s meta-analysis integrated 313 studies (over 35,000 participants) and confirmed a moderate positive correlation (r = .39) between attitude similarity and attraction. This effect has been consistently observed in both laboratory studies and real-couple research.
Regarding personality trait similarity, Malouff et al. (2010)'s meta-analysis reports that for all five Big Five factors, between-couple similarity has positive correlations with relationship satisfaction. Effects of agreeableness and conscientiousness similarity are particularly large.
Several mechanisms explain why similarity is advantageous. (1) Self-validation: similar partners give the sense of affirming one's values. (2) Predictability: similar partners' behavior is easier to predict, generating reassurance. (3) Shared activities: when interests and values align, more activities can be enjoyed together. (4) Reduced conflict: when fundamental values align, daily decision-making conflicts are fewer.
Limited Validity of the Complementarity Hypothesis
The complementarity hypothesis ("opposites attract") isn't entirely refuted. However, its scope of application is very limited.
Winch (1958)'s proposed complementarity hypothesis assumed specific complementary interpersonal needs: "dominant people are drawn to submissive people," "caregivers are drawn to those needing care." But subsequent replication research found limited support for this hypothesis. Related books can be found at related books (Amazon).
Modern research shows complementarity functions mainly in these limited contexts. (1) Early attraction stage: feeling freshness or admiration toward partners with traits one lacks. (2) Specific role divisions: when functional division of labor works (one extraverted handling socializing, the other introverted maintaining the home). (3) Short-term relationships: in short-term relationships where stimulation and novelty matter, dissimilarity can be attractive.
However, in long-term relationships, the initial "freshness" fades and daily value clashes come to the foreground. "Interesting because we're opposites" easily becomes "exhausting because we're opposites" within months.
Optimal Compatibility - Similarity + Moderate Difference
Recent research suggests that "high similarity + moderate difference" is optimal, not "complete similarity."
Gonzaga, Carter & Buckwalter (2010)'s large-scale study (eHarmony data, tens of thousands of couples) reports that while couples with higher personality similarity have higher relationship satisfaction, couples with "core values aligned but moderate differences in hobbies and interests" achieve the highest satisfaction - more than couples with "everything the same."
This is intuitively understandable. Two completely identical people cannot offer each other new perspectives. Moderate difference brings stimulation and growth opportunities to the relationship. What matters is "where to be similar and where to differ."
Areas to align: values, life goals, conscientiousness, agreeableness, emotional stability
Areas where difference is acceptable: hobbies, level of extraversion, openness (attitude toward new experiences)
This site's compatibility assessment reflects this insight, evaluating similarity on all traits while assigning higher weights to similarity in agreeableness, conscientiousness, and emotional stability.
Practical Advice
Specific advice for applying scientific findings to daily life.
Partner selection stage: Beyond initial "butterflies" and "freshness," prioritize alignment in values and lifestyle. "Can we naturally agree about daily meals, weekend activities, and how to spend money?" is an important indicator predicting long-term relationship quality.
Improving existing relationships: If you already have major differences with your partner, it's important to "understand" rather than "change" them. Acknowledging the existence of differences and respecting each other's domains - this "agreed-upon difference" doesn't threaten relationships. What threatens them is the pressure to "make the partner conform to oneself."
Importance of self-understanding: Before considering compatibility, accurately understanding your own personality traits is crucial. Knowing what you value and in what environments you feel most comfortable, the characteristics of compatible partners naturally become visible.