<p>This study examined whether similarity in personality traits between romantic partners is associated with their individual trait levels and life history strategy. We focused on two distinct personality frameworks relevant to mating dynamics: five-factor personality traits and the Dark Triad traits, each capturing different facets of personality functioning. Both self- and partner-reports were used as data sources to ensure comprehensive assessment. The first hypothesis tested whether personality similarity reflects competition for desirable partners and was examined on 565 couples. The second hypothesis explored whether personality similarity aligns with a slower life history strategy, which promotes endogamy, and was tested on 393 couples. Intra-pair similarity scores for each trait were computed and correlated with individual trait levels and life history strategy scores. Additionally, we assessed actor and partner effects and the impact of (dis)similarity on life history strategy using dyadic response surface analyses. Findings provided limited support for the first hypothesis, with slightly stronger effects observed for Dark Triad traits than five-factor traits, more pronounced in partner-reports than self-reports, and more evident in men than women. The second hypothesis was more strongly supported, demonstrating that personality similarity in both trait domains was linked to a slower life history strategy. Additionally, the results overwhelmingly indicate no congruence effects of personality traits on the level of life history strategy. Overall, while effect sizes were weak to moderate, the findings suggest that a slow life history strategy favors selecting partners with similar traits, and the ones showing reliable and valid fitness indicators.</p>