Personality traits that influence on individuals' communication and relationship behavior, assertiveness and responsiveness, are presumed to be important components for sexual behavior. We investigated the relationships of assertiveness and responsiveness to sexual behavior and tested the relative contributions of personality and physical traits (e.g., body shapes, fluctuating asymmetry, and digit ratios) to the prediction of sexual behavior. Additionally, we examined whether the effect of these personality traits on sexual behavior interacts with each other and with body attractiveness. Assertiveness had substantial effects on sexual behaviors for both sexes, whereas responsiveness had a negative effect in men and null effect in women. The finding of the relative contributions of personality and physical traits indicate that variation in sexual behavior is much bigger for personality traits than physical traits, probably gaining insight into the magnitude of variation in traits under frequency-dependent selection and those under positive selection. Moreover, interaction effects indicate that mating success is also an outcome of the concerted interplay of personalities and body attractiveness. Findings support the evolutionary hypotheses that personality traits are alternative adaptive strategies evolved as a means of ensuring fitness.

Key words: assertiveness, responsiveness, body attractiveness, fluctuating asymmetry, 2D:4D, sexual behavior