Postmating sexual selection favors males that sire offspring with low fitness.

Despite the costs of mating, females of most taxa mate with multiple males. Polyandrous females are hypothesized to gain genetic benefits for their offspring, but this assumes paternity bias favoring male genotypes that enhance offspring viability. We determined net male genetic effects on female and offspring fitness in a seed beetle and then tested whether fertilization success was biased in favor of high-quality male genotypes in double mating experiments. Contrary to expectations, high-quality male genotypes consistently had a lower postmating fertilization success in two independent assays. Our results imply that sexually antagonistic adaptations have a major and unappreciated influence on male postmating fertilization success. Such genetic variation renders indirect genetic benefits an unlikely driver of the evolution of polyandry.