Studies of postcopulatory sexual selection typically estimate a male's fertilization success from his paternity success (P₂) calculated at hatching or birth. However, P₂ may be affected by differential embryo viability, thereby confounding estimations of true fertilization success (F₂). This study examines the effects of variation in the ability of males to influence embryo viability upon the inequality between P₂ and F₂. It also investigates the consequences of this inequality for testing the hypothesis that polyandrous females accrue viability benefits for their offspring through facilitation of sperm competition (good-sperm model). Simulations of competitive mating trials show that although relative measures of male reproductive success tend to underestimate the strength of underlying good-sperm processes, good-sperm processes can be seriously overestimated using P₂ values if males influence the viability of the embryos they sire. This study cautions the interpretation of P₂ values as a proxy for fertilization success or sperm competitiveness in studies of postcopulatory sexual selection, and highlights that the good-sperm hypothesis needs empirical support from studies able to identify and separate unequivocally the males' ability to win fertilizations from their ability to influence the development of embryos.