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The relationship between fitness and parental similarity has been dominated by studies of how inbreeding depression lowers fecundity in incestuous matings. A widespread implicit assumption is that adult fitness (reproduction) of individuals born to parents who are not unusually closely related is more or less equal. Examination of three long-lived vertebrates, the long-finned pilot whale, the grey seal and the wandering albatross reveals significant negative relationships between parental similarity and genetic estimates of reproductive success. This effect could, in principle, be driven by a small number of low quality, inbred individuals. However, when the data are partitioned into individuals with above average and below average parental similarity, we find no evidence that the slopes differ, suggesting that the effect is more or less similar across the full range of parental similarity values. Our results thus uncover a selective pressure that favours not only inbreeding avoidance, but also the selection of maximally dissimilar mates.