1. Ecology and Evolution Group, School of Biology, University of Leeds, Leeds LS2 9JT, UK

Correspondence to: Tom Tregenza Correspondence and requests for materials should be addressed to T.T. (e-mail: Email: T.Tregenza@leeds.ac.uk).

Why do females typically mate with more than one male? Female mating patterns have broad implications for sexual selection^{1, 2}, speciation³ and conflicts of interest between the sexes⁴, and yet they are poorly understood. Matings inevitably have costs⁵, and for females, the benefits of taking more than one mate are rarely obvious. One possible explanation is that females gain benefits because they can avoid using sperm from genetically incompatible males, or invest less in the offspring of such males^{6, 7}. It has been shown that mating with more than one male can increase offspring viability^{8, 9, 10, 11, 12}, but we present the first clear demonstration that this occurs because females with several mates avoid the negative effects of genetic incompatibility¹³. We show that in crickets, the eggs of females that mate only with siblings have decreased hatching success. However, if females mate with both a sibling and a non-sibling they avoid altogether the low egg viability associated with sibling matings. If similar effects occur in other species, inbreeding avoidance may be important in understanding the prevalence of multiple mating.