Molecular techniques used to assign paternity have revealed previously unknown incidences of extra-pair paternity in socially monogamous bird species. DNA fingerprinting has now been used sufficiently often for mating-system biologists to appreciate the natural variation in the frequency of broods showing extra-pair young. The variation between species and between populations of the same species is surprisingly marked. Explaining this variation may help us to understand the factors promoting sexual selection. Recent comparative studies and detailed behavioural studies suggest that factors such as breeding density, genetic variation in the population and the intensity of sexual conflicts determine the costs and benefits to males and females of engaging in extra-pair copulations, and therefore contribute to the variation among populations.