We examined the claim that sperm competition is an important selection pressure operating in human populations. We recruited 222 men and 194 women to complete a survey of their sexual behaviour. Of these, 28% of men and 22% of women reported engaging in extrapair copulations (EPCs). A review of the literature suggests that rates of extrapair paternity are in the region of 2%. These values suggest that the risk of sperm competition in humans is relatively low, in line with comparative studies of relative testis sizes of humans and other primates. Testis volume was positively correlated with the number of sperm ejaculated. However, we found no support for a recent controversial claim that the within-population frequency distribution of testis size reflects a balanced polymorphism between men who specialize in sperm competition through EPCs and men who are monogamous. (C) 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.