Paternal uncertainty has shaped human behavior both in evolutionary and cultural terms. There has been much research investigating parenting as a function of genetic relatedness to the child, with a focus on male behavior, but the nature of these sex differences is hard to evaluate. We devised a hypothetical scenario that was as similar as possible for men and women to test whether, even in such a scenario, sex differences would remain strong. Participants were presented with the discovery that a child that s/he believed to be theirs was not carrying their own genes. Irrespective of sex, participants (n = 1007) were more upset when the baby was not genetically related to them than when the child was genetically related but the sex gamete was not from a chosen donor. Women were more upset than men in both scenarios, but were more likely to want to keep the baby. The results are discussed with reference to evolved and rational mechanisms affecting parenting.