However, contemporary sexual selection theory[121],[122] suggests that a number of factors, such as sex-biased mortality, population density, and variation in mate quality, will affect how competitive and choosy males and females are, with sex roles expected to vary considerably within and between societies. For example, this theory predicts that, in human beings, both sexes will be choosy when encounter rates with potential mates are high, particularly where the parental investment levels of both sexes are large and not too different, and/or where variation in mate quality of both sexes is high, and males are likely to be choosy in populations with a female-biased adult sex ratio and considerable paternal investment.