created for themselves in their evolutionary history, and argued that genetic variation in personality traits is most likely to be actively maintained by balancing selection by environmental heterogeneity, often mediated by negative frequency-dependent selection on life-history strategies. Another perspective (Tooby and Cosmides 1990) is that genetic variation in personality is a side effect of pathogen-driven balancing selection, whereby rare alleles are of higher fitness because pathogens are usually poorly adapted to attacking the rarest host genotypes (Garrigan and Hedrick 2003) – this would be an example of `pleiotropic balancing selection' (Turelli and Barton 2004).
