The third possibility for explaining the maintenance of genetic variation in personality traits is balancing selection. Under balancing selection, genetic variation is maintained rather than depleted by selection; for example by selection pressures that fluctuate over time and space (environmental heterogeneity), that differ between the sexes (sex-dependent selection), or that favour rarer trait values (negative frequency-dependent selection) or heterozygotes (overdominance). Investigating the relationship of exploratory personality with survival and reproduction rates in Great Tits, Dingemanse et al. (2004) found that selection pressures were opposite in males and females and fluctuated from year to year depending on food and space availability. They argued that this variation in selection was likely to maintain the substantial heritable component of exploratory behaviour in these birds. Similarly, Penke et al (2007) noted the varied and changing physical and social environments that humans have experienced and 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
