al. 2009; Alvergne et al. 2010; Jokela et al. 2010). However, positive correlations with one fitness component can be counterbalanced by negative correlations with other fitness components (e.g. Nettle 2005; Alvergne et al. 2010), which could potentially result in a zero net effect on fitness (Roff and Fairbairn 2007). In this vein, MacDonald (1995) proposed that human personality dimensions each represent a continuum of alternative strategies for maximising fitness, so that average fitness would be approximately uniform (selectively neutral) across the normal personality range. Expanding on this view, Nettle (2006) proposed concrete cost-benefit trade-offs associated with five of the major dimensions of personality variation in humans. For example, he proposed that high extraversion conferred the benefits of greater mating and social success, which were balanced by increased risk of accident and injury due to greater novelty seeking behaviour. In line with this type of view, recent theoretical work has emphasised that genetic variants affecting multiple traits can be invisible to selection when multivariate genetic constraints result in little or no variation in fitness effects; this can occur even when the individual traits correlate with fitness and have substantial genetic variation (Walsh and Blows 2009).
