Personality traits are basic dimensions of behavioural variation, comprising various more specific characteristics that tend to correlate together. In humans, much of the behavioural variation between individuals is thought to be accounted for by between three and seven roughly independent personality dimensions (Eysenck and Eysenck 1976; Cloninger 1987; Digman 1990; Almagor et al. 1995), and more than 50 years of twin, family, and adoption studies indicate that around 30% or more of the personality variation between individuals can be accounted for by genetic variation (see Johnson et al. 2008 for a recent review). In other animals, personality traits (or `behavioural syndromes') have been the subject of fewer genetic studies, but there is ample evidence in several species that inter-individual variation in behavioural tendencies is also due substantially to genetic variation (Bakker 1986; Drent et al. 2003; Sinn et al. 2006). The proportion of total trait variation that is accounted for by genetic variation is called broad-sense heritability. This consists of the additive component of heritability (due to the accumulation of the average allelic effects) and may also include nonadditive genetic
