variance within individuals over time. In a meta-analysis of published, peer-reviewed repeatability estimates, reviewers found that, in general, approximately 35% of behavioural variation among individuals could be attributed to individual differences. Five Factor Model The Five Factor Model, or the Big Five personality trait model, has been used to assign personality archetypes to some animals. The 5 categories for the five factor model for personality are openness to experience, conscientiousness, extraversion, agreeableness, and neuroticism. Each of these categories identifies personality factors at the broadest level of abstraction. Because the assessment of personality using the five factor model is often a self-reported measure for humans, applying this model to animals can be difficult to standardize. Researchers often measure personality by assessing the behavior of the being over a period of time to establish the pattern. Naturally, some animals may not have as wide a range of personality as humans do. The species of the animal determines how the personality manifests itself. Likewise, a species may be predisposed to exhibit a category of personality more than other categories. So far, chimpanzees are the only animal shown to exhibit conscientiousness. Chimpanzees are also the only non-human species shown to demonstrate a hierarchical structure of personality, with two dimensions of affect corresponding to negative and positive emotionality, and a third dimension of disinhibition (vs. constraint), which is thought to comprise a regulatory system that is known to play a role in the perception and interpretation of incoming stimuli. Neuroticism, agreeableness and extraversion are the most commonly found personality traits among measured animals. For example, chimpanzees show emotional stability, agreeableness and surgency, audiovisual reactivity, affect-extraversion, excitability-agitation, aggression affinity and social play. Correlated traits and behaviours Some behaviours are correlated into a set of personality traits that remain constant throughout different situations and contexts. These traits are referred to as behavioural syndromes. For example, the aggressiveness-boldness syndrome refers to the correlation between an individual's aggressiveness with conspecifics and boldness in novel environments. Many correlated behaviours are species specific. For instance, a study in 2014 reported that for horses, personality (quantified using a validated questionnaire) and tolerance to pain (using a Likert scale) indicated that neuroticism is negatively related to stoicism whereas extroversion was positively related to levels of expressed lameness. This suggests that pain may be more easily identified in highly extrovert horses. In zebrafish (Danio rerio), Proactive and Reactive personalities express different thermal preferences and