This thesis proposes a new theory of personality in human and nonhuman primates, integrating neuroscience with ontogenetic and phylogenetic developmental perspectives. In this theoretical proposal, the most important goals for the study of personality are to understand: a) the ontogeny of personality, including the development of individual differences; b) the phylogeny of personality, including the common brain areas and functions that underlie personality ontogeny across primate species; and c) how individual differences regarding idiosyncratic aspects of individuals’ personalities reflect adaptations to the individuals’ life-histories that brought them about. Three essential premises are at the core of this theory: first premise: Personality is a dynamic structure, changing through the individual’s ontogeny; second premise: Personality organizes interactions with the external world into a complex internal model; and third premise: Personality evolved through primates’ phylogeny along with the brain structures that sustain it. At the empirical level, the hypotheses were that animals with higher cognitive abilities related with the referred brain areas, would have more diverse, flexible and complex personalities. A group of species covering the main branches of the primate phylogenetic tree were studied: a) Strepsirrhines: red-ruffed lemurs (Varecia rubra) and ring-tailed lemurs (Lemur catta), b) Haplorrhines - Platirrhines: squirrel monkeys (Saimiri boliviensis) and tufted-capuchin monkeys (Sapajus apella), and c) Haplorrhines - Catarrhines: mandrills (Mandrillus sphinx) and white-handed gibbons (Hilobates lar). The choice of species was limited to access availability. Data were collected via direct observation of behavior of animals from the Lagos’ Zoo, Badoca Park and Maia’s Zoo. Data were analized statistically and mathematically to obtain information on three personality aspects: 1) Diversity, 2) Flexibility, and 3) Complexity. The personality study methodology used was innovative. The data obtained are a first corroboration of the hypothesis defended. Further studies with other groups, species, and outside the context of captivity are required
