Evolution and Development of Individual Behavioral Variation
What makes individuals unique? The answer to this question lies in understanding why and how individuals respond to numerous internal and external factors that they experience over their lifetimes. This fundamental question lies at the heart of the study of human and animal behavior and is best addressed by integrating both proximate and ultimate perspectives. From a proximate perspective, we need to understand the molecular, hormonal, and physiological pathways involved in enacting behavioral changes within individuals. From an ultimate perspective, we need to understand when and why behavior changes in response to different internal and external factors and whether such changes are adaptive, a result of constraints, or pathological. Research on this topic draws links across several fields including developmental and abnormal psychology, personality in humans and animals, developmental plasticity, and parental effects. The development of individual behavioral variation encompasses many different processes because there are so many ways that behavior can vary within and across individuals. This article considers behavioral variation in both of these respects, that is, within-individual plasticity and among-individual differences. Within an individual, behavioral plasticity describes the way in which behavior can change across the lifespan as a result of changes in internal factors such as maturational state, or as a result of salient experiences. Among individuals, differences in average behavior can be a result of individual differences in internal factors, such as genetic variation, variation in their experiences, and variation in how they respond to the same experiences. Traditionally, students of behavioral variation in humans and animals focused on describing the mean levels of behavior expressed by groups of individuals as a function of changes in age or in response to specific experiences. More recently evolutionary and behavioral ecologists have become interested in patterns of within-individual plasticity and among-individual differences in behavior, and the factors that contribute to their development, which is the focus of this review here.