The central unit of development for Bronfenbrenner is the child interacting with the different settings or micro-systems of daily life (Bronfenbrenner, 1994). Jean Piaget, the most influential theorist of cognitive development, elucidated how children and adolescents are active and creative agents in these interactions and, ultimately, of their own development (Piaget, 1964). Even when they give answers to questions that an adult perceives as wrong, answers typically come from intelligent deductions using previous knowledge. Thus, we need to always ask how an adolescent is thinking about and conceptualizing his or her current experience.

While Piaget focused almost exclusively on the solitary individual interacting with an inanimate environment, other specialists in cognitive development gave more attention to the interpersonal environment in which learning and development take place. Vygotsky showed how learning was typically not solitary, but collaborative (Vygotsky, 1978). Children often don’t think in isolation: teachers, parents, and apeers provide support and scaffolding that contributes to their thinking and learning. Another very different school of developmental theory, object relations and attachment theory, emphasizes the emotional quality of the interpersonal environment in which development takes place, particularly children’s relationships with their primary caretakers. Mahler, Winnicott, Bowlby, and others have shown how the warmth and responsiveness of a child’s significant others facilitate development, particularly development of a sense of self and the capacity to engage in healthy relationships in the future. Hostility or lack of trust creates anxiety, which disrupts development (Mahler et al., 1975; Winnicott, 1975; Bowlby, 1969).

These theories suggest that people are extremely malleable, that given the right environment, children can remake themselves as they wish. Research and theory in behavioral genetics, however, suggests there may be limits on how much individuals can change—although the nature of these limits are beyond the range of current knowledge (Plomin, 2000). The development of some human traits, like extroversion and novelty seeking, appear to be significantly constrained by genes, although other traits, such as prosocial and antisocial behavior, are clearly not so constrained (Plomin, 1994). It must be emphasized that the state of knowledge about the role of genes in psychological and social development is