The scientist best known for research on cognitive development is Jean Piaget (see pages 72–75), who proposed that children’s thinking goes through a set series of four major stages. Piaget believed that children’s cognitive skills unfold naturally as they mature and explore their environment.

Psychologist Lev Vygotsky believed that children’s sociocultural environment plays an important role in how they develop cognitively. In Vygotsky’s view, the acquisition of language is a crucial part of cognitive development. After children acquire language, they don’t just go through a set series of stages. Rather, their cognitive development depends on interactions with adults, cultural norms, and their environmental circumstances.

Current research indicates that children have complex cognitive abilities at much younger ages than Piaget suggested. As early as four months of age, infants appear to understand basic laws of physics. For example, a four-month-old infant can recognize that solid objects cannot pass through other solid objects and that objects roll down slopes instead of rolling up. At five months of age, infants can recognize the correct answers to addition and subtraction problems involving small numbers. These observations have led some researchers to speculate that humans are born with some basic cognitive abilities.

Critics argue that researchers who find these results are overinterpreting the behavior of the infants they study.