Photo: Jean Piaget

Jean Piaget (1896-1980) always considered himself a natural scientist, not a psychologist. As a boy he quickly gave up play and pretend to take refuge in "work" -- exploring internal combustion engines, studying fossils, shells, and birds. "I have always detested any departure from reality, an attitude which I relate to my mother's poor mental health," he recalled.

Strangely enough, after his successful undergraduate and graduate studies concentrating on mollusks, he began to work with children and did so for the rest of his life. His godfather had introduced him to philosophy and he found it so compelling that he decided "to consecrate my life to the biological explanation of knowledge."

After his education in Switzerland, he worked in France for Théodore Simon, Alfred Binet's collaborator, where he helped standardize tests for gauging the reasoning abilities of five- to eight-year-old Parisians. He was fascinated with finding that at a certain age, children could solve a particular reasoning problem, but, more than that, at an earlier age, they nearly always gave the same wrong answer. He set to find out more.

Over the next 60 years, in numerous prestigious academic posts, he continued to talk with children, play with them, ask them questions, and try to understand their thinking. Gradually he pieced together a "blueprint" for normal cognitive development in children, and presented findings that were amazing for their simplicity, insight, and endurance to the test of time.

For example, he found if he showed a baby a toy, but then covered the toy with his hat, the baby would forget about the toy -- if the baby were younger than nine months old. At about nine months of age, most babies understood that though it was hidden, the toy still existed.

Piaget found four major developmental stages (with many subdivisions). For the first year and a half or two years of life, infants are only aware of sensorimotor experience, and do not connect it to things outside of themselves. They do not know how things will react, and so are always experimenting -- shaking things, putting them in their mouths, throwing -- to learn by trial and error. The stage from 18-24 months to 7 years Piaget called preoperational, where children can think about things in symbolic terms. They can pretend, verbalize, and understand past and future. Still, cause-and-effect, time, comparison, and other complex ideas are out of reach. From 7 to 12 years, children gain new competence in thinking and are aware of events outside of their lives. But tackling a problem with several variables in a systematic way is unusual at this age. From 12 years old and up, people are able to think about abstract relationships (as in algebra), understand methodology, formulate hypotheses, and think about possibilities and abstractions like justice.

Piaget is widely recognized as the greatest developmental psychologist of the century. His ideas have been refined and added to, but they remain the foundation of child psychology.