The process by which the human body changes and matures over time, especially the means by which dying cells are not replaced in sufficient numbers to maintain current levels of function; the process by which human behavior alters with time.

Psychological studies of aging populations began in earnest in the late nineteenth century when psychologists found that mental abilities deteriorated with age. These abilities included memory and the types of mental performance measured in IQ tests. In some individuals, verbal abilities were shown to deteriorate with advanced age, although at a slower rate than other skills; with others, verbal abilities, especially vocabulary, may increase with age. Such data have often been corroborated in tests with chimpanzees, where younger animals perform better in tests of memory and other such areas of...