American biologist who helped pioneer the discovery and study of biorhythms (biological rhythm) and who showed that humans' biological processes can be strongly influenced by learned behaviour.

Richter attended Harvard University (B.S., 1917), and after a year as first lieutenant in the U.S. Army he entered Johns Hopkins University, where he was awarded his Ph.D. and joined the faculty, both in 1921. In 1922 he became director of Johns Hopkins' psychiatric clinic, a post that he held until becoming professor of psychobiology in 1957. He introduced the concept of the biological clock in a 1927 paper on the internal cycles that govern animals' drinking, eating, running, and sexual behaviour. In studying the influence of learned behaviour on human biology, Richter reported that ancient peoples' discovery of fire probably drastically changed their habits; as a result, their brain structure was changed, and their ability to learn and communicate was increased. He helped discover the relationships between behaviour and biochemistry that govern such diverse aspects of life as sleep, stress, and the onset of disease. Altogether Richter wrote over 250 research papers, and though he retired in 1960 he continued to work in his laboratory until shortly before his death.