Thomas J. Bouchard Jr. is an American psychologist known for his research in the field of behavioral genetics. He gained prominence for his work on twin studies, particularly those involving separated twins, which helped to illuminate the roles of genetics and environment in human behavior. Bouchard is best known for the Minnesota Study of Twins Reared Apart, which began in the 1970s. This landmark study provided valuable insights into the heritability of various traits, including intelligence and personality, and has significantly influenced the understanding of the nature versus nurture debate in psychology.