Mark Lepper

Mark R. Lepper (born 1944) is a social psychologist and emeritus Albert Ray Lang Professor of psychology at Stanford University. He is known for research on attribution theory and confirmation bias, and for helping to explain the overjustification effect, in which an external reward can reduce a person's interest in an activity they already enjoyed. With Lee Ross and Robert Vallone he carried out the first study of what became known as the hostile media effect, and with Ross and Charles Lord he co-authored a study on attitude change that is cited for disconfirmation bias. Lepper studied at Stanford, took his doctorate at Yale in 1970, returned to Stanford in 1971 and chaired its psychology department in the 1990s and again after 2000.