He is particularly known for his research on attribution theory and confirmation bias, and for his collaborations with Lee Ross.
Lepper is primarily responsible for the elucidation of the overjustification effect, alongside Richard Nisbett.
With frequent collaborator Lee Ross, and Robert Vallone, he authored the first study to identify the hostile media effect.
With Ross and Charles Lord he also authored an important study on attitude change and what is now called disconfirmation bias.
He subsequently earned a Ph.D. in Social and Developmental Psychology at Yale University in 1970, returning to Stanford in 1971 as an assistant professor.