Sidanius, who was of African American heritage, grew up in New York City. He earned a bachelor's degree in psychology from City College of New York in 1968. He went on to pursue a Ph.D. in psychology at the University of Stockholm, Sweden. His dissertation, passed in 1977, was titled: "Cognitive functioning and Socio-political Ideology: Studies in political psychology." He changed his last name from Brown to Sidanius upon moving to Sweden (where he also obtained citizenship), but returned to the United States in the 1980s as a permanent resident. He was married twice.
