Robert B. Zajonc, a distinguished psychologist who illuminated the mental processes that underpin social behavior and in so doing helped create the modern field of social psychology, died on Wednesday at his home in Stanford, Calif. He was 85.

The cause was complications of pancreatic cancer, his son Michael said.

At his death, Professor Zajonc (pronounced ZYE-unts) was emeritus professor of psychology at Stanford University, where he had taught since 1994. He previously had a long association with the University of Michigan.

Until the mid-20th century, social scientists seeking the impetus for human behavior tended to look reflexively to people’s environments. That, in an era when behavioral psychology reigned supreme, was precisely what they had been trained to do. Professor Zajonc, by contrast, also looked to the mind.

Published widely in professional journals and cited often in the news media, Professor Zajonc’s work ranged across the mental and social landscape. Among the subjects he investigated over five decades were the effect of birth order on intellectual performance; whether the mere presence of spectators can influence a performer for good or ill; and whether smiling can be a cause, as well as a consequence, of a good mood.