Philip Zimbardo, psychologist who led Stanford prison experiment ...

Stanford announced the death but did not provide a cause.

In a career spanning five decades, Dr. Zimbardo served as president of the American Psychological Association and pursued research on topics including shyness, the psychological roots of evil and the way dwelling on the past can affect decision-making. At the “core” of his interest, he told the publication Psychology Today, was “the process of transformation of human nature.”

To his chagrin, his prison experiment was the one that garnered the greatest attention, including questions about the ethics of that study. His findings on de-individuation — relinquishing one’s own identity to that of a group — were used by scholars and psychologists seeking to understand genocide in Rwanda, the rise of Nazism in Germany and other atrocities.

Decades after he brought the prison experiment to an early end, he drew upon it again when explaining why U.S. soldiers tortured detainees at Abu Ghraib prison in Iraq after the U.S.-led 2003 invasion.

The Stanford experiment followed up Yale social psychologist Stanley Milgram’s work in the 1960s on obedience to authority, in which subjects were willing to administer severe electrical shocks to “learners” in another room. The shocks were imaginary, but the subjects did not know that.

Milgram found that many of the subjects would obey orders to continue administering shocks, even at levels that would have been fatal. If that behavior occurred after just a short period with Milgram, Dr. Zimbardo recalled wondering, how would people act if given more time?

For his prison experiment, conducted over a school break, Dr. Zimbardo recruited 24 male students, screening them to ensure that they had no history of crime or violence and that they were emotionally stable. He randomly assigned half to be guards and half to be prisoners.

For $15 a day, the participants were to spend two weeks in a simulated prison in the basement of the psychology department’s building. Dr. Zimbardo, who assumed the role of superintendent, and a few colleagues observed and recorded.

The guards were instructed not to physically harm the prisoners but to otherwise maintain control as they saw fit. Wearing uniforms and sunglasses, they forced the prisoners to follow strict rules, and punishments for violators included solitary confinement in a converted closet and push-ups. The prisoners, wearing nylon stocking caps, smocks and no underwear, were referred to by numbers, and the guards were “Mr. Correctional Officer.”

After an attempted revolt on the second day, the guards began asserting their authority by turning fire extinguishers on the prisoners and demanding that they insult or embrace one another, strip naked and clean toilet bowls with their bare hands.

Inmates’ loved ones were given a day to visit. Dr. Zimbardo told the Stanford Historical Society Oral History Program that guards warned the prisoners to tell their visitors that “everything is wonderful, hunky dory. If you don’t, we’re going to make things worse for you when they leave.”

“It was a different era,” Zimbardo told the London Independent in 2008. “If I had done the study right now, there is no question that I would be sued by every guard and every prisoner. These studies are in ethical time capsules. They cannot be done in a legitimate way now.”

Several prisoners were removed from the experiment after they showed signs of extreme distress. Christina Maslach, a graduate student who later married Dr. Zimbardo, was sickened by what she saw during a visit and convinced the professor that he needed to end the experiment before conditions degenerated further.

The experiment lasted six days instead of the intended 14 and, over the decades, drew strong criticism.

In his 1973 book, “The Anatomy of Human Destructiveness,” social psychologist Erich Fromm accused Dr. Zimbardo of blurring the line between reality and experiment for the participants and of failing to make clear that prisoners were free to quit at any time.

The fact that not every guard behaved sadistically also undermined Dr. Zimbardo’s premise that bad situations transform people into monsters, Fromm added. “The difference between behavior and character matters very much in this context,” he wrote. “It is one thing to behave according to sadistic rules and another thing to want to be and to enjoy being cruel to people. The failure to make this distinction deprives this experiment of much of its value.”

Dr. Zimbardo defended his work, writing that the experiment was “a cautionary tale” about what can happen when “we underestimate the extent to which the power of social roles and external pressures can influence our actions.”

He saw parallels between the guards’ behavior during his exercise and the conduct of U.S. soldiers who had abused prisoners at the Abu Ghraib prison. He testified as an expert witness on behalf of Staff Sgt. Ivan L. Frederick II, who in 2004 was sentenced to eight years in prison for his role in the scandal. Rather than a few bad apples, Dr. Zimbardo said of Frederick and his fellow guards, they were good apples put in a “bad barrel.”

“The military created a situation and then absolved themselves of any responsibility,” Dr. Zimbardo told the London Independent in 2016. “It’s not the grunts who should be punished. It’s the entire chain of command which has to be called to question.”

Frederick, a former corrections officer, was paroled after three years.

Dr. Zimbardo drew on his prison experiment and the events at Abu Ghraib in his 2007 book, “The Lucifer Effect: Understanding How Good People Turn Evil.”

“Although you probably think of yourself as having a consistent personality across time and space, that is likely not to be true,” he wrote. “You are not the same person working alone as you are in a group; in a romantic setting versus an educational one; when you are with close friends or in an anonymous crowd.”

“Anyone,” Dr. Zimbardo told Fordham University students during a 2013 lecture, “can be evil.”

Philip George Zimbardo was born to a Sicilian American family in the South Bronx on March 23, 1933. His father was often unemployed, Dr. Zimbardo told Psychology Today, and the family frequently moved out of apartments under cover of night because they could not keep up with rent payments.

He recalled being taunted for being poor and for his dark complexion. That disparagement — along with an extended, isolating hospital stay for pneumonia and whooping cough as a young child — sparked his interest in how people formed connections and beliefs and whether those beliefs could be changed.

In 1954, he graduated from Brooklyn College with a triple major in psychology, sociology and anthropology. He received a master’s degree in 1955 and a doctorate in 1959, in psychology from Yale.

After teaching at New York University and Columbia University, he joined Stanford’s faculty in 1968. He retired in 2003 but continued to lecture and appear at workshops. He was a familiar face in high school and college classes as host of the “Discovering Psychology” video series, introducing students to influential psychologists, experiments and concepts. Actor Billy Crudup played Dr. Zimbardo in the 2015 movie “The Stanford Prison Experiment.”

His first marriage, to literature scholar Rose Abdelnour Zimbardo, ended in divorce. In 1972, he married Maslach, who became a psychology professor at the University of California at Berkeley. In addition to Maslach, survivors include a son from his first marriage, Adam; two daughters from his second marriage, Zara and Tanya; and four grandchildren.

Dr. Zimbardo wrote books on shyness, and he created a clinic in the Bay Area to study and help those who were extremely shy. He also helped start the Heroic Imagination Project, a nonprofit that by its own description seeks to prepare individuals to “act with integrity, compassion and moral courage [during] crucial moments in their lives.”

In 2007, Dr. Zimbardo, by then widely known among popular audiences, wrote an article in O, the Oprah Magazine, reflecting on human phenomena he had observed over the course of his career.

Personal values are constantly tested within families, schools, workplaces and large social institutions, he wrote, and people often submit to the group for the sake of harmony and acceptance.

“When everyone else is doing the bidding of unjust authorities or bending to the will of corrupt systems, the few who resist are heroes,” Dr. Zimbardo wrote. “But you don’t have to be a Mahatma Gandhi, Nelson Mandela, or Martin Luther King Jr. You can be a Joe Darby, the army reservist who revealed the Abu Ghraib photos to a criminal investigator and thereby ended the abuse.”