The historian Raul Hilberg described the Holocaust through three roles: perpetrators, victims and bystanders. Later researchers added a fourth, the rescuers. Studying the choices people made in each role is one of the most unsettling and important parts of this subject.

Ordinary perpetrators

It is comforting to imagine that atrocities are committed by monsters. The evidence suggests otherwise. In his 1992 study Ordinary Men, the historian Christopher Browning examined a German reserve police battalion that took part in mass shootings of Jews in Poland. Its members were mostly middle-aged men from ordinary jobs, few of them committed Nazis, and a minority refused to take part. Before their first killing operation their commander offered to excuse anyone who did not feel up to it; only a few stepped out. Most went on to participate, while Browning estimated that roughly 10 to 20 percent refused or found ways to avoid killing, without facing serious punishment.

Researchers point to many motives working together: conformity and not wanting to abandon comrades, deference to authority, careerism, fear of seeming weak, ideology and prejudice, dehumanizing propaganda, and the way each small step makes the next one easier. The political thinker Hannah Arendt, reporting on the 1961 trial of the Nazi official Adolf Eichmann, coined the phrase the banality of evil. Her point, in paraphrase, was that great evil can be done by thoughtless, careerist people, not only by fanatics. Later historians have argued that Eichmann himself was more ideologically committed than she believed, but her broader warning remains influential.

What psychology experiments suggest

In the early 1960s at Yale University, the psychologist Stanley Milgram asked volunteers to give what they believed were increasingly strong electric shocks to another person (actually an actor who received no shocks) whenever an experimenter told them to. In one well-known version, about two-thirds of participants continued to the highest shock level. The study is often cited to show how readily people obey authority, but it needs careful handling:

• Obedience fell sharply in some variations, for example when the experimenter gave orders by telephone or when other participants refused.
• Many participants protested, showed great distress, or doubted that the shocks were real, and later reviews of the archives found that experimenters sometimes departed from the script.
• A laboratory is not a war, and the study itself raised serious ethical concerns.

Some researchers now read the results less as blind obedience and more as people continuing because they identified with the goals of the person in charge. Earlier work by Solomon Asch showed that people sometimes go along with a group's plainly wrong judgment. Both lines of research suggest that social pressure is powerful, but not irresistible.

Bystanders

Bystanders range from neighbors to institutions to foreign governments. Their silence can tell perpetrators that no one will stop them, and some bystanders profit, taking property or jobs left behind. Yet bystanders also have the most room to change course, because they are not yet committed.

Rescuers

Some people chose to help at great risk. In Le Chambon-sur-Lignon, a French village on a remote plateau, Protestant residents led by pastors André Trocmé and Édouard Theis sheltered thousands of Jews, many of them children, and helped some escape to Switzerland. Israel's Holocaust remembrance center, Yad Vashem, honors such people with the title Righteous Among the Nations, given to non-Jews who risked their lives to save Jews; more than 28,000 have been recognized. Researchers who interviewed rescuers found that many had been raised to care for others and to see outsiders as fellow human beings, and many insisted they had simply done what any decent person should.

Worked example: the rescue of Denmark's Jews

Rescue was usually the work of individuals or small groups. In Denmark in 1943, it became something close to a national act.

Germany occupied Denmark in 1940 but at first left much of the Danish government in place, and Danish Jews were not forced to wear badges or give up their property. In late September 1943, a German diplomat in Copenhagen, Georg Ferdinand Duckwitz, learned that a roundup of the country's roughly 7,000 Jews was planned for the start of October and passed the warning to Danish politicians. The news spread through Jewish community leaders, churches, neighbors and colleagues.

Within days, Danes from all walks of life hid Jewish families in homes, hospitals and churches, then helped them reach the coast. Fishermen ferried them across the narrow strait to neutral Sweden, which had announced it would take them in. More than 7,000 people escaped. About 470 were caught and deported, and most of them survived, partly because Danish officials kept asking about them.

Several lessons stand out. One person's warning mattered, but so did thousands of small acts by people who could have looked away. Geography helped: Sweden was close and willing. The occupiers' own reluctance to provoke Danish resistance played a part. And the rescuers did not see Jews as outsiders but as fellow citizens and neighbors. At the request of the Danish resistance, Yad Vashem chose to honor it as a whole rather than single out individuals.