The mass murderers of the Nazi regime were ideologically motivated Germans who were inured to their crimes by a spiral of political violence, according to the first systematic analysis of the chief perpetrators of the genocide.
The research goes beyond previous studies that conclude that "ordinary people" motivated by anti-Semitism and the Nazi machine carried out the killings.
Michael Mann, professor of sociology at the University of California, Los Angeles, surveyed the biographies of 1,581 men and women known to have been involved in Nazi genocide to find common traits.
His findings, published in the journal Holocaust and Genocide Studies , show a high proportion of the killers were western Germans from regions lost to, or occupied by neighbouring countries after the first world war, and a relatively large number of Catholics and people whose professions were not destroyed by class struggle.
However, Professor Mann shows that those responsible for the genocide of 20 million people were not from one particular group but from many, joined by an ideological glue.
Ethnic German regions outside the country, except for the Sudetenland of Czechoslovakia, were disproportionately highly represented, with westerners from Alsace-Lorraine and areas lost to Denmark and Belgium particularly prominent. A large proportion fled to Germany as political refugees before their homes were "liberated", living initially in refugee camps where extremism flourished.
Within Germany, the perpetrators tended to come from regions adjacent to lost, occupied or demilitarised territories. Most came from areas where there were very few Jews.
Professor Mann believes this suggests "the origins of mass murder lay in part in a rather generalised embittered revisionism, whatever specific bite local anti-Semitic settlements may have added".
Catholics -probably lapsed -tended to be better represented than Protestants, reversing the pattern found among the mainly Protestant pre-coup Nazis.
By occupation, professionals such as teachers, doctors, lawyers and the police were found in high numbers while those from industry and agriculture were far less common.
Two-thirds of the group might loosely have qualified as "real Nazis" before the war, with just over one-third adding a track record of violence or fanaticism.
Nevertheless, Professor Mann observed: "This is not just a bunch of hardline Nazis, ready to kill from the beginning. The vast majority never dreamt they'd be killing in large numbers but were inured through escalating violence and then legitimated by war."
Professor Mann, who is writing a comparative study of ethnic cleansing, believes this core were able to draw in larger numbers of more ordinary Germans into the killing through hierarchy and comradeship.