It’s a question that social scientists have struggled with for years: Why do some groups enjoy privileged status in a society while others are left behind?

One possible explanation, scientists say, may lie in what’s known as Social Dominance Theory, the idea that human societies are organized in group-based social hierarchies in which some enjoy greater access to resources and opportunities than others.

To study that, social scientists from Harvard University, the University of Oslo, Aarhus University in Denmark, and Victoria University in Wellington, New Zealand, polled people from 30 U.S. states and 27 countries, and found evidence linking social-dominance orientation to individual, national, and international data indicators for inequality and political realities. The study is described in a paper in the Proceedings from the National Academy of Sciences.

“We looked to see whether or not there’s a relationship between crime rates, incarceration rates, levels of income inequality, murder rates, and an individual’s taste or preference for group-based inequality,” said Jim Sidanius, the John Lindsley Professor of Psychology and professor of African and African-American studies at Harvard, and an author of the paper.

“What we see is a self-fulfilling process where greater societal inequality motivates the group at the top to use even violent means to maintain such inequality” said Lotte Thomsen, a former Harvard postdoctoral fellow in the Sidanius lab and current associate professor in psychology at the University of Oslo, the paper’s senior author. “This, in turn, may lead to even more inequality and even extremist violence. This results in a vicious circle.”

The study is among the first to examine social-dominance orientation across individual psychological, national, and international lines and attempt to make sense of how these different levels contribute to the continuation of group-based inequality.

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