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.

“If you have a society that has a lot of people with high social-dominance orientations, that’s going to be reflected in the policies of that society, especially in a democracy. At the same time, the aggregate features of that society are going to affect people and shape personalities,” said Ryan D. Enos, an associate professor in Harvard’s Department of Government and an authority on political psychology, race, and ethnic politics, who was not involved in the study. “This paper is particularly significant because it tries to close the loop between the personal, the contextual, and the institutional.”

The next steps for the researchers include expanding the study into every U.S. state, and surveying a broader group of countries.

“This paper is really sort of a first attempt,” said Sidanius. “Going forward, we hope to collaborate with researchers in other parts of the country to get a more ambitious attack on the problem.”

Jonas Kunst, a postdoctoral fellow in the Sidanius laboratory, is the paper’s first author. Roland Fischer, an associate professor in the School of Psychology at Victoria University in Wellington, is also an author. The study was funded by young-researcher awards from the Danish and Norwegian research councils.