(PhysOrg.com) -- Arguably the worst feature of societies with class structures  the disproportionate suffering of the poor  may have been the driving force behind the spread of those stratified societies across the globe at the expense of more egalitarian societies. During hard times, a society in which the bulk of the suffering is borne by the poor can survive and expand into new territory more readily than can egalitarian societies.

Why do most cultures have a class structure  rich, poor and sometimes middle  instead of being egalitarian, with resources shared equally by everyone?

According to Stanford University researchers, it is the very inequities of the class structure that appear to have been behind the spread of those societies and the displacement of more egalitarian cultures during the early era of human civilization.

The researchers used a computer simulation to compare demographic stability and rates of migration for both egalitarian and unequal societies. They found that class structure provided unequal access to resources, thereby contributing a destabilizing effect on the population, and driving migration and the expansion of stratified societies.

"This is the first study to demonstrate a specific mechanism by which stratified societies may have taken over most of the world," said Marcus Feldman, an evolutionary biologist at Stanford. He is a co-author of a research paper on the topic, published online this week by the Public Library of Science in the journal PLoS ONE.

Feldman and his colleagues determined that when resources were consistently scarce, egalitarian societies  which shared the deprivation equally throughout the population  remained more stable than stratified societies. In stratified societies, the destabilizing effect of unequal sharing of scarce resources gave those societies more incentive to migrate in search of added resources.

In environments where the availability of resources fluctuated from year to year, stratified societies were better able to survive the temporary shortages because the bulk of the deprivation was absorbed by the lower classes, leaving the ruling class  and the overall social structure  intact. That stability enabled them to expand more readily than egalitarian societies, which weren't able to adapt to changing conditions as quickly.

Many possible causes for the development of socioeconomic inequality have been proposed by scientists, such as a need for hierarchical control over crop irrigation systems, or the compounding of small differences in individual wealth over time through inheritance.

"The fact that unequal societies today vastly outnumber egalitarian societies may not be due to the replacement of the ethic of equality by a more selfish ethic, as originally thought by many researchers," said cultural evolution specialist Deborah Rogers, lead author of the study. "Instead, it appears that the stratified societies simply spread and took over, crowding out the egalitarian populations." The study is a product of her PhD thesis project at Stanford. Feldman was Rogers' adviser.

"This is not just an academic exercise," Rogers said. "Inequalities in socioeconomic status are increasing sharply around the world. Understanding the causes and consequences of inequality and how to reduce it is one of the central challenges of our time."

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