Like any primate species worthy of its opposable thumbs, we humans like our social hierarchies. Yet too much inequality wrecks our health, rocks our politics, and chafes our social ties, find scholars across the social sciences. These same scholars also hotly debate where inequality comes from, yet arrive at little consensus.

A new study of 21 modern small-scale societies around the world, however, finds a clear pattern in the disparities: “How much inequality there is in a society depends on how inheritable the wealth is, which in turn depends on the kind of wealth that it is,” says economist Samuel Bowles, director of the behavioral sciences program at the Santa Fe Institute and one of the study’s lead authors.

Specifically, parents in crop farming and herding economies tend to value and create material wealth (such as land, cows, and money), which they then pass on to their offspring. Over time, this inherited wealth accumulates in certain households, widening the gaps between the haves and have-nots.

In contrast, parents in subsistence farming and hunting-gathering economies tend to rely on and generate other kinds of capital, including embodied wealth (like height, strength, and skills) and relational might (such as social alliances and connections to powerful people). Because these riches are harder to hand off to children, horticulturalists and hunter-gatherers have more egalitarian societies.

“Once you have private property, the whole game changes because you now have material wealth that you can transmit to your offspring,” says Monique Borgerhoff Mulder, professor of anthropology at the University of California, Davis, and another lead researcher of the 23-author team. “Intergenerational transmission [of wealth] then becomes a pernicious force that perpetuates inequality,” she says.

Yet economy is not destiny: Societies not only make wealth, they also create institutions that decide how to mete and dole that wealth among their people. Denmark, for instance, is a modern capitalist state built on agricultural foundations. Yet inequality in Denmark (as captured by the Gini coefficient) is similar to that of the most egalitarian hunter-gatherers and subsistence farmers. The Danes have chosen a set of institutions that limit inequality and the inheritance thereof, explains Bowles, “including good public education, widespread access to health care, and policies that protect against low wages in some sectors.”

Meanwhile, Bowles says, the United States “has adopted policies that allow parents to pass on most of their material wealth to their children.” As a result, the United States is one of the most unequal capitalist nations in the world. “People do have choices,” he adds.

Monique Borgerhoff Mulder, Samuel Bowles, Tom Hertz, et al., “Intergenerational Wealth Transmission and the Dynamics of Inequality in Small-Scale Societies,” Science, October 30, 2009.

Read more stories by Alana Conner.