Researchers at Wake Forest University in Winston-Salem, N.C., housed 20 male monkeys by themselves for 18 months. The monkeys' brain chemistry, hormonal activity and behavior were monitored.

Next, researchers moved the monkeys into groups of four. Within three months, dominant monkeys emerged in the five groups, and a hierarchy was established.

When the researchers introduced cocaine to the monkeys and allowed them to self-administer the drug, the scientists found that monkeys who were subordinate were more likely to give themselves doses of cocaine than dominant monkeys.

Researchers said that social rank is reflected in the brain chemical dopamine, which is closely linked to drug use. As a result, dominant monkeys showed an increase in the dopamine receptor found to be involved in brain pathways for reward processing. These monkeys were less vulnerable to cocaine addiction than the subordinate monkeys.

Michael Nader, who led the study, said the findings could prove beneficial in determining why some people who use cocaine end up as addicts while others do not. "Environmental enrichment can produce rapid changes in the brain that, in this particular case, protected the individual from drug abuse. And that is the applicability to people," said Nader.

The study is published in the journal Nature Neuroscience.

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