But--is it all genetically determined? This study in non-human primates shows how social factors can influence the level of dopamine receptors and the propensity to self administer drugs. Illustrated above are PET images of the brains of monkeys that were initially individually housed (left) and then group housed (right). Those emerging at the top of their social rank – experiencing less stress and more access to natural rewards – expressed more dopamine receptors and took less cocaine than did submissive monkeys.