Another alternative hypothesis is that the selective pressure at this locus was on social behavior. Cocaine addiction is characterized by a dampened reward response to social interaction, meaning that it inhibits the positive emotions that accompany social interaction or feelings of belonging. A recent study demonstrated that cocaine users process social gaze (joint attention on an object) differently than controls, resulting in a reduced activation of the reward system during social interactions [31]. Using fMRI, these authors showed that cocaine users had decreased activation of the medial orbitofrontal cortex, a region of the brain central for reward processing. If alleles that alter cocaine dependence risk alter an individuals’ natural reward system during social interactions, these observations could explain why alleles that protect against cocaine dependence could have provided advantage to carriers in the ancestral environment. Since nicotine sensitizes the animal to the effects of cocaine, which blunts the reward of social interactions, alleles that reduced the ability of nicotine to enhance the effects of cocaine would have undergone positive selection. In this scenario, the nicotine and cocaine dependence phenotypes are not hitchhiking with memory or learning, but rather with phenotypes protecting against antisocial and therefore maladaptive behavior.
