ABSTRACT
Drug reward is an evolutionary conundrum. It is not surprising that neural circuitry evolved to reward or reinforce behaviors leading to the essentials of survival and reproduction, like food, water, and sex. Why, though, would these same circuits reward and reinforce the consumption of drugs of abuse, which is often harmful? Here we briefly review the history of reward-based learning, which resulted in a widely accepted evolutionary account of drug reward that we term the hijack hypothesis. We then critique the evolutionary bases of the hijack hypothesis. We conclude by sketching an alternative evolutionary model of human drug use grounded in drug toxicity. Specifically, avoidance of toxic drugs is a compelling hypothesis for the low use of drugs by children and women relative to men. In addition, the regulated ingestion of small quantities of toxins might have provided important medicinal and other benefits to humans and non-human animals over the course of their evolution.
