These pathways, which rely on the neurotransmitter dopamine, are often referred to as the reward system, but they are also involved in motivation, attention, decision making, and other complex functions. In their studies, Wang and Volkow found that both drug addicts and obese people are less sensitive to dopamine’s rewarding effects. Being relatively numb to the pleasure and motivation signal may make them more likely to chase after a stronger thrill: more food or a bump of cocaine. Excessive stimulation further desensitizes dopaminergic neurons, and the compulsion snowballs.