The number of receptors don’t just predict drug usage; they are also affected by it. In that same study, Dr. Volkow discovered that people addicted to cocaine, heroin, alcohol and methamphetamines experience a significant reduction in their D2 receptor levels that persists long after drug use has stopped. These people are far less sensitive to rewards, are less motivated and may find the world dull, once again making them prone to seek a chemical means to enhance their everyday life.

Drug exposure also contributes to a loss of self-control. Dr. Volkow found that low D2 was linked with lower activity in the prefrontal cortex, which would impair one’s ability to think critically and exercise restraint.

The same neuroscience helps us understand compulsive overeating. Food, like drugs, stimulates the brain’s reward circuit. Chronic exposure to high-fat and sugary foods is similarly linked with lower D2 levels, and people with lower D2 levels are also more likely to crave such foods. It’s a vicious cycle in which more exposure begets more craving.

Dr. Volkow and colleagues showed that morbidly obese individuals had reductions in their D2 receptors and that the reduction was proportional to their body mass index. The implication of this blunted reward circuit is that they find normal food consumption insufficiently rewarding. At the same time, when exposed to pictures or smells that predict a food reward, they experience more intense cravings than non-obese people. And just like drug addicts, obese people with fewer D2 receptors also show decreased activity in their prefrontal cortex, making it harder to exert self-control.

At this point you may be wondering: What controls the reward circuit in the first place? Some of it is genetic. We know that certain gene variations elevate the risk of addiction to various drugs. But studies of monkeys suggest that our environment can trump genetics and rewire the brain. The good news is that while we can’t change our genetics, we can change our environment.

Michael Nader at the Wake Forest School of Medicine showed this in a study of monkeys and cocaine. When monkeys are moved from an individual cage and housed in a group, some become dominant and others assume a submissive role. For those that become dominant — meaning they get more attention, more grooming and more access to food and treats — this is a positive change. They now have more D2 dopamine receptors and are less interested in self-administering cocaine. But for submissive animals, the group setting is a stressful change, and they respond by increasing their use of cocaine.