As a key motivator, pleasure is central to learning; if we did not find food, water and sex rewarding we would not survive and have children. Pleasure evokes neural signals that converge on a small group of interconnected brain areas called the medial forebrain pleasure circuit — tiny clumps of neurons in which the neurotransmitter dopamine plays a crucial role.

This dopamine-using pleasure circuitry, refined over millenniums of evolution, can also be artificially activated by some, but not all, psychoactive substances that carry a risk for addiction, like cocaine, heroin, nicotine or alcohol. Our brain’s pleasure circuits are also hard-wired to be activated by unpredictable rewards: While a roulette wheel is spinning or horses are on the track, we get a pleasure buzz even if we don’t get a payout in the end. Uncertainty itself can be rewarding — clearly a useful attribute for high-risk, high-reward business ventures.

So why do some people become addicted to drugs, alcohol, gambling or sex while others can indulge in a moderate, noncompulsive manner? One hypothesis is that addicts feel those pleasures unusually strongly and are motivated to seek them more intently. It’s reasonable, but wrong. Evidence from animal experiments and human brain scans indicates that the opposite is true: Addicts want their pleasures more but like them less.

We’re now starting to understand the biology behind the blunted pleasure of addicts. From studies comparing identical and fraternal twins, it is estimated that genetic factors account for 40 to 60 percent of the variation in the risk for addiction. But we are only in the early stages of understanding the role of genes in addiction; there is no one “addiction gene,” but it is likely that a large number of genes are involved in this complex trait.

Crucially, genetic variants that suppress dopamine signaling in the pleasure circuit substantially increase pleasure- and novelty-seeking behaviors — their bearers must seek high levels of stimulation to reach the same level of pleasure that others can achieve with more moderate indulgence. Those blunted dopamine receptor variants are associated with substantially increased risk of addiction to a range of substances and behaviors.