SAN DIEGO, CALIFORNIA—Compulsive gamblers aren’t necessarily greedier than the rest of us—their brains may just be wired to favor money over sex. That’s the conclusion of a study presented here today at the Society for Neuroscience conference. This tendency to prioritize money over more basic desires resembles other addictions like alcoholism, researchers say, and could point toward new therapies.

Of the millions of people who gamble for fun or profit, about 1% to 2% qualify as pathological gamblers. They can't quit despite encountering serious negative consequences—going into debt, damaging relationships, and even smashing up slot machines and getting arrested when the habit gets out of control. This inability to stop even after sustained loss is one reason gambling recently became the first behavioral addiction to be recognized by psychiatry's most frequently used diagnostic manual, the DSM-5, says Guillaume Sescousse, a neuroscientist at the Radboud University Nijmegen in the Netherlands who led the new study. After all, he says, professional poker players can play for 10 hours a day and not be considered addicts—so long as they can stop when their luck runs out.

Researchers have long hypothesized that the basis for gambling addiction might be hypersensitivity to the highs of winning money, caused by dysfunctional wiring in neural circuits that process reward. Studies have produced conflicting results, however, so Sescousse decided to investigate an alternative hypothesis. He wondered if instead of being overly sensitive to monetary reward, compulsive gamblers were less sensitive to other rewarding things, like alcohol and sex.

To test this idea, he and his team recruited 18 male pathological gamblers by posting ads that asked, "Do you gamble a lot?" The researchers also recruited 20 healthy controls. After undergoing evaluations to establish how much they gambled, the volunteers were asked to lie inside a functional magnetic resonance imaging (fMRI) scanner that recorded brain activity during a task that required them to push a button as quickly as possible in order to win money or to see sexy pictures of women. The faster participants pushed the button, the more motivated they were thought to be to get the reward. This experimental paradigm is more objective than a questionnaire and has been extensively tested in humans and animal models, Sescousse says.

Prior to the task, the majority of gamblers reported that they valued money and sex equally. Their results, however, showed an unconscious bent toward cash. Their reaction times when trying to win money were about 4% faster than when trying to see erotica, an effect that "might seem small, but is actually highly significant" in this type of research, Sescousse says. As the participants performed the task, the researchers watched their brain responses on the fMRI scanner, which tracks blood flow as a measure of brain activity. They found that gamblers had much diminished responses to erotic images compared with monetary images in the ventral striatum, a brain region that processes reward. The difference in response was much smaller in controls, Sescousse says.

Next, the researchers looked at the participants' brain activity in another key brain region involved in reward, the orbitofrontal cortex. In previous studies of healthy people, they had noticed that different parts of the orbitofrontal cortex respond to erotic and monetary stimuli—a division they think reflects a dissociation between innate rewards such as food and sex, which are key to survival, and secondary rewards such as money and power, which we must learn to value. In compulsive gamblers, the same region that normally only lights up in response to sex was activated when the participants looked at monetary cues, suggesting that they interpreted money as a more primary reward, the researchers say. Cognitive therapies that enhance sensitivity to nonmonetary rewards and change how gamblers think about money—for example to think of it as a tool, rather than as a reward in itself—might help address this distortion, Sescousse says.

The study results are "convincing," says neuroscientist George Koob, an alcoholism expert at the Scripps Research Institute in San Diego, California. It's possible that the gamblers' sensitivity to rewarding activities such as sex may be so blunted that gambling is the only thing that still brings pleasure, he says. "Maybe it's all that's left."