Mind & Body

Your Ability to Feel Pleasure May Be Partially Due to Genetics

Whether it's to explain their love of blackjack or just their coffee habit, we all know someone who blames their vices on their "addictive personality." Despite the fact that there's no scientific evidence for such a thing, it sure does seem like some people get a bigger kick than most out of gambling, alcohol, food, and other pleasures. A new study may point to one reason why: genetic variation. Differences in people's ability to feel pleasure may be at least partly controlled by genes.

Comfortably Numb

The study, which was published last month in the journal Psychological Science, wasn't just out to explain why some people get a bigger thrill from cocktails and casinos than others. In fact, it was the opposite: People with neuropsychiatric disorders like schizophrenia, depression, and bipolar disorder often experience anhedonia, or the inability to feel pleasure, and the scientists wanted to understand where that comes from.

This all seems to come down to the brain's reward system, which is located in the nucleus accumbens, a pair of raisin-shaped structures in the middle of the brain. Dopamine activity in this region is responsible what scientists call "salience attribution of motivation" — when you have that familiar urge to go after a reward, whether it's a personal record at a 5K or the donuts in the break room, it's what tells you that reward is valuable.

Brain scan studies have found some link between genes that control dopamine and the activation of the reward system, but it's generally understood that no one gene is responsible for any given psychiatric condition. If you really want to find the genetic basis for particular brain activity, you're going to have to take a wider view.

fMRI for the Whole Family

When scientists want to figure out how much of a trait is genetic and how much is environmental — in other words, how much is nature and how much is nurture — they often turn to twins. Pairs of twins usually share the same environment, but the genes they share varies. While identical twins come from the same egg and therefore have nearly identical genes, fraternal twins come from separate eggs and share only about 50 percent of their genes. If a trait happens more often in both identical twins than in both fraternal twins, that suggests that the trait has a genetic basis.

To that end, the researchers recruited 43 pairs of same-sex identical twins and 44 pairs of same-sex fraternal twins, striking a roughly even mix of males and females. The participants sat in an fMRI machine that monitored their real-time brain activity while they performed the monetary incentive delay task, a sort of computer game that's standard in reward-system research. In the task, participants are shown different shapes that indicate different amounts of money they could win or lose — for example, a triangle might mean that hitting a target will win them five points, while a square might mean that missing it will lose them five points. Each trial starts with a random shape flashed on the screen in the blink of an eye, followed by a few seconds of delay. Then a target appears, which the participant has less than half a second to click on in order to get their reward (or avoid their punishment). Finally, the participant sees how much they won or lost.

While they waited for their reward or punishment, the fMRI monitored the activity of their nucleus accumbens, and that told researchers a whole lot. On trials where the participants expected a reward, activation of the nucleus accumbens was moderately heritable — in other words, the difference in brain activation between the types of twin pairs appeared to have some genetic basis. The participants also filled out questionnaires about their levels of physical and social anhedonia and their experience of pleasure, and these answers told the researchers something too: nucleus accumbens activation seemed to share genes with physical anhedonia, and physical anhedonia appeared to share genes with the experience of pleasure. The researchers didn't find any genetic basis for nucleus accumbens activation on trials where the participants expected to lose points, though.

Next, the researchers want to figure out exactly how genetics shape the brain's reward center. Finding the places in human DNA that are linked to reward responses could one day help scientists understand exactly what causes that lack of pleasure that people experience with so many psychiatric conditions. Better understanding will hopefully lead to better treatments.

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Learn more about the incredible power of dopamine in "The Molecule of More: How a Single Chemical in Your Brain Drives Love, Sex, and Creativity―and Will Determine the Fate of the Human Race" by Daniel Z. Lieberman and Michael E. Long. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Ashley Hamer August 30, 2019

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