Why Sex Sells and Diets Fail

It's still early in the year, but already many of us find our New Year's resolutions in shambles. A firm plan to work out at the gym three times a week has degraded into a weekly "maybe." A healthful diet is spoiled time and again by the office candy machine. The aspirations for quitting smoking cold turkey are replaced by guilt-laced cigarette breaks.

Why do we say we'll do one thing, but actually do something else? Humans are supposed to be rational beings, but time after time our emotions and their messy ways interfere with our goals.

To understand how this happens, Brian Knutson, Ph.D., at Stanford University, tracks what is going inside the brain when we make up our mind. By scanning the brains of people as they make financial decisions, he has found a brain region that pushes us towards taking a risk or buying something—and another that reins us in.

Knutson's studies have implications beyond the world of finance because they tap into what happens in the brain when we are motivated to do something. As a fairly universal motivator, "money is a convenient way to study that," Knutson says.

The neurobiology of 'going for it'

Knutson and his colleagues have scanned brain activity in people as they make investing or purchasing decisions. To understand how the brain promotes one choice over another, they focused on brain activity that happens immediately before making a decision.

In one study, participants played a game in which they had to decide whether to invest in stocks or bonds. As in real life, the stocks were risky, potentially yielding higher—though uncertain—gains than bonds, which guaranteed a lesser return but are safer. Knutson's study found that before choosing stocks, people had a strong activation in a brain area called the nucleus accumbens (NAcc), which is located within the basal ganglia.

This was a bit of a surprise, since the NAcc is known as the brain's pleasure center, responding to rewarding experiences, such as eating and drinking. Finding that this area–shared by mammals, birds and reptiles alike–was involved in the cognitive exertions of a financial decision shows how the brain uses old parts for new tricks.

Knutson's study also found that the NAcc can herald risky choices, even mistaken ones. When a person chose a stock, but ideally should have chosen a bond, NAcc activity still preceded that choice. Right or wrong, this area seemed involved in urging people to go for it.

Knutson also found an anxious alter ego for the NAcc in a region called the anterior insula. Just before people conservatively chose the safer bonds, this region, which is tucked between the frontal and temporal lobes, became active.

This neural push-pull between pleasure and worry also occurred when people made purchasing decisions. In a different study, Knutson and his team showed people desirable merchandise on a video screen, like a box of chocolates or an MP3 player. A few seconds later, the price of the item appeared. NAcc activation predicted that a person would decide to purchase, whereas insula activation predicted that they would not.

Deciding to buy something like a shirt doesn't initially seem as risky as losing one's shirt while playing the stock market. But in both situations, we anticipate pleasure in gaining something, whether money or possessions—or we anticipate losses so great that we opt for bonds, or decide against opening our wallets. What's striking is that these opposing outlooks involve two distinct brain areas.

Titillation and risky choices

NAcc activity may actually nudge us toward risky choices, says Knutson. In a study published in March of 2008 in Neuroreport, he showed that incidental increases in NAcc activity—in this case, caused by showing erotic images to college-aged men—biased them to make riskier financial investments immediately afterward. The more titillated they were, the riskier their choices.

This may begin to explain why we say one thing, but do another: Our emotions can override our best intentions, and this may take place in the NAcc. Rewarding things like listening to our favorite music, eating sweets or even seeing erotic images can boost our mood and NAcc activity alike, and apparently, also influence our imminent choices. So as we shop in a store and decide whether to buy something, the music, lighting, and other seemingly innocuous decorations may prod the NAcc (and our credit card) into action.

Rewards may also undermine our health goals: A single jelly bean might propel a dieter to binge on ice cream; a nice cold beer might induce a former smoker to light up.

Does this mean we have to live like monks, avoiding all things rewarding? Knutson says probably not, though if a person is trying to quit smoking and tends to take risks, then avoiding smoky bars and other happy reminders of smoking would be a good idea.

And keeping in mind that our NAcc has a knack for being susceptible to outside influences may empower us to get a handle on our urge to splurge and indulge.

More Brain & Body Columns:

• How to Gain Self-Control
• Marijuana's Memory Paradox
• Stimulating Confidence

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After spending 15 years in the lab doing her own neuroscience research, Michele Solis is now putting her Ph.D. to work as a science writer. Her work covers a variety of topics including autism, linguistics, and animal communication. She contributes regularly to the Autism Speaks, Simons Foundation, and Crosscut Web sites.