Thinking is, well, hard. Why else do we put off studying for tests, procrastinate doing work, or offload splitting the bill at the restaurant onto our friend? Because mental grunt work requires real, concentrated effort.

But how does your brain make the decision whether to put in that cognitive work in, or just slack off instead? A new study might have the answer.

The choice of whether to partake in some mental heavy-lifting comes down to the neurotransmitter dopamine, this new research suggests.

When you weigh up the benefits versus the costs of putting in the cognitive effort, dopamine ups your sensitivity to the benefits of doing it — and distracts you from the drawbacks.

“People have understood there’s a link between dopamine and willingness to expend physical effort for a long time. And we thought, well, could it be that dopamine also underlies your willingness to expend cognitive effort?” Andrew Westbrook, a neuroscientist at Brown University and author of the study, tells Inverse.

The findings were published Thursday in the journal Science.

Cost-benefit analysis

In the study, Westbrook and his colleagues asked 50 young adults to complete a performance task commonly used to assess working memory, called the N-back task.

Westbrook says they chose that specific exercise because it feels awful to do: “I really hate doing it myself.”

Then the participants were asked: Would they be willing to repeat the difficult task for more money, or an easier version of it for less money? While they made a decision, the researchers tracked their eye gaze. Those who gazed at the potential benefits were more likely to choose to repeat the tricky task than those who focused on the costs.

Westbrook explains using the example of deciding whether you should study for a hard test for class. “You know that doing so is going to get you a higher grade, so there's certain benefits you can pay attention to. But on the other hand, you could think about how much you dislike studying and how much you'd rather spend time on Reddit or Twitter or whatever.”

And so, he says, the question is: “If we play around with your dopamine levels, or if you have higher dopamine function, just at baseline, is it that you spend more of your attention space on thinking about the benefits versus the cost? Or is it that when you think about the benefits, that just has more of an influence over your choice?”

To try and answer these questions, the researchers then looked at levels of dopamine signaling in the participants' brains, finding that those who were more willing to expend effort for a reward had higher levels of dopamine signaling in their striatum, a brain region found deep in the midbrain.

“When you're thinking about the good grades you might get, that does make you more motivated, as obviously it would, but that effect is amped up when you have higher dopamine levels,” Westbrook says.

Smart drugs?

The researchers then zoomed in on the participants with the lowest levels of dopamine signaling. Their experience may reflect that of certain brain conditions, like attention deficit hyperactivity disorder, or ADHD. People with ADHD struggle at making the cost-benefit choice, and are thought to be in possession of fewer dopamine receptors.

Methylphenidate, more commonly known as Ritalin, is a stimulant medication often used to treat ADHD. The drug works by boosting levels of the neurotransmitters dopamine and norepinephrine in the brain, helping people with ADHD to focus. Up until now, how exactly these drugs work as cognitive enhancers was a mystery. But this new study may finally have cracked it.

When subjects with lower levels of dopamine were given the stimulant, the researchers found their decisions largely matched those with average or higher levels.

Westbrook believes this is because, by boosting dopamine levels, their attention shifts toward the task’s outcome, making them less focused on the slog of actually doing it.

Essentially, the drugs may enhance your willingness to exert cognitive effort — rather than your ability.

Ritalin and stimulants like it are often used (and abused) by people who do not have ADHD — most commonly, college students, who might pop a pill in order to pull all-night study marathons in the library. This has earned them a reputation as “smart drugs.” But the study reveals that, in people who do not have naturally low baseline dopamine levels, the stimulant medication did not improve their cognitive efforts.

If you have high dopamine functioning anyway, the drug “doesn't really have much of an effect,” Westbrook explains.

“And there's even a hint that it might worsen the situation.”

He hopes these findings serve as a “cautionary tale” for those who use these types of drugs without a prescription.

The major takeaway of the study is that “thinking hard about things is really about cost-benefit decision making, and dopamine seems to make you more reward-motivated, more driven to the benefits of thinking, and less sensitive to the costly effort of thinking," Westbrook says.

Abstract: Stimulants such as methylphenidate are increasingly used for cognitive enhancement but precise mechanisms are unknown. We found that methylphenidate boosts willingness to expend cognitive effort by altering the benefit-to-cost ratio of cognitive work. Willingness to expend effort was greater for participants with higher striatal dopamine synthesis capacity, whereas methylphenidate and sulpiride, a selective D2 receptor antagonist, increased cognitive motivation more for participants with lower synthesis capacity. A sequential sampling model informed by momentary gaze revealed that decisions to expend effort are related to amplification of benefit-versus-cost information attended early in the decision process, whereas the effect of benefits is strengthened with higher synthesis capacity and by methylphenidate. These findings demonstrate that methylphenidate boosts the perceived benefits versus costs of cognitive effort by modulating striatal dopamine signaling.