New research underscores the value of studying individual differences in performance in response to widely used drugs.

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Want to help your underperforming workers take on more challenging tasks? Amphetamine will do the trick, but it will also cause your high achievers to slack off — at least if your employees are rats.

A new study examined whether rats chose to take on harder mental work for double the amount of sugary snacks, or stick with undemanding tasks for smaller rewards. The rats were trained to poke their noses into specific holes in response to light flashes. They could push levers to determine how long the flash would last — a full second, making it easy to spot, or just a fifth of a second, which required more concentration during the task.

When rats chose the more mentally demanding task, they received twice as many sugar pellets as they did for the easier version. In general, the rats tend to pick the harder challenge, but there ere measurable individual differences, just like in people: some “slacker” rats tended to go for the easy job more frequently, while “workers” were more highly motivated by the larger reward.

Then, the researchers introduced amphetamine, alcohol and caffeine into the equation. Under the influence of the stimulants, the rats’ preferences changed. Without the drugs, workers chose the hard task about 85% of the time, while slackers made the bigger effort just 55% of the time. On low doses of amphetamine, the difference between the groups began to narrow, with slackers working harder and workers slacking off. On the highest dose of the drug, the groups’ choices became similar, with all rats selecting the hard work about 70% of the time. Overall, amphetamine didn’t affect accuracy.

High doses of caffeine, in contrast, reduced effort in workers, but had no effect on the slackers. Alcohol didn’t change the rats’ responses at all — not even affecting accuracy, oddly enough.

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Why do we care how rats behave under the influence? Because many college students and executives attempt cognitive enhancement by using similar drugs non-medically, and the U.S. military actually prescribes amphetamines to pilots on some critical missions to combat the effects of sleep-deprivation. So understanding the influence of such substances on individual performance matters.

And indeed, research in humans suggests similarly varying effects. For example, on some tests of memory, taking amphetamine enhances performance among those who usually score average or low, but slightly diminishes scores for people who ordinarily excel. Further, taking stimulants (like Ritalin or Adderall) clearly improves impulse control among people with ADHD, yet some studies find the drugs have the opposite effect for some people on certain tasks. The current rat study found that both slacker and worker rats responded more impulsively on amphetamine.

Some researchers theorize that the differences may have to do with the sweet spot of the neurotransmitter dopamine in the brain, which is affected by these drugs. Not enough dopamine may hinder performance, but too much can also be a problem. The authors write:

[M]ost of the crucial decisions that determine success in an industrialized society involve varying degrees of cognitive, rather than physical, effort. For example, when deciding whether to perform the bare minimum at the office or to exceed expectations in the hope of promotion, one of the key costs we are evaluating is the cognitive effort required for the potentially more lucrative task.

For high-reward tasks, however, people who are already highly motivated may not need any extra stimulus. Taking an added stimulant — including that extra cup of coffee — may just result in cognitive disruption.

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The research was published in Neuropsychopharmacology (everyone’s favorite journal name to spell).

Maia Szalavitz is a health writer for TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland‘s Facebook page and on Twitter at @TIMEHealthland.