Anyone who's ever taken a preschooler to the doctor knows they often cry more before the shot than afterward. Now researchers using brain scans to unravel the biology of dread have an explanation: For some people, anticipating pain is truly as bad as experiencing it.

How bad? Among people who volunteered to receive electric shocks, almost a third opted for a stronger zap if they could just get it over with, instead of having to wait.

More importantly, the research found that how much attention the brain pays to expected pain determines whether someone is an "extreme dreader" — suggesting that simple diversions could alleviate the misery.

The research, published Friday in the journal Science, is part of a burgeoning new field called neuroeconomics that uses brain imaging to try to understand how people make choices. Until now, most of that work has focused on reward, the things people will do for positive outcomes.

"We were interested in the dark side of the equation," explained Dr. Gregory Berns of Emory University, who led the new study.

"Dread often makes us make bad decisions."

Standard economic theory says that people should postpone bad outcomes for as long as possible, because something might happen in the interim to change improve the outlook.

In real life, the "just get it over with" reaction is more likely, said Berns, a professor of psychiatry and behavioral sciences. He offers a personal example: He usually pays credit card bills as soon as they arrive instead of waiting until they're due, even though "it doesn't make any sense economically."

So Berns designed a study to trace dread inside the brain. He put 32 volunteers into an MRI machine while giving them a series of 96 electric shocks to the foot. The shocks varied in intensity, from barely detectable to the pain of a needle jab.

Participants were told one was coming, how strong it would be, and how long the wait for it would be, from 1 to 27 seconds.

Later, participants were given choices: Would they prefer a medium jolt in 5 seconds or 27 seconds? What about a mild jolt in 20 seconds vs. a sharp one in 3 seconds?

When the voltage was identical, the volunteers almost always chose the shortest wait. But those Berns dubbed "extreme dreaders" picked the worst shock if it meant not having to wait as long.

The MRI scans showed that a brain network that governs how much pain people feel became active even before they were shocked, particularly the parts of this "pain matrix" that are linked to attention — but not brain regions involving fear and anxiety. The more dread bothered someone, the more attention the pain-sensing parts of the brain were paying to the wait.

In other words, the mere information that you're about to feel pain "seems to be a source of misery," George Lowenstein, a specialist in economics and psychology at Carnegie Mellon University, wrote in an accompanying review of the work.

"These findings support the idea that the decision to delay or expedite an outcome depends critically on how a person feels while waiting," Lowenstein added.

The National Institute on Drug Abuse funded the research. What's the link between dread and drug use? It's indirect, but now that scientists know how healthy people's brains anticipate unpleasant consequences, future studies can compare how drug abusers process such information.