This is Scientific Americans 60 Second Science.

I'm Karen Hopkins.

Getting a vaccine can be a painful experience, especially when you're a kid.

But getting told the shot might hurt a bit could actually make it worse.

We know that expectation affects pain experience in adults, but we don't really know whether this is

also true for children. Kalina Mejowska, a developmental neuroscientist at the University of California, Riverside.

She led a study to find out.

The study included 25 adults and 48 children, and 27 of the kids had a pre-existing anxiety disorder.

Because medical procedures make pretty much all kids anxious,

but those who are anxious to start with

tend to find the experience even more painful.

The researchers used a hand-held wand to apply heat to the forearm of each participant.

And they asked subjects to rate the temperature in terms of discomfort.

The hottest setting was about the temperature of very warm tap water.

Uncomfortable perhaps, but not damaging.

But during the experiment, we were most interested in only one temperature, the one that each subject rated as medium.

And that's where the anticipation part of the experiment comes in.

Subjects were played one of two tones.

One tone meant that low heat was coming,

the other meant that high heat was upcoming.

But here's the sneaky part. No matter what tone was played,

participants got the same heat applied, the one rated as medium.

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