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Everyone makes snap decisions from time to time, but now researchers think they may know why some people are more impulsive than others. NPR's Geoff Brumfiel reports that it may come down to a kind of chemical thermostat in your brain.

GEOFF BRUMFIEL: It's late at night. You're watching TV.

(Soundbite of infomercial)

Unidentified Man: Would you love healthy, delicious snacks that are good for you? Do you love beef jerky, but hate the preservatives and, most of all, the price?

BRUMFIEL: You don't need a food dehydrator, but there's a part of you that wants it, anyway. You look at your phone.

(Soundbite of infomercial)

Unidentified Man: Call now.

BRUMFIEL: What happens next may come down to how impulsive you are. Josh Buckholtz is a researcher at Vanderbilt University. He studies why some people will buy a food dehydrator without hesitation.

Mr. JOSHUA BUCKHOLTZ (Vanderbilt University): These people with high levels of impulsive traits tend to leap before they look.

BRUMFIEL: Impulsiveness is about much more than shopping. The same people are vulnerable to substance abuse and some forms of mental illness.

Researchers like Buckholtz think that the brains of impulsive people have too much dopamine. Dopamine is a chemical that our brain releases when it wants something. In Buckholtz's case, that's a drink of water.

Mr. BUCKHOLTZ: Ah. Yeah, that's refreshing. OK.

BRUMFIEL: So do you think dopamine was involved in your decision to drink?

Mr. BUCKHOLTZ: Certainly. Certainly. Well, you know, I had a thirst. I was driven to obtain the reward of that water. Dopamine was all over that behavior.

BRUMFIEL: Dopamine is produced by neurons in the midbrain. That's the lower-middle bit of the brain. It's then piped out to other regions, where it creates the drive to get the things you want. Normally, sensors in the midbrain, called autoreceptors, keep dopamine at the right level.

Mr. BUCKHOLTZ: And you can think of it as very similar to how a thermostat works. In your house, you have a furnace that produces heat and sends it through pipes all over your house, and a thermostat that sends a signal to your furnace to stop producing heat, or to produce more heat if it's too cold.

BRUMFIEL: The Vanderbilt researchers suspected that the dopamine thermostats of highly impulsive people are broken. To find out, they took 32 healthy volunteers - some impulsive, and some not. They scanned their heads and found that, on average, impulsive people had fewer thermostats.

To test the idea still further, the team gave volunteers a drug that releases dopamine, and scanned their brains again. Sure enough, impulsive people released more dopamine.

Mr. BUCKHOLTZ: The people who scored highest on our trait measure of impulsivity had upwards of four times the amount of dopamine released, compared to the people with the lowest levels of impulsivity.

BRUMFIEL: The work is published this week in the journal´┐ŻScience. But some researchers believe that there's more to impulsiveness than the dopamine thermostat. Ahmad Hariri is at Duke University. He thinks that other brain chemicals, with their own thermostats, also play a role.

Professor AHMAD HARIRI (Duke University): It's becoming clear that the ability of these chemicals to regulate themselves through these thermostat mechanisms is absolutely crucial for how they regulate the brain and behavior.

BRUMFIEL: Eventually, researchers hope that understanding how the brain's thermostats work could help them to develop better drugs and therapies for treating destructive behavior. Until then, try counting to 10 before deciding to...

(Soundbite of infomercial)

Unidentified Man: Call now.

BRUMFIEL: Geoff Brumfiel, NPR News.

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