Ouch! Did you do that on purpose?

Psychologists have long concluded that people tend to judge others more harshly for their negative actions—assigning fault and blame more often than praise or accolades. Now we know why.

New research shows the human brain is wired to react more emotionally to the bad things people do. That’s why we tend to assign blame much faster than we give compliments.

It turns out that the labels of blame and praise are processed in different parts of the brain, by different mechanisms. While blame is assigned from a very emotional place, praise comes from a more logical spot.

The end result? People are more likely to assume that the good acts of others are simply happenstance, but bad things are done on purpose.

The research, published Friday in Scientific Reports, comes from a mix of brain imaging and behavioral research by a team from Duke University. In the study, more than 660 people took traditional surveys that included reading about different scenarios with negative and positive outcomes (like spreading weed killer and hurting a neighbor’s crops, or spreading anti-fungals and protecting a neighbor’s crops). Twenty others had their brains scanned as they read the scenarios and assessed how intentional the actions had been.

The negative stories were more likely to trigger a reaction stemming from the amygdala, an emotional center deep inside the scanned brains. Positive actions were more likely to set off a more statistical, reasoned approach, without lighting up the amygdala at all.

Study co-author Scott Huettel, a psychology and neuroscience professor at Duke, says “blame and credit – are not two sides of the same coin, but two different processes.”

This is not the way that other systems in the brain work. The brain’s reward system, for example, controls a whole spectrum of positives to negatives—from how much value people assign to a good meal to how much they’ll pay to avoid foods they don’t like.

Huettel says we may be built this way because ‘good actions’ aren’t as important for us to track. It’s the people taking ‘bad actions’ we must look out for, evolutionarily speaking. The researchers suggest the phenomenon may even affect the decisions by judges and juries: The more egregious an act, the more intentional people tend to assume it was.

Aristotle dealt quite the challenge to humanity when he issued his moral philosophy about the ‘perfect man,’ saying he does not “concern himself that others should be blamed.” Aristotle must have missed the brain scans.

For more about research on human behavior, watch this Fortune video: