This is Hidden Brain. I'm Shankar Vedantam.

When Saru Najarian was about 10, his pastime was collecting baseball and basketball cards.

These were hard to come by in Cyprus, where he grew up.

So when Saru's cousin pestered him to share his cards with her, he always said no.

But she didn't give up.

As she pestered and begged, then pleaded,

it came to a boiling point where I got so angry

that everything blacked out

and I slapped her really hard.

Saru's arm seemed to act of its own volition.

A second later,

I came back into the reality

and I saw her crying and had no idea what I had done.

Paula Reid experienced something similar at the same age.

She was a budding environmentalist with a peace ecology flag hanging on her bedroom wall.

One afternoon, she heard the cracking of trees and a low rumble.

She realized that her neighbor was knocking down trees to build himself a shorter driveway.

He was using a bulldozer.

This neighbor came up the road in the bulldozer and was pushing over trees and something

in my head just snapped.

Paula's dad had bought a machete in his travels.

Without thinking, Paula seized the weapon.

...