Our lives are full of instances where have to hold ourselves back. We stop ourselves from eating that tempting slice of cake to avoid putting on weight. We bite our tongues to avoid insulting our friends. We slam on the brakes to avoid killing a pedestrian.  To quote Yoda: “Control! Control! You must learn control.”

People with drug problems clearly have a problem with this. Their ability to resist their own impulses falters at the promise of the next hit. Now, scientists are starting to understand the changes in the brain that underlie these problems.

This is far from the first study to examine the brains of drug users. But it’s never been clear whether changes in such brains were caused by