Nir’s Note: This guest post is by Dr. Marc Lewis, who studies the psychology and neuroscience of addiction. After years of active research, Marc now talks, writes, and blogs about the science and experience of addiction and how people outgrow it. Visit his website here. 

You’ve just obliterated the last seven or eight zombies. It was a narrow escape and you’re flushed with satisfaction. But you didn’t see that horrendous creep, weaping sores and oozing pus, because he was hidden behind the dustbin in the shadow of a bombed out building. You get slimed, you’re dead. Or worse than dead. So you touch the “play again” bar at the bottom of the screen. Now you start further ahead than last time. You know you’re going to meet the slime-master again. Soon. Be prepared.</