"There's more similarities than differences [between drug addiction and gambling addiction]," says Dr Andrew Jones, a psychologist with Liverpool University's addiction research group. "There are similar underlying behaviourial traits to both. Addiction is mainly an imbalance between the reward system and our control system."
Our brains reward us with a feeling of pleasure when we eat, or have sex, and other things that, in our evolutionary history, helped us survive and reproduce. "We have a reward system for good reasons," says Munafo. "It evolved to force us to pay attention to where we found food, to learn from our experiences.
"That's obviously a necessary system," he says. "Where it goes wrong is where that is hijacked by non-natural systems. The classic example is cocaine." Cocaine stimulates the reward system directly.
The other factor is our impulse control system – that is, our ability to stop ourselves from doing things that feel good right now, to defer reward. The classic example is if someone is asked whether they want £5 now or £50 in six months' time. "People do vary in reward deferral," says Munafo, "and it seems to be associated with addictive behaviour."