to addictive drugs, according to research. “It’s a huge minority – one large enough to create one of the greatest public health issues we have on this planet – but it’s still a minority,” says Markus Heilig at Linköping University in Sweden. To find out why some people get hooked and others don’t, Heilig and his team trained rats to press a lever in return for a reward. They then gave the rats a choice: either press a lever that delivers a few drops of alcohol or one that delivers a sweet solution. About 15 per cent chose alcohol. Rats cannot be labelled as addicted, because it is a complex disorder, perhaps with uniquely human aspects. But they do show behaviours that closely mimic some of the key features of clinical addiction, and the team theorised that the 15 per cent would continue to choose alcohol, even if it led to negative consequences. In the next experiment, every time such a rat pressed the lever delivering alcohol, it received an electric shock. Did it stay hooked to the alcohol despite the painful zap? “The answer, to our delight, was a crystal clear yes,” says Heilig, “and that’s a first.” Having shown addiction-like behaviours in rats, Heilig wanted to know whether there were differences between the brains of the “hooked” rats and the others, and discovered a striking difference in the amygdala, a key part of the brain for dealing with emotions. Rats that behaved like people
