The human brain may be "wired up" for addictive behaviour according to a study that shows how some people are more likely than others to become addicted to crack cocaine.
Scientists have found specific abnormalities in the brains of regular cocaine users which are likely to have been present in early childhood rather than coming about as a result of the drug misuse.
The researchers also found similar abnormalities in the brothers and sisters of cocaine addicts - even though the siblings were not themselves drug users - but did not find the same brain patterns in the general population.
The discovery of specific brain abnormalities in the families of drug addicts suggests a genetic basis for addictive behaviour. But it also implies that some people can overcome this predispositiion to remain free of drugs, said Karen Ersche of the University of Cambridge.
"Cocaine is a highly addictive drug but only some people get hooked on it.
However, your chances of getting hooked rise about eight times if you have a family member who is addicted," Dr Ersche said.
"Our findings suggest that drug addiction is not a failure of character or a lifestyle choice. It's a problem with the brain. If your brain is wired for addiction it's easier for the drugs to take over, but the good thing is that this is not inevitable," she said.
The study, funded by the Medical Research Council and published in the journal Science, used hospital scanners to analyse the brains of 50 cocaine addicts and compared them against the brain scans of their non-addicted siblings. As an overall control, the researchers also scanned the brains of 50 unrelated, healthy volunteers.
The scans showed that both the addicts and their siblings shared defects in the nerve fibres that communicate with the front part of the brain, the temporal cortex, which is known to be involved in controlling impulsive behaviour.
Previous research has shown that drug addiction is linked with brain abnormalities involved with self control but it was not known whether the drug misuse was the cause or the result of the irregularities in the brain.
"Given that some forms of drug addiction are thought to develop out of bad habits that get out of control, it's intriguing that siblings who don't abuse drugs show similar brain abnormalities as the ones who have been abusing drugs for many years," Dr Ersche said.
"Our findings now shed light on why the risk of becoming addicted to drugs is increased in people with a family history of drug or alcohol dependence - parts of their brains underlying self-control abilities work less efficiently," she said.
"The use of drugs such as cocaine further exacerbates this problem, paving the way for addiction to develop from occasional use," she added.
The next stage of the research is to find out why the drug-free siblings were able to avoid getting hooked on drugs. They may have developed other interests that led them away from drug taking, or have been influenced by older family members.
"While we still have more work to do to fully address the reasons why some family members show a greater resilience against addiction, our results will provide the scientific basis for the development of more effective prevention and treatment for people at risk," Dr Ersche said.
Professor Les Iversen of the University of Oxford said: "These new findings reinforce the view that the propensity to addiction is dependent on inherited differences in brain circuitry, and offer the possibility of new ways of treating high-risk individuals to develop better 'self control'."