Abnormalities in the brain may make some people more likely to become drug addicts, according to scientists at the University of Cambridge.
They found the same differences in the brains of addicts and their non-addicted brothers and sisters.
The study, published in the journal Science, suggested addiction is in part a "disorder of the brain".
Other experts said the non-addicted siblings offered hope of new ways of teaching addicts "self-control".
It has long been established that the brains of drug addicts have some differences to other people, but explaining that finding has been more difficult.
Experts were unsure whether drugs changed the wiring of the brain or if drug addicts' brains were wired differently in the first place.
This study, funded by the Medical Research Council, attempted to answer that by comparing the brains of 50 cocaine or crack addicts with the brain of their brother or sister, who had always been clean.
Both the addicts and the non-addict siblings had the same abnormalities in the region of the brain which controls behaviour, the fronto-striatal systems.
The suggestion is that these brains may be "hard-wired" for addiction in the first place.
Lead researcher Dr Karen Ersche said: "It has long been known that not everyone who takes drugs becomes addicted."
She told the BBC: "It shows that drug addiction is not a choice of lifestyle, it is a disorder of the brain and we need to recognise this."
However, the non-addicted siblings had a very different life despite sharing the same susceptibility.
"These brothers and sisters who don't have addiction problems, what they can tell us is how they overcome these problems, how they manage self-control in their daily life," Dr Karen Ersche said.
Dr Paul Keedwell, a consultant psychiatrist at Cardiff University, said: "Addiction, like most psychiatric disorders, is the product of nature and nurture.
"We need to follow up people over time to quantify the relative risk of nature versus nurture."
It is possible that the similarities in the sibling's brains may not be down to genetics, but rather growing up in the same household. Research on the relationship between addiction and the structure of the brain is far from over.
However, many specialists believe these findings open up new avenues for treatment.
"If we could get a handle on what makes unaffected relatives of addicts so resilient we might be able to prevent a lot of addiction from taking hold," said Dr Keedwell.
The chief pharmacist for Derbyshire Mental Health Trust, David Branford, said the study, "implies that addiction does not produce noticeable changes to brain structure and function which means that there may be provision for looking at new treatment techniques for addiction".
Prof Les Iversen, from the department of pharmacology at 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'."