London (CNN) An ancient retrovirus that predates modern humans may explain why people suffer from addiction, scientists have said.

Researchers studied drug users in two cities and found they were up to about three times more likely than the general population to have remnants of the HK2 virus within a particular gene in their DNA.

The virus dates back at least to Neanderthals and is present in the RASGRF2 gene -- which a researcher called the "pleasure gene" because it increases the activity of dopamine in the brain -- in about 5% to 10% of people.

That proportion of the population could therefore be more disposed to addictive behavior, the scientists said.

The team, from the universities of Oxford and Athens, found the virus in 34% of drug users tested in Glasgow, Scotland, compared to 9.5% of the local population, and in 14% of Greek patients, compared to 6% of that country's population.

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