They say people with a family history of heavy drinking quickly develop a tolerance to alcohol.

The development of tolerance may maintain or increase drinking in people with a family history of alcoholism

Sandra Morzorati, Indiana University School of Medicine

This means they have to drink more to feel the same effects.

An underlying genetic susceptibility is the most likely explanation, say researchers at the Indiana University School of Medicine.

In a laboratory experiment, 116 social drinkers were given intravenous infusions of alcohol or a placebo at least a week apart.

The 58 with a family history of alcoholism reported feeling more intoxicated that those without a family history.

They also adapted to the alcohol more quickly.

Genetic factors

"This suggests that the development of tolerance may maintain or increase drinking in people with a family history of alcoholism," says the study's lead author, Sandra Morzorati.

"In other words, they imbibe more to maintain the same effects."

Dr Morzorati says the study is based on the premise that increased risk for alcoholism is related to genetic factors.

The results, she says, provide additional support for that premise.

She told BBC News Online: "The people who have a family history of alcoholism have an inherited predisposition to alcoholism."

'Hopeless alcoholic'

The research may contribute to our understanding of the vulnerability of certain individuals, says Richard Phillips, Assistant Director (Policy and Services) of Alcohol Concern in the UK.

But even assuming genetic factors play a part, it seems highly likely that other factors will be just as or more important, he adds.

"There is a reasonable amount of evidence to suggest that there is some genetic influence over the risk of developing alcohol problems," he told BBC News Online.

"The idea that some individuals are 'doomed' to become heavy drinkers is a powerful story, supported by the image of the 'hopeless alcoholic', though not justified by the current evidence base."

The research is published in the August issue of Alcoholism: Clinical and Experimental Research.

Dr Morzorati says she plans to carry out studies on twins "to pursue this genetic association further".