"The flies choose to consume alcohol to intoxicating levels, they will do so even if alcohol is made unpalatable, and they relapse to drinking high levels of alcohol after being deprived of it," said Ulrike Heberlein of the University of California, San Francisco. "Addiction is a purely human condition, but, surprisingly, flies show several key features of it."

Heberlein’s group has been studying the genes underlying alcohol response and addiction using flies as a model for many years. (They are the team that earlier this year brought us the "happy hour" gene, which influences flies’ susceptibility to alcohol’s sedative effects.) Heberlein says she is a strong believer that the fly is a useful model of human conditions. Even so, she was "amazed" to find that flies will make such complex decisions when it comes to alcohol.

The researchers found that flies prefer to consume ethanol-containing food over regular food, and that the insects’ preference increases over time. Flies are attracted to the smell of the alcohol, the researchers show, but they actually don’t like its taste. Their attraction to alcohol isn’t explained by the immediate sensory experience of it, or by its calories. Nevertheless, flies self-administer ethanol to intoxicating levels. They will put aside their aversion to particular tastes or smells in order to consume it, and they will rapidly return to high levels of ethanol consumption after a period of imposed abstinence.

Heberlein doesn’t really know in the case of the flies what drives the behavior, but she says she can only assume that they find alcohol rewarding and therefore choose to drink it despite its adverse consequences.

"Previously, we studied simple behaviors, such as intoxication and development of tolerance," Heberlein said. "This work opens the door for us to study much more complex alcohol-related behaviors, such as ‘use despite adverse consequences’ and ‘relapse.’"

The researchers include Anita V. Devineni and Ulrike Heberlein, of the University of California, San Francisco.

During the studied year, two percent of Americans used alcohol either daily or near daily. The survey also showed alcohol users were more likely than other Americans to be alcohol dependent in the past year. Of all admissions to substance abuse treatment centers, 40 percent reported alcohol as the primary substance of abuse.

Of those 40 percent, 22 percent reported alcohol abuse only and 18 percent reported alcohol and additional substances. During the study year, alcohol-only treatment admissions reporting daily use were, on average, more than six years older than those reporting less frequent use.

For alcohol-only admissions younger than 21 years of age, those who reported less than daily use outnumbered daily users by 15.6 to 1. For those in the age range of 21 to 24, the ratio declined to 7.1 to 1. For alcohol-only admissions among those in the age range of 25 to 34 and 35 to 44, the ratios were 3.7 to 1 and 1.7 to 1.

Alcohol-only treatment admissions in 2006 had, on average, a first intoxication age of 16 years. Most alcohol-only admissions were first intoxicated before the age of 18.

Almost half, or 49 percent, of alcohol-only treatment admissions in 2006 who reported daily use were referred to treatment by themselves, a family member or a friend. Only 19 percent of those reporting less frequent use were referred this way. More than half, or 55 percent, of alcohol-only treatment admissions reporting less than daily use were referred by the criminal justice system. Only 15 percent of daily users were referred to treatment this way.

Those who sought treatment for alcohol only daily use were more likely than those using less frequently to be treated in a detoxification type setting. The variance here was 55 percent versus 17 percent. Daily users were also more likely to be treated in a rehabilitation or residential service setting at 17 percent versus 8 percent. Ambulatory was more common for less frequent users.

During the study year, nearly half – or 47 percent – of alcohol only treatment admissions in the Northeast reported daily use, while 29 percent were reported in the South, 26 percent in the Midwest and 25 percent in the West. Daily users were also more likely to not be employed. Overall, daily alcohol users did not vary substantially in educational attainment, race/ethnicity or gender.