Moss and colleagues studied data from 1,484 U.S. adults who took part in a national survey conducted by the NIAAA from 2001 to 2002. The study focused on alcohol dependence and also included questions about personality, family history of alcoholism and other substance use.

The researchers applied the survey's findings to the U.S. population. They estimate that in the year before the study, nearly 8 million people in the U.S. met the standard for a diagnosis of alcohol dependence.

Alcoholism is the nonmedical, popular term for alcohol dependence, notes Moss.

The young adult subtype accounts for about 32 percent of U.S. alcoholics. They're young adults who rarely seek help for alcohol dependence. About 24 years old, they became alcoholics by age 20, on average. They drink less frequently than other alcoholics, but they tend to binge drink when they drink. This is the largest subtype.

The young antisocial subtype comprises 21 percent of U.S. alcoholics. They are 26 years old, on average. More than half have antisocial personality disorder. They tended to start drinking at 15 and became alcoholics by 18 - earlier than other subtypes. They are

more likely to smoke tobacco and pot. The young antisocial subtype and the young adult subtype don't overlap, Moss tells WebMD.

The functional subtype accounts for about 19 percent of U.S. alcoholics. They're generally middle-aged, working adults who tend to have stable relationships, more education, and higher incomes than other alcoholics. They tend to drink every other day, often consuming five or more drinks on drinking days.

The intermediate familial subtype makes up nearly 19 percent of U.S. alcoholics. Nearly half have close relatives who are alcoholics. Alcoholics in this subtype typically began drinking by 17 and became alcoholics in their early 30s.

The chronic severe subtype is the rarest subtype, accounting for about 9 percent of U.S. alcoholics. This subtype mainly includes men, has the highest divorce rate, and frequently includes users of illicit drugs.

"When most people think of alcoholics, they think of middle-aged men with a profile similar to our chronic severe subtype," Moss tells WebMD. "Our data shows that alcoholism is more a disorder of youth than previously suspected."