Alcohol dependence (AD) is influenced by both genetic and environmental factors, and its development involves "transitioning" through multiple stages of drinking behaviors. A unique study using twins to investigate genetic and environmental influences on the rate at which young women progress to AD has found that genetic and individual-specific environmental influences are evident in all transitions. By contrast, environmental influences common to members of a twin pair (for example, exposure to parental conflict) are evident primarily in the transition from non-use to first alcohol use.
Results are published in the April issue of Alcoholism: Clinical & Experimental Research.
"Women have historically been underrepresented in alcohol research, in part because alcohol-related problems are more prevalent in men, but also because the early notions about alcohol-use disorders were narrower and more focused on stereotypically male traits than they are today," explained Carolyn E. Sartor, postdoctoral research fellow at Washington University School of Medicine and corresponding author for the study. "The decrease we are seeing in the gender gap in alcohol research is a reflection of a general increase in attention to women's health issues ... as well as the closing gender gap in alcohol consumption."
Examining the development of AD in terms of how rapidly individuals transition among stages of use is a relatively new approach that evolved out of an extensive literature on the course of alcohol use and "drinking milestones," said Sartor. "Our aim is to go beyond predicting who will eventually develop alcohol problems to depicting the pathways that lead there."
Study participants were 3,546 female twins, 18 to 29 years of age, from a longitudinal twin-based study of alcohol-related problems and associated psychopathology in female adolescents and young adults. Retrospective reports of alcohol-use histories were collected through telephone interviews and used to determine transition times between drinking milestones: from non-use to initiation, initiation to onset of first alcohol-related problem, and first problem to onset of AD.
Genetic factors were found to contribute significantly to all three transition times, accounting for 30 to 47 percent of the variance. Environmental factors unique to individuals also contributed significantly to the timing of all three transitions, but environmental factors shared by twins were influential only in the rate of progression from non-use to initiation of use.
"Our results indicated that heritable influences are traceable to a common factor, but the degree to which they shape the timing of transitions varies by stage in drinking course," said Sartor. "Genetic factors appear to play a larger role in later-stage transitions than in the age at which girls begin drinking. By contrast, this first stage was the only one influenced significantly by aspects of the environment that are common to members of a twin pairs, such as shared peer influences."
Sartor cautioned that study results should not be interpreted to mean that alcohol-related problems are pre-determined. "Drinking behaviors are influenced in large part by environment at all stages of alcohol use and are therefore modifiable," she said. "That being said, the substantial contribution of genetic factors to the rate at which problem drinking develops does mean that individuals with family histories of alcohol-use disorders are at increased risk and therefore especially important targets for prevention efforts."
By examining AD development in terms of transition points, Sartor noted, the most potent genetic and environmental influences at each stage along this "pathway of risk" can be identified and interventions tailored accordingly.
"For example, we found that environmental factors that make members of a twin pair more similar play a significant role in the age at which girls begin to drink," she said. "This suggests that the most effective strategies for delaying first alcohol use would be those that focus on such environmental factors as parental attitudes toward drinking, friends' alcohol use, and parental monitoring of adolescents' activities."
Sartor and her colleagues are planning to extend their research to the examination of additional stages of drinking behaviors, such as cessation of alcohol use, as well as genetic and environmental influences on the timing of transitions in the development of other substance-use problems. Future studies will include both men and women so that possible gender differences can be detected.
Alcoholism: Clinical & Experimental Research (ACER) is the official journal of the Research Society on Alcoholism and the International Society for Biomedical Research on Alcoholism. Co-authors of the ACER paper, "Genetic and Environmental Influences on the Rate of Progression to Alcohol Dependence in Young Women," were: Arpana Agrawal, Michael T. Lynskey, Kathleen K. Bucholz, and Andrew C. Heath of the Washington University School of Medicine. The study was funded by the National Institute on Alcohol Abuse and Alcoholism, the National Institute of Child Health and Human Development, and the National Institute on Drug Abuse.