To understand birth cohort effects on gender differences in the likelihood of alcohol use disorders, overall birth cohort shifts in prevalence must be understood. U.S. per capita alcohol consumption began an upward trend at the repeal of Prohibition in 1933 that peaked in the early 1980’s (Lakins et al., 2004). While National Alcohol Survey data indicate that cohort effects may vary by beverage type (Kerr et al., 2004), most large-scale cross-sectional studies have indicated an increase in the prevalence of alcohol use and alcohol use disorders by birth cohort. The National Household Surveys on Drug Abuse indicated a cohort effect for any alcohol use, with individuals born in cohorts after 1950 particularly more likely to use alcohol than those born previously (Johnson and Gerstein, 1998). The National Longitudinal Alcohol Epidemiologic Survey (NLAES) and the National Comorbidity Study (NCS) both suggest that birth cohorts after World War II evidence heightened risk for alcohol use (Grant, 1997) and lifetime alcohol disorders (Grant, 1997; Grant, 1996; Warner et al., 1995; Anthony et al., 1994) compared with earlier birth cohorts. Although cross-sectional studies cannot empirically establish the existence of a cohort effect as opposed to a period (factors impacting risk among all members of a population at a given point in time) or age effect (factors impacting risk at specific ages), these shifts in the prevalence of alcohol disorders in the population are consistent with a cohort effect because respondents within birth cohorts are similar to each other with respect to alcohol consumption and alcohol use disorders across the lifecourse, as opposed to all cohorts being similar at a single point in time (a period effect) or all individuals across birth cohorts having the same risk at a given age (age effect).

While studies of adolescent alcohol use have consistently shown a convergence in rates of alcohol use initiation in younger birth cohorts (Johnson and Gerstein, 1998; Johnston et al., 2004; Johnston et al., 2005; Engs and Hanson, 1990; Schulenberg et al., 2001), studies of adults are less consistent. In the Netherlands, convergence was suggested by linear increases in average weekly alcohol consumption during a time when consumption among men decreased (Neve et al., 1993; Saelan et al., 1992), although this effect was not seen for weekly heavy (≥6 drinks) drinking (Neve et al., 1996). Converging rates of mean weekly drinking due mainly to increases among women by cohort were found in a survey in Finland, but not in Germany, Switzerland, or the Netherlands (Bloomfield et al., 2001). In New Zealand, gender differences in alcohol consumption decreased from 1995 to 2000, due mainly to increases among women in typical quantity/frequency (e.g., consuming more than 20 liters of alcohol per annum and drinking enough to feel drunk once per week (McPherson et al., 2004)). In surveys of U.S. women from 1981 to 2001, complete abstention and heavy episodic drinking (≥6 drinks/day) both decreased over time (Wilsnack et al., 2006), but these surveys did not include males, so direct gender comparisons could not be made. These studies of alcohol consumption differed widely in their measures and study design, and thus their lack of consensus on gender differences in alcohol consumption is difficult to interpret.

In contrast to the literature on alcohol consumption, studies of DSM-IV alcohol use disorders have been more consistent, although most evidence comes from family genetic studies, which are not representative samples. Among relatives of 300 Caucasian alcohol dependent individuals (Reich et al., 1988), secular trends in the consumption of alcohol contributed to an increased prevalence of alcohol dependence over time, and sex differences in alcohol consumption and problem alcohol behaviors decreased. In a Midwestern Caucasian twin study, individuals born after 1951 were at higher risk for DSM-IV alcohol dependence than those born earlier, a risk that was more pronounced for women (Holdcraft and Iacono, 2002). The Collaborative Study on the Genetics of Alcoholism (COGA) noted a decrease in the gender gap in the prevalence of DSM-IV defined alcohol dependence among those born in younger cohorts compared to earlier-born cohorts, but did not directly test for the presence of effect modification (Rice et al., 2003). These studies were all limited in terms of generalizeability. In contrast, the NLAES, a 1991-1992 U.S. national survey of over 42,000 adults, provided a substantial advantage in terms of sample size and representativeness. Descriptively, male and female prevalences by birth cohort suggested that gender differences in alcohol use disorders had decreased in younger cohorts (Grant, 1997). However, this apparent difference was never directly tested. These studies suggest birth cohort effects on the gender ratio in the prevalence of DSM-IV alcohol use disorders, and point to the need for direct testing in a recent, large, representative sample.

This research represents an important contribution to the study of gender differences in alcohol disorders; using the best cross-sectional data available, this work supports and extends accumulating evidence in less representative samples that gender differences in alcohol disorders are decreasing as suggested previously by Reich et al., 1988; Holdcraft & Iacono, 2002; and Rice et al. 2003 for alcohol dependence, indicating support for a cohort effect on gender differences in the population. Additionally, we extended the earlier work of Grant in the NLAES (Grant, 1997) by testing directly for a gender by birth cohort interaction in predicting alcohol abuse and dependence. We additionally showed that frequent binge drinking increased monotonically among women by birth cohort. This finding is contrary to other large studies of women only (Wilsnack et al.,2006; Neve et al., 1996) that suggested declines in heavy episodic drinking (6+ drinks once per week or more). The present study is perhaps more representative of recent overall trends in frequent binge drinking in the United States due to 1) a larger, epidemiologic sample of both males and females in the U.S. population and 2) a cohort of more recently born participants. Finally, we observed differences in the magnitude of gender convergence across measures of consumption and disorder, underscoring the concept that different alcohol measures are tapping into distinct constructs. Differences in the effect of gender across alcohol-related constructs is an important avenue for further research, as it may have implications for understanding the biological underpinning of gender differences in alcohol. However, similar to previous literature in this area, it should be noted that a cross-sectionally designed study cannot empirically test for the presence of a birth cohort effect as opposed to an age or period effect; while these data are consistent with a cohort effect, age and period effects could be simultaneously influencing the results.

There are several possible explanations for the decreased male/female differences in alcohol use disorders. First, there could be a true cohort effect such that drinking behaviors and risk for alcohol disorders among women and men are converging in more recent cohorts. If so, the potential mechanisms, both social and biological, involved in alcohol-related gender differences should be considered. While early twin and adoption studies suggested greater genetic contribution to alcoholism among men (Cloninger et al., 1981; Jang et al., 1997; Light et al., 1996), larger, population based twin samples show no gender difference in heritability (Heath et al., 1997; Prescott et al., 1999; Prescott and Kendler, 2000). Other biological factors include male-female differences in alcohol metabolism (Jones and Jones, 1976; Lieber, 1997; Sutker, et al. 1987; Thomasson, 1995), greater sensitivity to adverse health effects due to heavy drinking among women. Despite the decreases in gender differences in cohorts shown above, the prevalence of all alcohol disorders remained higher in men than women in all birth cohorts. Thus, these biological differences may explain some part of the remaining gender gap, although they clearly do not account for the changes by birth cohort. For such changes, social-environmental explanations must be sought.

Some investigators hypothesized that stress among women due to pursuing both career and family leads to increased alcohol use and misuse (Fillmore, 1984; Johnson and Gerstein, 1998). However, since other studies indicated that women with multiple roles were at lower risk for alcohol disorders, this explanation seems unlikely (LaRosa, 1990; Wilsnack and Wilsnack, 1991). An association between frequency of alcohol consumption among women and the number of men in their workplace (Haavio-Mannila, 1991) was interpreted as showing an imitation effect (Holmila and Raitasalo, 2005). A study of medical students in the 1980s found that at the start of medical school, female students had fewer alcohol-related problems than men, but by the start of clinical training, the gender difference had disappeared (Richman and Rospenda, 1992). Perhaps imitation as well as increased socialization to traditionally male medical roles decreased constraints against drinking originally shown by the women at the beginning of medical school. Finally, from 2001 to 2002, the proportion of young girls exposed to print advertising of low-alcohol beverages (e.g. wine coolers) increased by 216% (Jernigan et al., 2004). These and other time trends in advertising exposure to young women may have increased the social acceptability of alcohol use by women in younger generations.

We noted above clear increases in the proportion of women working outside the home and decreases in the proportion of women having children (Thronton and Freedman, 1983; Echols, 1989; Rosen, 2000). We also noted changes in gender based drinking norms. More specifically, social norms for drinking in various situations were compared over three national surveys conducted between 1979 and 1990. During those years, there was no change in the proportion of respondents who felt that “a man drinking at a bar with friends” was acceptable. However, there was a significant increase in the proportion that felt “a woman drinking at a bar with friends” was acceptable (Greenfield and Room, 1997). This indicates a decrease in the negative perception associated with drinking in women, potentially leading to greater opportunities for them to experience alcohol problems.

The changes in social norms for drinking and drunkenness lead to a second potential explanation for the observed decrease in gender differences in alcohol disorders; that differential changes in stigma by gender associated with the reporting of drinking could give rise to the appearance of a cohort effect on gender differences due to social desirability effects on self-reports of drinking. Qualitative historical research has focused on adherence by women and men to norms of “moral” behavior. Alcohol researchers observed that in the early 20^th century, female sex roles were characterized by greater “conventionality” and “acceptance of the dominant ‘official’ standards of morality and propriety” that included alcohol consumption (Clark, 1967). Women that appeared to abstain or drink very little were thus more closely following the official standards of morality and propriety in the time period, while men, less bound by these standards, were more likely to drink and develop chronic alcohol problems. As these norms changed, the reduced need to adhere to such moral norms was greater for women, potentially affecting not only drinking but also self-reports of drinking among women. Such ideas are intriguing, but unfortunately, few means to verify them empirically are available in the absence of time trend data comparing reported versus true alcohol use among men and women. To definitively rule out social desirability as an explanation of the findings reported above, alternative indicators of alcohol use not based on self-report should be identified and analyzed, work that is currently in progress. However, this issue does not negate the importance of statistically demonstrating a decreased gender difference in successively younger birth cohorts, as we have done above.

A third alternative hypothesis could be that the observed increase in the prevalence of alcohol disorders in younger cohorts is due to the inability of the cross-sectional design to capture all cases of alcohol disorders in the older cohorts (i.e. those respondents in the oldest cohorts are those that survived to be surveyed between 2001-2002). As adults with active alcohol disorders have a higher mortality rate than the general population, some lifetime cases of alcohol dependence are probably missed in older cohorts, underestimating the prevalence of disorder in these groups. However, as we are comparing men to women within each cohort, the observed odds ratios will only be affected if there is differential alcohol-related mortality between men and women in the oldest cohorts. While there is some evidence that among those with active, chronic alcohol dependence, women have a greater sensitivity to adverse health effects due to heavy drinking (Deal and Gavaler, 1994; Hanna et al., 1997; Singletary and Gapstur, 2001; Hommer et al., 2001), this literature is based on men and women with unremitting long-term cases of alcohol dependence. As most cases of alcohol dependence in the general population across gender remit at some point (Dawson et al., 2005), it is unlikely that the death of women in the general population with alcohol use disorders account for the observed effect. However, the extent of differential mortality by gender among lifetime cases of alcohol dependence in the general population is unknown and should be determined.

As with all cross-sectional data, these data are limited by recall bias. The conclusion that these results are most consistent with a birth cohort effect relies on the assumption that the lifetime measure of alcohol use and alcohol use disorder is accurate. Previous literature has established that recall bias is an issue in the reporting of past alcohol consumption (Liu et al., 1996; Caldwell et al., 2006), thus these results should be interpreted with caution. This issue is exemplified in the NESARC sample as we observed few lifetime cases of alcohol disorders in respondents older than 90 at the time of survey, although this effect may be a combination of recall bias and selective mortality. However, to account for this possibility we removed those respondents at or older than 90 years old. Additionally, since our findings for binge drinking and alcohol abuse follow the patterns of the yearly per capita alcohol consumption in the United States, it is unlikely that the observed cohort effect is entirely a reflection of poor recall among heavy drinkers in the older cohorts (Johnson and Gerstein, 1998; Lakins et al., 2004). Additionally, because the NESARC sampled those 18 and older, some people in the youngest-born cohort may have been misclassified as unaffected because they have not had time to develop alcohol disorders. If this is true, however, then the prevalence of alcohol disorders in the younger cohort was underestimated. This could be re-examined using data from the three-year follow-up of the NESARC sample that will become available in the next few years. Finally, future large-scale surveys using the AUDADIS instrument could also be used to simultaneously estimate the effect of cohort, age, and period effects on trends in alcohol consumption and alcohol use disorders in the United States, an important area for continued follow-up research.

Despite the necessity to test alternative hypotheses, the study has several substantial strengths that make these findings a contribution to our understanding of the epidemiology of alcohol disorders. First, the state-of-the-art instrument used for data collection (the AUDADIS-IV) increases the sensitivity and specificity of the estimates of alcohol consumption and alcohol disorders. Second, the large, representative sample of men and women represent an improvement over previous cross-sectional studies attempting to examine birth cohort effects on gender differences, which previously were conducted in samples from family genetic studies. Third, in contrast to information from previous epidemiologic samples, this study specifically tested a hypothesis regarding a cohort by gender interaction, and had sufficient power to detect such interactions.

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