The effects of US state income inequality and alcohol policies on symptoms of depression and alcohol dependence

Abstract

Mental health is likely to be influenced by contextual variables that emerge only at the level of the group. We studied the effect of two such group-level variables, within-state income inequality and alcohol tax policy, on symptoms of current depression and alcohol dependence in a US national sample, controlling for state-level and individual characteristics. A cross-sectional US national probability sample provided the individual-level data. State income data were obtained from the 1990 US census. The Gini coefficient (raw and adjusted) indicated income inequality. Outcome measures included current symptoms of depression and alcohol dependence. Controlling for individual-level variables and state median income, the odds of depressive symptoms was not positively associated with state income inequality. Controlling for individual-level variables, state median income and alcohol distribution method, a weak negative association between Gini and alcohol dependence was observed in women, but this association disappeared after additional adjustment for beer tax. No association was observed in men. Higher state beer tax was significantly associated with lower prevalence of alcohol dependence symptoms for both men and women. The results suggest that state income inequality does not increase the experience of alcohol dependence or depression symptoms. However, evidence was found for a protective effect of increased beer taxation against alcohol dependence symptoms, suggesting the need to further consider the impact of alcohol policies on alcohol use disorders.

Introduction

The recent resurgence in interest in social epidemiology (Link & Phelan, 2000) has included the identification of contextual variables that emerge only at the level of the group. Such variables are aspects of the environmental context of health, which is omitted if only individual-level data is measured (Susser, 1994). For example, the notion that inequality in the distribution of income may be related to health has recently attracted much attention. Wilkinson's (1992) original report of an association between income inequality and mortality in developed countries has since been replicated for states and metropolitan areas within the US (Lynch et al., 1998; Kaplan, Pamuk, Lynch, Cohen, & Balfour, 1996; Kennedy, Kawachi, & Prothrow-Stith, 1996). These associations, however, do not appear to be as strong in some contexts, as demonstrated by the weaker relationship between income inequality and mortality observed across Canadian provinces (Ross et al., 2000). Further work has examined the extent to which the relation between income inequality and health persists once individual-level factors such as income are controlled for. The relationship has been found to persist in some (Kennedy, Kawachi, Glass, & Prothrow-Stith, 1998; Kahn, Wise, Kennedy, & Kawachi, 2000; Diez-Roux, Link, & Northridge, 2000; Lochner, Pamuk, Makuc, Kennedy, & Kawachi, 2001) but not all studies (Fiscella & Franks, 1997; Daly, Duncan, Kaplan, & Lynch, 1998; Osler et al., 2002; Shibuya, Hashimoto, & Yano, 2002; Sturm & Gresenz, 2002). There is still substantial ongoing debate regarding the presence and strength of an inequality effect, over and above the well-established effect of individual-level income (Wilkinson, 2002; Lynch & Davey Smith, 2002).

Kennedy et al. (1992), Wilkinson (1997a), Wilkinson (1997b) has suggested that income inequality affects both physiology (for example, due to endocrine changes) and stress-related behaviors that affect health, for example, stress-related smoking, drinking, comfort eating and interpersonal violence. It has also been hypothesized that the negative effects of living in an unequal society may be greater in persons of lower income (Kennedy et al., 1998; Kahn et al., 2000; Diez-Roux et al., 2000). To the extent that stress is causally related to common mental health problems (for example, Kendler et al., 1995), this interpretation implies that those living in areas of greater income inequality will have higher rates of mental health problems, particularly those on lower incomes. One study on maternal mental health (Kahn et al., 2000) was consistent with this hypothesis, while two more, on adults in the US (Sturm & Gresenz, 2002) and the UK (Weich, Lewis, & Jenkins, 2001) did not find the predicted relationship. All three of these studies addressed mental health outcomes involving depression or anxiety, but did not include alcohol, specifically suggested by Wilkinson as being one of the health conditions affected by income inequality.

To date, state-level information based on a nationally representative sample has not been used to address the relationship of income inequality to common mental health problems including both depression and alcohol. State-level data offers a number of advantages in addressing this question. According to Wilkinson, income inequality becomes important only in areas large enough to contain the relevant social stratification, while in small neighborhoods with little income gradient, morbidity and mortality are more closely related to the average income. The choice of US state to define the geographic area thus provides large geographic units with considerable social heterogeneity (Wilkinson, 1997a). State-level analysis also allows incorporation of potential state-level confounders, such as state alcohol policies, that may influence mental health outcomes such as alcohol dependence through their effects on consumption. An example of such an alcohol policy is excise tax on alcohol, relevant because studies consistently show an inverse relationship between alcohol price and alcohol consumption (Toomey & Wagenaar, 1999; Chaloupka, Grossman, & Saffer, 2002). Another such policy includes alcohol distribution methods, which may affect consumption by restricting access (Toomey & Wagenaar, 1999). Because variation in both of these can be studied at the state level, US state represents a geographical unit of analysis that is meaningful in many respects.

The National Longitudinal Alcohol Epidemiologic Survey (NLAES) presents an unusual opportunity to study the relationship of income inequality and state alcohol policy to depression and alcohol dependence at the state level because the sample is very large and the response rate very high (Grant et al., 1994). Further, well-validated and reliable measures of depression and alcohol disorders were used (Grant & Hasin, 1992; Grant, Harford, Dawson, Chou, & Pickering, 1995; Hasin et al., 2003). We therefore assessed the contextual effect of state income inequality on symptoms of depression and alcohol dependence, controlling for potential state- and individual-level confounding variables. The gender distribution of major depression and alcohol dependence are almost mirror opposites of each other (Hanna & Grant, 1997). Therefore, we investigated each of these disorders separately for men and women.