covariates (e.g., divorce rates, per capital alcohol consumption, female labor force participation, family size, and cohort size), Mathur and Freeman found that increases in gun dealerships per capita predicted increases in the overall youth suicide rate. Finally, Duggan (2003) used two decades of gun magazine sales with controls for state and year fixed effects to explain the trends in suicide rates across all age groups. Duggan found no association between magazine subscription rates and either gun suicide or overall suicide rates across time (b = .046, s.e. = .064, and b = .004, s.e. = .051, respectively).

Assessment of Ecological Studies

Overall, the body of ecological studies has firmly established that firearms access is positively associated with gun suicide, but the association between firearm access and overall suicide is less certain.

In particular, gun suicide rates are strongly correlated with gun prevalence across space and possibly across time, in the United States and across countries. Likewise, many ecological studies do report a cross-sectional association between gun ownership rates and overall suicide rates in the United States. However, gun ownership rates do not seem to explain overall suicide trends across countries or across time in the United States. Moreover, the results seem to vary according to the firearm measure used, the age group being studied, and the covariates included.

To further improve our understanding of the effects of firearms on suicide, researchers need to be increasingly sensitive to the possibility of confounding factors and substitution. Moreover, these ecological studies introduce two additional problems that must be considered. First, the analyses are conducted at the aggregate level, rather than at the individual level, and second, direct measures of access to firearms are often not available, thus forcing researchers to rely on proxies. We consider each of these issues in turn.

Substitution and Confounders

As with all empirical analyses, researchers and policy makers must be sensitive to unobserved confounders when attempting to draw causal inferences (see Box 7-1). To what extent would suicidal persons substitute other methods if firearms were less available? Unmeasured and confounding factors associated with both suicide risk and gun ownership might lead to a spurious association between guns and suicide. For example, if persons who are prone to own guns because of their mistrust of others were also at greater risk for suicide, whether or not they owned guns, there could be a noncausal statistical association between gun ownership and suicide. Likewise, high levels of “social capital” might be associated with lower rates of

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