Cross-Sectional Associations

К оглавлению
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 
51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 
85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 
102 103 104 105 106 107 108 109 110 111 112 113 114 115 

Almost all ecological studies using cross-sectional data, both within the

United States and across countries, have found that both gun suicide rates

and the fraction of suicides committed with a gun are higher in geographic

areas with a higher prevalence of household gun ownership. This association

has been reported by investigators across the spectrum of the gun

control debate. It has been found across cities, states, regions, and nations

(Kleck and Patterson, 1993; Azrael et al., 2004; Killias, 2001), and it contrasts

with the more variable association between gun ownership rates and

the fraction of homicides committed with a gun.

1Studies were identified using various search engines, by a search for book chapters and

unpublished studies identified through personal communication with researchers in the field,

and by review of the reference lists of previous publications. A particular effort was made to

find studies in the firearms policy literature, reviewed for other chapters of this volume, which

may have examined suicide as a secondary focus of the investigation.

However, the most important policy question is not whether gun access

increases the risk of gun suicide, but whether gun access increases the

overall risk of suicide. Many cross-sectional studies have reported a positive,

bivariate association between gun ownership rates and overall suicide

rates across cities, states, and regions of the United States, but the relationship

is much smaller and less precise than the association between gun

ownership rates and gun suicide rates. The association between gun ownership

and overall suicide also appears to be sensitive to the details of the

measures and the statistical models being used.

U.S. Studies

Several ecological studies by Birckmayer and Hemenway (2001) and by

Miller et al. (2002a, 2002c) have focused on age-specific suicide rates by

region and state. Their gun ownership measures include survey estimates of

handgun and overall gun ownership from the GSS and, as a proxy measure,

the fraction of suicides committed with a firearm. Before controlling for

other social variables, Birckmayer and Hemenway find a positive association

between regional GSS-reported rates of gun ownership and age-specific

rates of suicide in every age group. After controlling for divorce, education,

unemployment, urbanization, poverty, and alcohol consumption, they find

a modest positive association between gun ownership and suicide risk for

youth ages 15 to 24 (b = .35, 95% confidence interval .05 to .65) and for

adults age 65 and over (b = .62, 95% C.I. .40-.84), but not for working-age

adults between ages 25 and 64. Subsequent studies from the same research

group use other model specifications, with varying results. For example,

Miller et al. (2002a) do not incorporate control variables; they find a

positive association between gun ownership and overall suicide rates in all

age groups (incidence rate ratio 1.14; 95% CI 1.01-1.24) and a negative

association between gun ownership and nongun suicide (IRR .87, 95% CI

.77-.97) that is more pronounced for persons 45 years and older, suggesting

greater substitution among methods in older age groups.

Duggan (2003) undertook a similar age-specific analysis, using subscriptions

to the gun magazine Guns & Ammo as his proxy for gun ownership.

Like Miller et al., Duggan did not include other covariates in his

regression models and, like Miller et al., he found a positive and significant

bivariate association between gun ownership and suicide across states. But

Duggan also found a significant positive association between gun magazine

subscription and nongun suicide for youth ages 10 to 19. The association

between the gun proxy and nongun suicide shifts from positive to negative

between ages 20 and 69 and becomes negative and statistically significant

for persons over age 69. He concludes that the positive association between

gun magazine subscriptions and nongun suicide among youth is evidence

TABLE 7-1 Ecological Studies of Associations Between Firearms

Prevalence and Suicide in the United States

Unit of Gun Subjects;

Source Analysis Measure Strata

Duggan 50 states Proxy: Guns 10 yr. age

(2003) 1996 & Ammo groups

Hemenway 9 regions Survey: GSS

and Miller 1988-1997 (household

(2002) handgun

ownership)

Miller et al. 9 regions Survey: GSS, Children

(2002b) 50 states BRFSS 5-14

1988-1997 Proxy: Cook

index, FS/S

(adult only)

Miller et al. 9 regions Survey: GSS, Adult

(2002c) 50 states BRFSS women

1988-1997 Proxy: Cook

index , FS/S

Miller et al. 9 regions Survey: GSS, 10-yr. age

(2002a) 50 states BRFSS groups

1988-1997 Proxy: Cook

index, FS/S

Birckmayer 9 regions GSS 10-yr age

and 1979-1994 groups

Hemenway

(2001)

Azrael et al. 9 regions Survey: GSS,

(2004) 50 states BRFSS, HICRC

1994-1998 Proxies: FS/S,

UFDR,

Guns & Ammo,

NRA

membership

continued

Results: Results: Results:

Guns and Guns and Guns and

Control Gun Nongun Overall

Variables Suicides Suicides Suicides

None all ages + 10-19: + all ages +

20-69: 0

70+: –

Major + – +

depression,

suicidal

thoughts, and

urbanization, OR

education, OR

unemployment,

OR

alcohol

consumption

Poverty, + 0 +

education,

urbanization

Poverty, + BRFSS:+ +

urbanization Others: 0

None all ages + <45: 0 all ages +

45+: –

Divorce, 15-24: + 0 15-24: +

education, 25-44: 0 25-64:0

unemployment, 45-84: + 65+: +

urbanization

None + n/a n/a

TABLE 7-1 Continued

Unit of Gun Subjects;

Source Analysis Measure Strata

Kaplan and 9 regions Survey: GSS Sex race

Geling 1989-1991

(1998)

Kleck and 170 U.S. OLS proxy:

Patterson cities gun crimes

(1993) IV proxy:

gun sport

Sloan et al. 2 cities Registry: Two age

(1990) 1985-1987 handguns groups,

Proxies: Cook race, sex

index

Strictness of gun

laws

Lester 48 states Proxy: gun

(1989) 1980 magazines

Lester 6 (of 7) Survey-household

(1988a) Australian gun ownership

states

Lester 9 regions Survey

(1988b) 1970 Proxy: gun laws

Lester 48 states Proxies: gun

(1987a) 1970 laws, UFDR

Proxy: Cook

index

Duggan 50 states Proxy: guns All ages

(2003) ammo sales rate

continued

Results: Results: Results:

Guns and Guns and Guns and

Control Gun Nongun Overall

Variables Suicides Suicides Suicides

None + Male:- n/a

Female: 0

Community traits: + 0 OLS: +

race, sex, age IV: 0

unemployment

rate, poverty,

income, home

ownership,

college

enrollment,

transience,

population

change, divorce,

place of worship,

etc.

None + – 0

None + 0 +

None 0 – 0

% black, median + 0 0

age, % urban,

divorce rate

None + UFDR:– 0

Other: 0

State, year fixed 0 0 0

effects

for an omitted variable, because any plausible causal effect of gun ownership

should be independent of, or negatively associated with, the nongun

suicide rate. There are several other possible explanations for Duggan’s

results; most obviously, it may be that Guns & Ammo subscribers are not

representative of all gun owners; his arguments about confounding would

also have been strengthened by the inclusion of some observable covariates.

All the same, both Miller’s and Duggan’s results support the view that

different gun proxies may yield different results, and all of the age-stratified

studies suggest that instrumentality effects, substitution, and omitted variables

may be playing different roles at different ages.

The most comprehensive effort to control for confounding factors was

published a decade ago. Kleck and Patterson (1993) undertook a crosssectional

study of the effect of firearms prevalence on crime rates and

firearm-related fatalities in 170 U.S. cities. Although the study did not

consider differences by age, the models included a set of 38 control variables

previously identified as predictors of violence rates. Like other investigators,

these authors found that higher levels of the proxy for gun owner-

TABLE 7-1 Continued

Unit of Gun Subjects;

Source Analysis Measure Strata

Mathur and 48 states Gun dealers Adolescent

Freeman per capita suicide

(2002) (15-19)

Azrael et al. 9 regions Survey: GSS

(2004) 50 states Proxy: FS/S

Clarke and Entire Survey: Gallup Type of

Jones United poll, GSS gun

(1989) States

NOTES: +, - indicate positive or negative effect (respectively), statistically significant at p <

.05; 0 indicates not significant.

BRFSS = Behavioral Risk Factor Surveillance System; GSS = General Social Survey; FS/S =

ratio of firearm suicide/total suicides; Cook Index = mean of firearm suicide/total suicide and

firearm homicide/total homicide; HICRC = Harvard Injury Control Research Center; UFDR

ship predicted higher rates of suicide (b = .132, p < .05). Kleck and Patterson

also found evidence that there might be a different association between

suicide risk and sporting gun ownership and suicide risk and defensive gun

ownership. In particular, they found no significant effect of sporting gun

ownership on the risk of suicide.

International Studies

Like the U.S. studies, the existing cross-national surveys have looked

for an association between rates of household gun ownership, overall suicide

rates, and the fraction of suicides committed with a gun. And, like the

U.S. studies, cross-national studies have found a consistent association between

gun ownership and the fraction of suicides committed with a gun

across countries; but in contrast to the U.S. studies, the cross-national

surveys do not reveal a consistent association between gun ownership and

overall suicide rates.

= unintentional firearm death rate; FLFP = female labor force participation; OLS = ordinary

least squares; IV = instrumental variable (two-stage least squares); NRA = National Rifle

Association.

When only one result is listed in column, all gun measures gave similar results. When

reported results include models both with and without covariates, only results with covariates

are presented.

Results: Results: Results:

Guns and Guns and Guns and

Control Gun Nongun Overall

Variables Suicides Suicides Suicides

State, year fixed Not stated Not stated +

effects

FLFP, divorce,

alcohol

consumption

family & cohort

size

Regional fixed + Not stated Not stated

effects

None Handgun + n/a

All guns: 0 All guns: 0

Handgun: +

Although gun ownership rates in the United States are much higher

than in most other developed countries, the rates of suicide in the United

States rank in the middle. Killias (1993), Killias (2001), and Johnson et al.

(2000) found that reported rates of household gun ownership were strongly

correlated with the fraction of suicides committed with a gun in each country

(Spearman’s rho = .79 to .92, p < .001). But the cross-country correlations

between household gun ownership and overall rates of suicide have

proven to be smaller and statistically imprecise (Spearman’s rho .25, p =

.27) (Killias, 2001). Likewise, in an often-cited study, Sloan et al. (1990)

compared the rates of gun and nongun suicides in Seattle, Washington,

with suicide rates in Vancouver, British Columbia, between 1985 and 1987;

they found higher rates of gun ownership are associated with higher rates of

gun suicide, lower rates of nongun suicide, and no significant difference in

the overall suicide rate between the two cities (relative risk .97, 95% CI .87

to 1.09).

Associations Between Gun Ownership and Suicide Rates Across Time

The fraction of suicides in the United States that are committed with a

firearm has increased from just over 35 percent in the 1920s to about 60

percent in the 1990s. Four studies have attempted to link this change in the

fraction of gun suicides with changes in gun ownership across time.

Three of these four studies have found positive associations between

proxies for gun ownership and the fraction of suicides committed with a

gun, but only one study, focusing on youth suicide, found an association

between gun ownership and overall suicide rates. Clarke and Jones (1989),

examined the national prevalence of household gun ownership reported in

polls by Gallup and the National Opinion Research Center between 1959

and 1984, comparing these reports with aggregate U.S. suicide rates over

the same period. This study found a positive association between the fraction

of households owning a handgun and the fraction of suicides committed

with a gun (b = .68, p = .001), but no association between household

gun ownership and overall risk of suicide (b = .04, p = .85). Azrael et al.

(2004) also report a strong linear association between individual and household

rates of gun ownership within regions and the fraction of suicides

committed with a gun between 1980 and 1998, with cross-sectional beta

coefficients ranging from .55 (for individual handgun ownership) to 1.02

(for household gun ownership of any kind), and an inter-temporal coefficient

between FS/S and household gun ownership of .905 (s.e. = .355). They

did not report the association between gun ownership and overall risk of

suicide. Mathur and Freeman (2002) used state-level per capita gun dealership

rates to predict adolescent suicide rates from 1970 to 1997. After

controlling for state and year fixed effects and number of other observed

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

defensive gun ownership and lower suicide rates (Hemenway et al., 2001).

Neighborhood levels of gun ownership could even conceivably be affected

by neighborhood suicide rates: suicide rates might contribute to a

community’s perceived level of violence, whether people are aware of making

such a link or not.

This concern is not unique to ecological studies, but has been generally

ignored in this literature. There have been few systematic efforts to explore

or model possible confounders of the association between gun ownership

and suicide risk. Two studies by Hemenway and associates are suggestive.

First, Hemenway et al. (2001) investigated the hypothesis that persons who

live in communities with higher levels of mutual trust may be at lower risk

of suicide (because of increased social support), and lower risk of gun

ownership and less likely to own firearms (because of decreased motivation

for defensive gun ownership). They found that, across U.S. states, lower

levels of mutual trust and civic engagement, as reported on the General

Social Survey and on the Needham Lifestyle Survey, were associated with a

higher fraction of suicides committed with a gun. This study did not examine

the association between social capital, firearm ownership, and overall

suicide rates. Hemenway and Miller (2000) investigated the hypothesis that

regions with higher rates of firearm ownership were characterized by higher

rates of major depression, which is known to be an important independent

risk factor for suicide. They found that the cross-sectional, regional association

between firearm ownership and suicide rates was not explained by

differences in the regional prevalence of major depression and serious suicidal

thoughts.