State handgun purchase age minimums in the US and adolescent suicide rates: Regression discontinuity and difference-in-differences analyses

Objective To evaluate the association between US state policies that establish age 18 or 21 years as the minimum purchaser age for the sale of handguns and adolescent suicide rate.
Design Regression discontinuity and difference-in-differences analyses.
Setting 46 US states without policy changes between 2001 and 2017; Missouri and South Carolina, which lowered the age for handgun sales in 2007 and 2008, respectively; and West Virginia and Wyoming, which increased the age for handgun sales in 2010.
Participants Adolescents aged 13 to 20 years(554 461 961 from 2001 to 2017) in the regression discontinuity analysis, and adolescents aged 18 to 20 years (168 934 041 from 2002 to 2014) in the main difference-in-differences analysis.
Main outcome measure Suicide rate per 100 000 adolescents.
Results In the regression discontinuity analysis, state policies that limited the sale of handguns to those aged 18 or older (relative to 21 or older) were associated with an increase in suicide rate among adolescents aged 18 to 20 years equivalent to 344 additional suicides in each state where they were in place between 2001 and 2017. In the difference-in-differences analysis, state policies that limited the sale of handguns to those aged 21 or older were associated with 1.91 fewer suicides per 100 000 adolescents aged 18 to 20 years (95% confidence interval −3.13 to −0.70, permutation adjusted P=0.025). In the difference-in-differences analysis, there were 1.83 fewer firearm related suicides per 100 000 adolescents (−2.66 to −1.00, permutation adjusted P=0.002), with no association between age 21 handgun sales policies and non-firearm related suicides. Separate event study estimates indicated increases in suicide rates in states that lowered the age of handgun sales, with no association in states that increased the age of handgun sales.
Conclusions A clear discontinuity was shown in the suicide rate by age at age 18 in states that limited the sale of handguns to individuals aged 18 or older. State policies to limit the sale of handguns to individuals aged 21 or older were associated with a reduction in suicide rates among adolescents. Increases in suicide rates were observed after states lowered the age of handgun sales, but no effect was found in states that increased the age of handgun sales.