ATLANTA, Georgia (CNN) — State laws that restrict access to guns could reduce the rate of firearm-related suicide, according to new research.

Researchers examined suicide rates in Connecticut and Missouri, two states that changed their permit-to-purchase handgun laws in recent decades. Connecticut passed a law in 1995 that requires people to apply for a permit with local law enforcement and take eight hours of gun safety training before they can buy a firearm. In 2007 in Missouri, the state repealed a 1921 law that required people to apply with the local police to buy a gun.

The rate of gun-related suicide in Connecticut in the 10 years after its law passed was 15 percent lower than what researchers predict it would have been had the law not been passed. The researchers made this prediction based on the suicide rate between 1995 and 2005 in Rhode Island and North Dakota, which have similar demographics as Connecticut, and which also had similar suicide rates as Connecticut in the years before its law (1981-1994).

In contrast, the gun-related suicide rate in Missouri was 16 percent higher from 2007 to 2011 than researchers predict it would have been based on the rates in the comparable states of North Carolina and Nebraska.

An unintended benefit to gun control laws

It is not surprising that permit-to-purchase laws have a protective effect against suicide, even though they are intended to prevent people with a history of crime, violence or mental illness from acquiring a firearm, said Cassandra Crifasi, assistant scientist in the Johns Hopkins Bloomberg School of Public Health department of health policy and management. Crifasi is lead author of the study, which was published in the September issue of the journal Preventive Medicine.

The study is the first to look at suicide rates in states across a period in which they changed their gun law, although previous research has found that suicide rates are lower overall in states that have permit-to-purchase or other laws. "It was a nice natural experiment that Connecticut passed a law and Missouri repealed, because we could see pre- and post- what the impact of the laws are," Crifasi said.

In addition to the decrease in firearm-related suicides in Connecticut, the researchers concluded there was probably a drop in the rate of suicide involving other means. Factors in the state other than the gun law could also be influencing the suicide rate, although it is unclear what those could be, Crifasi said.

Making 'a difficult thing harder'

Research suggests that non-firearm-related suicides do not skyrocket when firearm-related suicides drop, said Michael Anestis, professor of psychology at the University of Southern Mississippi, who was not involved in the study. "A lot of people think [those who want to commit suicide] will just reach for whatever is there, but suicide is difficult, and when you make a difficult thing harder, it's that much less likely to happen," Anestis said.

One study found that about half of people who attempt suicide made the decision after thinking about it for only 20 minutes. However, it would probably take some days or weeks to buy a handgun in Connecticut after its law went into effect, Crifasi said.

The impulsive nature of some suicides could be part of the reason Crifasi and her colleagues predicted the biggest drop in suicides in Connecticut --- and the biggest increase in Missouri --- would occur among people 20 to 29 years of age. "Young adults are far more impulsive, and that decreases as one ages," Crifasi said.

Other handgun laws, not just permit-to-purchase laws, could also reduce suicide rates. A study by Anestis of University of Southern Mississippi found that laws that prohibit open carrying of handguns in California and Oklahoma reduced suicide rates by 3.5 percent and 1.7 percent, respectively, in the year after they were passed. Open carry could make handguns seem more familiar and less scary in general, Anestis said.

Anestis' study, which came out shortly after Crifasi's, also found that a law in the District of Columbia that extended the waiting period to complete a handgun purchase was associated with a 2.2 percent drop in suicides, whereas South Dakota's repeal of a waiting period law was associated with a 7.6 percent increase in suicides.

Another reason state handgun laws could help reduce the suicide rate is that law enforcement agents are able to do more thorough background checks. Although federal law requires licensed gun dealers (but not private sellers) to do background checks, federal records can be incomplete because states do not report events, such as when a person is involuntarily committed to a mental health facility, Crifasi said.

A previous study by Johns Hopkins researchers concluded the rate of firearm-related homicide in Connecticut dropped by about 40 percent after the 1995 law, whereas Missouri's rate increased by 23 percent after its repeal.

Homicide was the cause of 16,000 deaths in the United States in 2013, whereas more than 40,000 people committed suicide.

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