Semiautomatic Rifle Ban Stopped Mass Shootings in Australia, Study Suggests

Ricki Lewis, PhD

March 14, 2018

There have been no mass shootings in Australia since the passage of a ban on semiautomatic rifles in 1996. The probability that no mass shootings have occurred in the 22 years since the ban went into effect is due to chance is one in 200,000, according to a statistical analysis published online March 13 in the Annals of Internal Medicine.

The National Firearms Agreement was enacted in the wake of the Port Arthur massacre in Tasmania, in which 35 people were killed and 23 were wounded. Port Arthur is an historic site that was formerly a prison colony site and is now a tourist attraction.

The legislation restricts private ownership of semiautomatic rifles, semiautomatic shotguns, and pump-action shotguns. It also requires uniform gun registration and removes self-defense as a legitimate reason to own a firearm. The act led to the surrender of more than a million illicitly held firearms, thought to be about a third of the guns in Australia at the time.

In the 260 months since the enactment of the gun control legislation, no mass shootings have occurred in Australia. In the 18 years (210 months) prior to and including the Port Arthur massacre, 13 mass shootings (defined as killing five or more people, excluding the shooter) occurred.

"Most people hear these starkly contrasting numbers and conclude that Australia's gun law reforms effectively stopped firearm massacres here. However, some scholars and members of the gun lobby have argued that since mass shootings are relatively rare events, the concentration of incidents in one decade and their absence in another decade is merely a statistical anomaly," said lead author Professor Emeritus Simon Chapman, PhD, from the University of Sydney, in a news release.

Chapman and colleagues at the University of Sydney and at Macquarie University tested the null hypothesis that the rate of mass shootings in Australia before and after the 1996 law reforms has not changed.

They used a rare events model that divides time into independent, nonoverlapping intervals. The P value of less than .001 indicates "strong evidence" of "a structural change in 1996." Even adding a simulated shooting to the time since the legislation went into effect still results in a P value of less than .001. A value of less than than .05 is considered strong evidence against the null hypothesis.

Had the prelegislation rate of mass shootings continued unabated, the investigators calculate, Australia would have experienced 16 additional incidents by February 2018.

In the absence of a randomized controlled trial in which firearms would be allowed in some parts of the country but not others and in which the massacre rates would then be compared, the "standard rare events model provides strong evidence against the hypothesis that this prolonged absence simply reflects a continuation of a preexisting pattern of rare events," the researchers conclude.

Coauthor Philip Alpers, associate professor at the University of Sydney, put the findings into perspective. "Gun lobby–affiliated and other researchers have been saying for years that mass shootings are such rare events it could have been a matter of luck they dropped off in the wake of Australia's gun control laws. Instead, we found the odds against this hypothesis are 200,000 to one. Australia followed standard public health procedures to reduce the risk of multiple shooting events, and we can see the evidence. It worked."

"I believe the authors show sound evidence that supports the perspective that the measures put in place for firearm control resulted in less risk of deaths from mass shootings. Their recognition that these are rare events is noted, but this helps explain the Australian experience and makes it less likely to have occurred by chance alone," Georges C. Benjamin, MD, executive director of the American Public Health Association, told Medscape Medical News.

Similarly, Marcello Pagano, PhD, professor of statistical computing at Harvard's T. H. Chan School of Public Health in Boston, Massachuesetts, told Medscape, "I have reread this article a few times. The beauty of statistics is that there are usually a large number of approaches one can take to opine about any natural occurrence. The one they have taken can be characterized as being abundantly cautious in their findings. One could quibble with minor details, I guess, but the overall finding is convincing and based on sound methodology."

Philip Alpers is director of GunPolicy.org, a global nonpartisan clearinghouse for firearms-related data. Dr Chapman was a member of the Australian Coalition for Gun Control from 1993 to 1996. The remaining authors and the commentator have dislosed no relevant financial relationships.

Ann Intern Med. Published online March 13, 2018. Full text

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