Swiss cheese and gun violence


Preventing tragedies like Newtown isn't just about reducing the availability of guns. We need to look at all of the variables.

In 2000, James Reason, professor of psychology at the University of Manchester, proposed a Swiss cheese model to describe the alignment of failures that must occur in a system to allow an unintended bad outcome.1

Such an alignment might occur because of an active failure (mistake, error, or procedure violation) on the part of a person combining with flawed conditions that are a part of a system but remain unnoticed or “latent” until they align with the active failure.

Reason argued that in organizations with outstanding records of safety such as nuclear aircraft carriers, air traffic control systems, and nuclear power plants, there is what he described as a “collective preoccupation with the possibility of failure.” He concluded: “For these organizations, the pursuit of safety is not so much about preventing isolated failures, either human or technical, as about making the system as robust as is practicable in the face of its human and operational hazards.” He stated further: “We cannot change the human condition, but we can change the conditions under which humans work.”

Reason’s model has been widely used in discussions of patient safety to illustrate the importance of recognizing system flaws that can lead to adverse patient outcomes. In these discussions, it is acknowledged that human error, however regrettable, is inevitable and that prevention of adverse outcomes requires development of systems that prevent human error from leading to harm.

On December 14, 2012, the system failed 20 children and 7 adults in Newtown, Connecticut. That same system also failed Adam Lanza. We do not know the details of Lanza’s psychological or emotional history. We also don’t know the circumstances that allowed him access to his mother’s weapons or why she had those weapons in her home.

We do know, however, that the combination of Lanza’s mental state and the availability of guns and ammunition sufficient to murder 27 people as well as for Lanza to kill himself was an alignment of human and system flaws that our society should not tolerate.

And, of course, it was not an isolated incident. In 2010 alone, homicide by firearm was the most common cause of violence-related death in persons aged between 1 and 22 years (3,236 deaths) and the second highest cause of death overall in that age group, exceeded only by unintentional injuries.2

The tragedy at Sandy Hook Elementary School has amplified the voices that had already been raised after other gun massacres in recent years. Calls for enhanced and accessible mental health services and for legislation to limit access to guns and ammunition will be synthesized in the work of a Presidential task force to be led by Vice President Biden.

As these efforts move forward, it will be important to keep the Swiss cheese image in mind and to recognize that flawed human beings are the only kind we have. If we want to prevent another Sandy Hook or Aurora, Colorado, tragedy, the systems that surround us must prevent even the most disturbed person from finding the succession of holes that lead to gun violence.


1. Reason J. Human error: models and management. BMJ. 2000;320(7237):768-770.

2. Centers for Disease Control and Prevention. National Center for Injury Prevention and Control. Welcome to WISQARS (Web-based Injury Statistics Query and Reporting System). Last updated December 17, 2012. Accessed January 10, 2013.

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