Two terrible incidents of violence occurred last week. Michael McLendon went on a killing rampage in Alabama that took the lives of 10 people before he killed himself. Half a world away in Germany, at about the same time, Tim Kretschmer attacked a school and murdered 15 people before killing himself. All told, 27 people died in these two incidents of mass murder.

The news media in the United States has spent enormous amounts of time covering both incidents. We have not watched the German media, but it too has probably focused a lot of attention on the two incidents. It is probably safe to assume that news watchers in both countries have received a healthy dose of mass murder during the past several days.

It will be interesting to see what results from these two incidents. To the extent that the past is prologue, we should expect to see plenty of public fear and extreme reactions from officials and politicians. Both can be traced, at least to some degree, to a cognitive shortcut called the availability heuristic.

We use the availability heuristic to estimate the frequency of specific events. For example, how often are people killed by mass murderers? Because higher frequency events are more likely to occur at any given moment, we also use the availability heuristic to estimate the probability that events will occur. For example, what is the probability that I will be killed by a mass murderer tomorrow?

We are especially reliant upon the availability heuristic when we do not have solid evidence from which to base our estimates. For example, what is the probability that the next plane you fly on will crash? The true probability of any particular plane crashing depends on a huge number of factors, most of which you're not aware of and/or don't have reliable data on. What type of plane is it? What time of day is the flight? What is the weather like? What is the safety history of this particular plane? When was the last time the plane was examined for problems? Who did the examination and how thorough was it? Who is flying the plane? How much sleep did they get last night? How old are they? Are they taking any medications? You get the idea.

The chances are excellent that you do not have access to all or even most of the information needed to make accurate estimates for just about anything. Indeed, you probably have little or no data from which to base your estimate. Well, that's not exactly true. In fact, there is one piece that evidence that you always have access to: your memory. Specifically, how easily can you recall previous incidents of the event in question? The easier time we have recalling prior incidents, the greater probability the event has of occurring - at least as far as our minds are concerned. In a nutshell, this is the availability heuristic.

Of course, any rational person understands that this method of estimation is flawed. Just because you happened to see a clown get run over by a dump truck yesterday and you can now easily recall this event, this doesn't mean that this sort of thing happens all of the time. Likewise, just because a plane crashed recently or two mass murders occurred last week, this doesn't make these events any more likely either. Nevertheless, studies on the availability heuristic consistently show that we estimate the probability of events occurring based in large part on how easily these events come to mind.

As this relates to the recent mass murders, it is likely that people will become, at least for a time, more fearful that they or someone they know will be the victims of the next shooting incident. Politicians, whose jobs depend upon being in tune with the concerns of their constituents, and who are likely themselves to overestimate the likelihood of the next mass murderer coming to their towns, will probably introduce heavy-handed policies, such as banning literature that might incite the next perpetrator (police in Alabama discovered a stash of videos in the home of the gunman that instructed how to, for example, shoot from a moving vehicle). While these interventions will likely have little to no effect on future occurrences of mass murder, they will make people feel like something is being done to protect them from the boogeyman that now seems certain to live in their neighborhood.

Although there are many problems associated with the availability heuristic, perhaps the most concerning one is that it often leads people to lose sight of life's real dangers. Psychologist Gerd Gigerenzer, for example, conducted a fascinating study that showed in the months following September 11, 2001, Americans were less likely to travel by air and more likely to instead travel by car. While it is understandable why Americans would have been fearful of air travel following the incredibly high profile attacks on New York and Washington, the unfortunate result is that Americans died on the highways at alarming rates following 9/11. This is because highway travel is far more dangerous than air travel. More than 40,000 Americans are killed every year on America's roads. Fewer than 1,000 people die in airplane accidents, and even fewer people are killed aboard commercial airlines. The bottom line is that being a passenger on a plane being flown by trained professionals who are being guided by a team of professionals (i.e., air traffic control) is much safer than driving your own car on streets surrounded by other amateur drivers who may or may not follow the rules of the road (and whose cars may or may not be fit to drive). Nevertheless, I (JF) almost always worry that my plane will crash, but I rarely even consider the dangers of driving - and I teach the availability heuristic every semester! It just shows how powerful this cognitive shortcut really is.

Back to the killings in Alabama and Germany...The probability that any of us or anyone we know will ever become the victim of mass murder is almost too low to imagine. If we focus too many resources on trying to prevent this from ever happening again, we will likely expose ourselves to more mundane but much higher probability dangers, such as accidental shootings (which take far more lives than all of the mass murders put together). And this goes for anything whose probability is influenced by the availability heuristic (which is just about everything).

Consider, for example, that the 2009 budget for homeland security (the folks that protect us from terrorists) will likely be about $50 billion. Don't get us wrong, we like the fact that people are trying to prevent terrorism, but even at its absolute worst, terrorists killed about 3,000 Americans in a single year. And less than 100 Americans are killed by terrorists in most years. By contrast, the budget for the National Highway Traffic Safety Administration (the folks who protect us on the road) is about $1 billion, even though more than 40,000 people will die this year on the nation's roads. In terms of dollars spent per fatality, we fund terrorism prevention at about $17,000,000/fatality (i.e., $50 billion/3,000 fatalities) and accident prevention at about $25,000/fatality (i.e., $1 billion/40,000 fatalities). This huge imbalance tells us that our priorities are seriously out of whack. (And don't even get us started on bigger killers like heart disease!)

The take-home message of all of this is that we should be a lot less afraid of many of the things that scare us. Yes, terrible things such as plane crashes, terrorism, and mass murder do happen. Likely each of these things will happen several more times before the year is finished. But the good news is that the chances that any of us will be affected by any of these events are so remote that we can safely relax and not worry about them. To the extent that we do try to prevent scary things from happening, we should put forth more effort to prevent real dangers like car accidents, heart attacks, and diabetes. Interestingly, many of the real dangers are things that we have a lot of control over (unlike mass murder). Therefore, to the extent that we try to prevent them, we might actually improve our quality of life.

(This post was co-authored by Ilan Shrira)