Installing metal detectors at movie theaters doesn’t make sense — there’s no reason to think the next crazy gunman will choose a movie theater as his venue, and how effectively would a metal detector deter a lone gunman anyway? — but understanding the reasons why the United States has so many gun deaths compared with other countries does. The particular motivations of alleged killer James Holmes aren’t relevant — the next gunman will have different motivations — but the general state of mental health care in the United States is.
Even with this, the most important lesson of the Aurora massacre is how rare these events actually are. Our brains are primed to believe that movie theaters are more dangerous than they used to be, but they’re not. The riskiest part of the evening is still the car ride to and from the movie theater, and even that’s very safe.
You know how people are always moaning that they don’t use mathematics in real life? This is one reason why they should. Probabilities are a branch of math, and you have to understand probabilities to make a decent assessment of risk. Most people don’t understand that.
There are plenty of other public policy issues that can’t be understood without knowing how probabilities work. People who reject biological evolution often think it’s obviously not true, but they have no idea of the math involved, and that’s where the real story is.