When horrific crimes hit the headlines, many people quickly demand that government "do something." Blunt instruments that they are, politicians act in really only one way: by writing and rewriting laws. "That'll be the end of that," they say, as the ink dries on their latest legislative brainstorm.

In the case of the Isla Vista murders, we've seen calls for tighter gun control (even though California already has more restrictive laws than most states), mental health screening, and implications that misogynistic websites should somehow be reined-in. And something about Seth Rogen movies. Maybe a waiting period?

But the truth is that law is a pretty ineffective way to prevent people from doing what you don't want them to do. While laws allow government officials to signal what they consider to be the boundaries of acceptable behavior, and to define the penalties for crossing the boundaries, they aren't very effective at preventing people from stepping over the line.

The idea of laws as deterrents to behavior is an old one; but the escalation in the United States of penalties (now running up against staggering human and monetary cost) for a host of crimes is ample evidence that the laws themselves aren't having the intended deterrent effect. So politicians try to up the ante.

But as Valerie Wright, a research analyst with the Sentencing Project, put it in a 2010 paper, "Research to date generally indicates that increases in the certainty of punishment, as opposed to the severity of punishment, are more likely to produce deterrent benefits."

It's harder to get more severe than killing an offender, but even capital punishment is an uncertain deterrent at best, according to Jeffrey A. Fagan of Columbia Law School. In crunching the numbers, he found "estimates of the deterrent effect of execution that are no different from chance." Given that each execution cost between $2.5 million and $5 million (in 2006), that's an expensive way to not prevent crime.

Actual incarceration seems to lower crime rates, if only by scooping up offenders and keeping them away from opportunities to do things the state doesn't want them to do. But that gets back to that whole "certainty of punishment" issue. That is, you have to catch lawbreakers to incarcerate them, and others then have to see that breaking the law carries a high risk of being caught.

And they have to care about being caught.

This can turn politicians' efforts weirdly futile. Sen. Richard Blumenthal (D-Ct.) took the opportunity of the Isla Vista murders to push for background checks (which the murderer, Elliot Rodger, passed), bans on "assault weapons" (which Elliot Rodger didn't use), and limits on magazine capacity (which already exist in California, so Rodger used legal 10-round magazines) in order to deter spree kiilers who, like Rodger, generally don't intend to survive their crimes.

Blumenthal and company would probably argue that laws more tightly restricting guns would deter others from providing firearms to the likes of Elliot Rodger. But Blumenthal's own state of Connecticut is currently enjoying mass defiance of its new restrictions on so-called assault weapons, with tens of thousands of people refusing to comply.

Which shouldn't be a revelation to anybody who has followed the history of gun laws, drug laws, and prohibitions of various sorts on things that many people want to do, despite the preferences of the powers that be. Instead of deterring violations, such laws create opportunities for black market profit and mass scofflawry. That's because there is minimal chance of any individual violator being caught since those individuals are engaged in violations in private, or with other willing participants. And if there's damned little chance of being caught, there's absolutely no certainty of punishment to act as a deterrent.

Which is to say, if you want laws to be effective at preventing crime, keep them targeted at violations that are generally agreed upon as wrong, easily understood, and easily detected. And even then, you'll only deter those who care about the consequences.

Politicians like to propose ever-tighter laws, because that's really all they have to offer. But much of life is just beyond the reach of the law.