Do car alarms really deter theft?

I have seen enough cars hauled off, alarms blaring, by towtrucks or flatbeds to wonder whether some of them were in fact being stolen.

But never in my life have I ever seen anyone poke their head out a window or come running out of their house or building when the alarm goes off. My neighbours did this, about 15 years ago - heard the alarm in the middle of the night, went out to have a look. Didn't see anyone. Discovered in the light of the morning that someone had been trying to jimmy the lock on their SUV. So, yes, alarms do work sometimes. They also immobilize the car so you can't drive it away, which is why a lot of thefts these days involve thieves putting the whole car on a flatbed and then dealing with the alarm in their workshop later.

And http://www.transalt.org/files/newsroom/reports/caralarms/11prevention.html one of the reports linked to in the Wiki article about why the NYPD doesn't like them. I don't know that an alarm going off after the breaking in has started is much of a deterrent, based on all these sources. But I do think that if you have a http://www.autos.com/aftermarket-parts/what-are-the-most-effective-auto-anti-theft-devices or such, it may have a slight deterrent effect and make the criminal more likely to pick the car next to yours without one.

I don't have a cite, but I believe a few states mandate that auto insurance rates are lowered with the presence of car alarms. Apply a little public choice theory and you can imagine where those regulations come from.

No. In my neighborhood, they serve as a skee-ball-style lights-and-sirens reward to squirrels that drop acorns on our cars. (there's a story about squirrels, underemployment, a pneumatic potato gun, and a City of St. Louis Police detective that I could tell, but it doesn't serve to answer the question...)

Not sure about non-anecdotal evidence, but when I hear a car alarm going off in the wee hours of the night for more than 10 minutes, there's nothing more I'd like to do than hit the car with a baseball bat until the beast is dead. So in the sense of others having lesser restraint than my own person, I would suggest that car alarms may increase the likelihood of angry revenge hate tampering.

Car alarms work pretty great as proximity detectors: An alarm sounding may also mean some idiot has scratched/dented your parked car and is making off in a hurry. I don't know that an alarm going off after the breaking in has started is much of a deterrent, based on all these sources. The parking spaces at work (university campus) are somewhat remote, and traffic is pretty thin in the off hours. We get sporadic car break-ins and it looks like that the thieves will keep breaking into more cars as long as things keep quiet - an alarm may not save the car it is attached to, but it definitely helps the ones around it.

My car alarm is extremely useful for finding my car in the parking lot. However, there have been times with the alarm going off with me inside the car due to forgetting to turn it off and no one came running to stop me or even looked twice.

A few years ago our car alarm went off. I opened the front door, didn't see anything suspicious, turned off the alarm, and went back to bed. The next morning I found evidence of somebody trying to jimmy the door, so I called the police. The police in turn lectured me about the stupidity of opening the door to investigate, as setting off the car alarm to get you to open the door would be an easy way to gain access to your house. So in my case, yes, the car alarm averted a crime.