Do home security alarm systems really deter criminals?

In my experience,yes, especially as concerns residential burglars. It's a matter of minimizing risk, why take the house with the alarm when there is less risk in choosing one that isn't alarmed. Burglars expect commercial property to be alarmed, so if they can';t disable it they count on a fast in and out before the police arrive. I had an attempted break in at my house last year. As soon as the sensors went off the offenders ran away. I wasn't home, but they didn't get anything.