There was a study by the University of Florida back in the early 90's on the death penalty as a crime deterrant. The researchers surveyed several hundred "career" criminals (i.e. 3 or more violent/armed convictions) and found that while the DP was not a deterrant, other things were to varying degrees.
Surveyed robbers, rapists and murderers had no real fear or arrest, prison or the chair. Locks and alarm systems were seen as challenges, but the number one thing that they admitted to fearing was an armed victim. Police are trained to shoot as a last resort. John Q. Citizen typically opens fire without hesitation.
BTW Dogs were found to be the best overall deterrant to robbers...dealing with a dog is just not worth the trouble apparently