When you install a burglar alarm in your home, you do so presumably because the cost of the alarm is less than the expected benefits. The cost of a burglary, if one occurs, is likely to be much higher than the cost of fitting an alarm. A clearly visible box on an outside wall of an alarmed house alerts and discourages potential thieves. Because your house is less likely to be burgled, your neighbour's house may be more vulnerable. Therefore, while there may be little impact on overall crime levels, expensive security devices reduce crimes against those with the devices but increase crimes against those without.