Quote from David D. Friedman,
"Suppose one little old lady in ten carries a gun. Suppose that one in ten of those, if attacked by a mugger, succeeds in killing the mugger instead of being killed by him -- or shooting herself in the foot. On average, the mugger is much more likely to win the encounter than the little old lady. But -- also on average -- every hundred muggings produces one dead mugger. At those odds, mugging is an unprofitable business -- not many little old ladies carry enough money to justify one chance in a hundred of being killed getting it. The number of muggers declines drastically, not because they have all been killed but because they have, rationally, sought safer professions."