Final answer:
Research on the effects of the death penalty and three-strikes sentencing on criminal behavior has provided mixed results, with some studies suggesting a deterrent effect and others showing the opposite. Therefore, deterrence has mixed effectiveness and is not consistently proven.
Step-by-step explanation:
The question is asking whether studies have found a significant deterrent effect on criminal behavior for the death penalty and three-strikes sentencing. Research on this topic has provided mixed results. While some studies suggest that harsh penalties can have a deterrent effect on crime, such as the decrease in gun crime in New York City after longer sentences were introduced, other studies show that certain programs like Scared Straight, which aim to deter criminal behavior in youths, can actually increase crime. Therefore, it can be said that deterrence has mixed effectiveness and is not consistently proven by studies.