Such research is also subject to a more fundamental difficulty affecting all research in this field: the effectiveness of the Criminal Law in preventing crime in general or in specific cases is inherently and notoriously difficult to prove and measure, and thus issues in establishing a causal link between gun control or particular gun control policies and violent crime must be understood to be an aspect of a more general empirical difficulty, which pervades the fields of Criminology and Law at large. It is not simple, for example, to prove a causal connection between the laws against murder and the prevailing murder rates, either. Consequently, this general background must be appreciated when discussing the causal and empirical issues here.