....Social scientists generally, and criminologists especially, often lack the ability (usually due to both ethical and practical concerns) to perform randomized controlled trials, the gold standard of research. We might expect, for instance, that having low levels of self-control is a cause of criminal behavior. In fact, some of the most powerful explanations of crime have been built on this idea, and there is much evidence to support it. We might also hypothesize that bad parenting causes children to develop low levels of self-control. Yet we can’t randomly assign people to have different levels of self-control, and we most assuredly...