There are many pieces of evidence that could be used to make this case. Perhaps the most important such evidence is the number of people who are in our prison systems for non-violent drug crimes.
In the United States today (one can argue), there is too much of an emphasis on punishing crime. This comes out of what Herbert Packer would call the “crime control model” of criminal justice. This model focuses on being as efficient as possible in catching and incarcerating people who commit crimes.
The problem with this is that it overcrowds prisons with people who (arguably) should not be seen as criminals. In this view, our resources would be better spent on trying to help minor offenders do things like getting off of drugs and finding work. This would be better for our country because it would reduce our prison costs and increase the number of people in productive work.
You could, then, use the overcrowding in prisons and the number of inmates who are incarcerated for minor crimes as evidence that we are too conservative when it comes to crime.