I affirm that the hypothesis is not solidly proven at this point. Although I also wonder what else could explain the widespread fall in criminality. Is it possible that the 90s was just the point where we reached economic sufficiency for a large swathe of people who would otherwise have been criminals to attain more? Maybe violent video games have diverted the little criminal in many of us (I jest but I have heard this idea floated in some places)? Did people switch to drugs that were less likely to cause criminal behavior? It would be nice to have an answer but I suspect that we will never have more than questions and conjectures on this one.