There are many statistics that "explain" many things such as what the chances are of something happening or how often it occurs. Some even tell you why things happen. There are many agents that cause these actions or prevent them from happening. One of the most common misonceptions is that drugs and alcohol cause violence. Richard J. Gelles and Mary M. Canavaugh wrote an article called Association is not Causation. Just because people who carry out violent acts do use these substances, it does not mean that it is caused by the drugs and alcohol.
I believe that drugs and alcohol do play a significant role in causing someone to be violent. whether it's the leading cause or not, it doesn't really matter. If it were, there would be a stricter control on the circulation of drugs and alcohol. However, people should not wait until something gets out of hand before they take the neccessary action against it.