Research studies show alcohol is present in a significant proportion of aggressive and violent events, but the same is not always true of illegal drugs. Some research suggests alcohol has physiological effects that increase aggressive behavior. Other theories of alcohol and aggression indicate alcohol reduces inhibitions and weakens brain mechanisms that normally restrain impulsive behavior. Research on the physiological effects of illicit drugs have not demonstrated increased levels of aggressive behavior. To the contrary, in the case of heroin and marijuana, aggressive behavior appears to decrease. With respect to violent crime, 50 percent of violent offenders in State prisons reported being under the influence of alcohol or drugs at the time of their offense. Substance abuse has also been implicated in child abuse, rape, and domestic violence cases. Reasons why causal relationships between substance abuse and violence have not been conclusively proven are noted. 39 references and 27 endnotes