Rates of insanity acquittals and the factors associated with successful insanity pleas

The frequency of use of the insanity plea, the probability of being acquitted by reason of insanity given the plea has been used, and the overall volume of insanity acquittees was determined for seven states. Across the seven states, there was an inverse relationship (r = -.67) between the frequency of use and the likelihood of success. As a result, the overall volume of insanity acquittees was stable. Based upon a sample of 8,138 people indicted for a felony and who raised an insanity plea at some point during the processing of their cases, the characteristics of insanity defendants and their cases differed by state, and a number of these characteristics, particularly diagnosis and type of crime were related to the likelihood of an insanity acquittal. The inverse relationship between the plea and success rates was at least partially explained by the differences in the composition of the cases. States with high plea rates tended to have a higher proportion of cases that involved defendants who were unlikely to succeed. States with lower plea rates tended to have higher proportions of cases that involved defendants who were likely to succeed.