Alcohol and Parasomnias: The Statistical Evaluation of the Parasomnia Defense in Sexual Assault, Where Alcohol is Involved.

Sleep sex may be a defense for alleged sexual assault. The International Classification of Sleep Disorders (ICSD3) states: "Disorders of arousal should not be diagnosed in the presence of alcohol intoxication… The former [alcohol blackouts] are exponentially more prevalent." A panel member of ICSD3, quoting ICSD3 asserts: "alcohol intoxication should rule out a sleep-walking defense". This implies extremely strong support for a prosecution hypothesis (H _p ) over a defense hypothesis (H _d ). I use Bayesian methodology to evaluate the evidential probity of alcohol intoxication. The likelihood ratio, LR, measures the amplification of prior odds of guilt, LR = Posterior odds of guilt after considering alcohol intoxication /Prior odds of guilt before considering alcohol intoxication . By Bayes' theorem, LR = p ( alcohol intoxication, given H p ) / p ( alcohol intoxication, given H d ) . I use data from cross-sectional studies of sexual assault and prevalence of alcohol use, in college students, with data from longitudinal studies, and data from the epidemiology of parasomnias to evaluate LR (alcohol). LR ~1.5 or 5, depending whether alcohol does, or does not, increase the risk of parasomnias. The proposition of extremely strong support for H _p implies a LR ~1,000,000, so the proposition in ICSD3 is not supported by formal analysis. The statistical reasoning in ICSD3 is unclear. There appears to be inversion of the Bayesian conditional (confusing intoxication given assault, and assault given intoxication) and failure to evaluate alcohol intoxication in H _d . Similar statistical errors in R. v Sally Clark are discussed. The American Academy of Sleep Medicine should review the statistical methodology in ICSD3.
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