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June 07, 2017

Involuntary paralysis prevents many rape victims from resisting, study finds

Police investigating rape cases seek visible signs of resistance in determining whether a sexual assault has occurred, and absent those signs are difficult to convince that a crime has taken place, studies have indicated.

But during an attack most victims of sexual assault may experience a state of involuntary paralysis, called "tonic immobility," that would make resistance impossible, according to a Swedish study out Wednesday.

The findings, published in Acta Obstetricia et Gynecologica Scandinavica, counter the idea that active resistance during rape is the "normal" reaction.

Tonic immobility was also associated with subsequent posttraumatic stress disorder (PTSD) and severe depression after rape. The study suggests that tonic immobility should be assessed in all sexual assault victims for health care follow-up and legal matters.

Tonic immobility in animals is considered an evolutionary adaptive defensive reaction to a predatory attack when resistance is not possible and other resources are not available. Little is known about tonic immobility in humans, however. Anna Möller, M.D., Ph.D., of the Karolinksa Institutet and the Stockholm South General Hospital in Sweden, and her colleagues assessed tonic immobility at the time of assault in 298 women who had visited the Emergency Clinic for Rape Victims in Stockholm within one month of a sexual assault. After six months, 189 women were assessed for the development of PTSD and depression.

Of the 298 women assessed in the study, 70 percent reported significant tonic immobility and 48 percent reported extreme tonic immobility during the assault. Among the 189 women who completed the six-month assessment, 38.1 percent had developed PTSD and 22.2 percent had developed severe depression. Tonic immobility was associated with a 2.75-times increased risk of developing PTSD and a 3.42-times increased risk of developing severe depression. Prior trauma and a history of psychiatric treatment were also linked with tonic immobility.

"The present study shows that tonic immobility is more common than earlier described," said Dr. Möller in a statement. "This information is useful both in legal situations and in the psychoeducation of rape victims. Further, this knowledge can be applied in the education of medical students and law students."

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