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Active resistance is often considered to be the “normal” reaction during rape, but a Swedish study has found that most victims may experience a state of involuntary paralysis, called tonic immobility. Tonic immobility was also associated with subsequent post-traumatic stress disorder (PTSD) and severe depression after rape. The findings, published in Acta Obstetricia et Gynecologica Scandinavica, indicate that for health care follow-up and legal matters, tonic immobility should be assessed in all sexual assault victims. 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 it in humans, however. The Swedish researchers 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 6 months, 189 women were assessed for the development of PTSD and depression. Of the 298 women, 70% reported significant tonic immobility and 48% reported extreme tonic immobility during the assault. Among the 189 women who completed the 6-month assessment, 38.1% had developed PTSD and 22.2% had developed severe depression. Tonic immobility was associated with a 2.75-times higher risk of developing PTSD and a 3.42-times higher risk of developing severe depression. Prior trauma and a history of psychiatric treatment were also linked with tonic immobility.