Survivors of sexual assault who come forward often confront doubt on the part of others. Did you fight back? they are asked. Did you scream? Just as painful for them, if not more so, can be a sense of guilt and shame. Why did I not resist? they may ask themselves. Is it my fault? And to make matters worse, although the laws are in flux in various jurisdictions, active resistance can be seen as necessary for a legal or even “common sense” definition of rape. Unless it is clearly too dangerous, as when the rapist is armed, resisting is generally thought to be the “normal” reaction to sexual assault.

But new research adds to the evidence debunking this common belief. According to a recent study, a majority of female rape survivors who visited the Emergency Clinic for Rape Victims in Stockholm reported they did not fight back. Many also did not yell for help. During the assault they experienced a kind of temporary paralysis called tonic immobility. And those who experienced extreme tonic immobility were twice as likely to suffer post-traumatic stress disorder (PTSD) and three times more likely to suffer severe depression in the months after the attack than women who did not have this response.

Tonic immobility (TI) describes a state of involuntary paralysis in which individuals cannot move or, in many cases, even speak. In animals this reaction is considered an evolutionary adaptive defense to an attack by a predator when other forms of defense are not possible. Much less is known about this phenomenon in humans, although it has been observed in soldiers in battle as well as in survivors of sexual assault. A study from 2005, for example, found 52 percent of female undergraduates who reported childhood sexual abuse said they experienced this paralysis.

The new study, published in Acta Obstetrecia et Gynecologica Scandinavica,reports that of nearly 300 women who visited the rape clinic, 70 percent experienced at least “significant” tonic immobility and 48 percent met the criteria for “extreme” tonic immobility during the rape. (The condition’s severity was assessed using a scale that measured feelings of being frozen, mute, numb and so on.)

This latest research is important because of its large sample size (298 women reporting, 189 of whom returned for a follow-up assessment after six months) and because they related their experience within 30 days of the assault, thus reducing the possibility of faulty recall. These findings strongly support previous research that links this involuntary paralysis with greater psychological harm following the assault. The 2005 study, for example, found an association between having experienced tonic immobility and significant psychological impairment.

The connection between this paralysis response and suffering greater PTSD and depression makes sense at the intuitive level, clinicians say. Women, men and children who are sexually assaulted and think they should have resisted but did not may also be prone to feeling guilt and shame. The correlation is strong although it does not prove causality. “I am not surprised that tonic immobility is common,” says University of Sydney psychiatrist Kasia Kozlowska, who has recently published, with her colleagues, a study in the Harvard Review of Psychiatry about the brain’s involuntary defense mechanisms in humans and other animals. “After all,” she wrote in an e-mail, “tonic immobility is designed to activate when there is contact with a predator (akin to the sexual abuse situation). Theoretically, one could expect it to activate when there is physical contact, high arousal and fear, and no possibility of running away.”

This “rape-induced paralysis,” she explains, is one of six automatically activated defense behaviors in animals and humans that make up the “defense cascade.” Typically, nonhuman animals are programmed to go through each of the states as the proximity of the danger escalates. The stages are: arousal (alertness to possible danger); freezing (momentarily putting flight or fight on hold while assessing danger); “flight or fight”; tonic immobility; collapsed immobility (fainting in fear); and quiescent immobility (a subsequent state of rest that promotes healing). People who experience sexual assault may go through several of these stages, or skip straight to tonic immobility.

Each of the defense reactions, she explains, involves activation of motor and arousal centers in the brain and changes in pain and sensory processing. When flight or fight is possible, motor programs for running or fighting are activated, the arousal system is switched to a high-energy setting and nonopioid analgesia is switched on. This helps the victim either run away or fight the predator. When flight or fight is not possible, immobility motor programs are activated, causing the paralysis. At the same time, the arousal system is switched to a low-energy setting, and the brain is flooded with “opioid analgesia” to reduce the intensity of the fear and pain.

Humans and other animals cannot control these defense mechanisms. In humans who are being raped, tonic immobility may be immediately triggered when their sensory inputs (touch, smell and so on) reach a critical threshold and they feel there is no escape.

The implications for rape survivors in the legal system are immense, experts say. If courts demand these people prove they resisted, says Kozlowska, “these courts are actually causing psychological harm to the women and failing to recognize the body’s innate response to serious attack.” Police and soldiers, she adds, also experience tonic immobility in traumatic situations and similarly suffer from unnecessary guilt.

The phenomenon of tonic immobility during an attack is not well known within the legal and judicial system, but people working with sexual assault survivors have long been aware of it, says James Hopper, a psychological trauma expert and teaching associate at Harvard Medical School. Since 2012 Hopper has been training civilian and military investigators and prosecutors around the country, and has found them very receptive.

It is critical, says Karolinska Institute gynecologist Anna Möller, the current study’s lead author, for rape survivors themselves to understand that their ability to fight was out of their conscious control. Education could be instrumental in altering their interpretations of their behavior after the fact, reducing their shame and guilt. It could provide them, the study authors say, “with evidence that they do not choose the path their bodies ultimately went down.”

For more on the prevalence of tonic immobility during sexual assault, go to this site. For more on the relationship between tonic immobility and PTSD, see this site. For more on the protective function of tonic immobility, see here.