Many people assume that during a sexual assault, the victim will fight back. But a new study from Sweden finds that, during an attack, it's common for victims to experience an innate, defensive reaction that renders them paralyzed.

This physiological response, called "tonic immobility," is normal in the face of extreme fear and happens involuntarily during an attack, according to the study. In other animals, this reaction is sometimes referred to as "playing dead."

In other animals, tonic immobility is a survival tactic: If an animal looks dead, a predator may not attack, said lead study author Dr. Anna Möller, an OB-GYN at Stockholm South General Hospital in Sweden. But during a sexual assault, this reaction does not often make the attacker stop the assault. [5 Misconceptions About Sexual Assault]

Although little is known about how the response works in humans, it's been described as a "catatonic-like state" in which a person cannot move, may be unable to speak and is unresponsive, the researchers wrote in the study.

Initially, when a person is attacked, the sympathetic nervous system, which is responsible for the fight-or-flight response, is activated, Möller told Live Science. But tonic immobility is the result of the body's subsequent response: It occurs when the parasympathetic nervous system gets activated, which acts as a counterbalance to the sympathetic nervous system, and prevents muscle movement.

Tonic immobility during sexual assault is quite common: In the new study, researchers interviewed female sexual assault survivors and found that 7 out of 10 reported "significant" immobility during an assault, and nearly half experienced "extreme" immobility. The findings are published today (June 7) in the journal Acta Obstetricia et Gynecologica Scandinavica.

The study "shows that tonic immobility is more common than earlier described," Möller said in a statement. "This information is useful in both legal situations and in the psychoeducation of rape victims."

In legal situations, courts may dismiss a sexual assault case if the victim or the attacker doesn't have any signs of physical struggle indicating that the victim fought back, the authors wrote in the study. For example, they pointed to a 2002 study from Florida that was published in the journal Annals of Emergency Medicine, which showed that attackers were more likely to be prosecuted if the victims showed signs of trauma.

But a victim's passiveness should not be considered consent, Möller said. Many survivors of sexual assault blame themselves for not having fought back more, so it may help them to understand that becoming immobilized is in fact normal, she said. Indeed, tonic immobility can even occur in people who have been trained to fight off an assault, Möller added.

In the study, the researchers gave questionnaires to nearly 300 women who visited the Emergency Clinic for Raped Women in Sweden between February 2009 and December 2011. The questions assessed whether the women had experienced tonic immobility during their assault, as well as whether they had post-traumatic stress disorder (PTSD), acute stress and depression. Six months later, the researchers followed up with the women and got responses from 63 percent of them, again evaluating them for PTSD, acute stress and depression. [6 Ways Sexual Harassment Damages Women's Health]

Seventy percent of the women reported that they had experienced tonic immobility during their assault, including 48 percent who said that the immobility was "extreme." In addition, 81 percent of the women reported having had significant fear during their assault.

The study also found that the effects of the immobility reaction extended far beyond the assault itself: Women who experienced tonic immobility were more than twice as likely to have PTSD six months after the assault, and 3.4 times more likely to develop severe depression, compared with the women who did not experience tonic immobility during their assault.

Women who had been sexually assaulted previously were twice as likely to be immobilized during their recent assault, the study found. And more severe assaults, such as those involving moderate or severe physical violence, were twice as common among the women who reported tonic immobility. Women who drank alcohol sometime during the 12 hours before the assault were less likely to experience this form of paralysis.

The researchers also found that women who experienced tonic immobility were twice as likely to have PTSD from a previous experience, three times more likely to have acute stress disorder and 7 percent more likely to have severe depression two weeks after the most recent assault, compared with the sexual assault survivors who did not experience tonic immobility during their assault.

Editor's Note: This article was updated on June 7 to include information from the study author.

Originally published on Live Science.