When a person is being raped, they may have every intention of fighting off their attacker, but it might not be possible — many victims experience an involuntarily state where they cannot move their bodies, called tonic immobility, according to new research.

It’s a type of paralysis that’s often been observed in animals as a way for them to avoid or deter predators, but there’s not much known about how it affects humans. While prior studies have indicated it may occur among rape victims, a team of researchers from Sweden wanted to see how often it occurs, and if it’s followed by post traumatic stress disorder (PTSD) and severe depression.

Read: Young Rape And Sexual Assault Victims Show Reduced Cognitive Function, Maternal Instinct

In the study, published in Acta Obstetricia et Gynecologica Scandinavica, researchers looked at data from nearly 300 women who visited a clinic for rape victims within one month of assault. Their findings revealed that 70 percent of the women reported tonic immobility and 48 percent said they experienced extreme tonic immobility during the assault.

"The present study shows that tonic immobility is more common than earlier described," study author Dr. Anna Möller said in a news release. "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."

Additionally, 189 of the women completed a 6-month assessment, which revealed about 38 percent had developed PTSD - a mental health condition that's triggered by a frightening event. Symptoms can include flashbacks, nightmares, anxiety, and uncontrollable thoughts about the event, according to Mayo Clinic. About 22 percent of the women also developed severe depression.

Read: Half Of Male College Athletes Admit History Of 'Sexually Coercive' Behavior Such As Sexual Assault, Rape

Michelle Bovin, a psychologist not involved in the study, says the research doesn’t necessarily show that tonic immobility was responsible for raising the risk of PTSD. But, feeling paralyzed during an assault is of course terrifying, she told Bustle.

“You can’t move, you know this horrible thing is happening to you, and you don’t know why,” said Bovin, who works at the Veteran’s Association National Center for PTSD. “So a lot of the people that I treat, who had what I assume was tonic immobility during a rape, experience huge levels of guilt, because they don’t understand why they didn’t move.”

Every 98 seconds, a person is sexually assaulted in the United States and nearly every eight minutes the victim is a child, according to the Rape, Abuse, and Incest National Network (RAINN). Additionally, a majority of the victims are female and are raped in their own home.

See also: Chessy Prout, St. Paul's School Sex Assault Victim: Advice To Rape Survivors From Experts

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