You’ve likely heard of the fight-or-flight response — but what about freeze?

A new study published in the journal Acta Obstetricia et Gynecologica Scandinavica suggests that many people who are raped experience involuntary paralysis during the assault.

This type of paralysis is also known as tonic immobility.

It’s a temporary motor inhibition that can make it impossible for someone to call out, flee, or fight back during a traumatic encounter.

“It’s actually a biological response,” lead author, Dr. Anna Möller, of the Karolinska Institutet and Stockholm South General Hospital in Sweden, told Healthline. “When a person or animal is exposed to a threat, the parasympathetic nervous system gets activated, meaning that you get a low pulse, you get low blood pressure, and you can get passive or paralyzed.”

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A precursor for depression and PTSD?

Möller and colleagues used the Tonic Immobility Scale to assess 298 women who visited the Emergency Clinic for Rape Victims in Stockholm within one month of being sexually assaulted.

After six months, they assessed 189 of those women for features of post-traumatic stress disorder (PTSD) and depression.

They found that 70 percent of the 298 participants reported experiencing symptoms of significant tonic immobility while being sexually assaulted. Forty-eight percent reported experiencing symptoms of extreme tonic immobility.

Among the 189 participants who took part in the six-month follow-up assessment, approximately 38 percent had developed PTSD, and 22 percent had developed severe depression.

Participants who reported experiencing tonic immobility were more than twice as likely to develop PTSD, and more than three times as likely to develop depression.

Those with a past history of sexual assault were at higher risk of experiencing tonic immobility, as well as PTSD and depression.

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Survivors don’t fit ‘ideal victim’ stereotype

Tonic immobility is thought to be an evolved defensive response to intense fear.

Most research on the phenomenon has focused on nonhuman animals.

But in recent decades, investigators have increasingly studied tonic immobility in humans, particularly among people who survive sexual assault.

While past studies have linked tonic immobility to PTSD, this is the first investigation to prospectively examine that association.

More research is needed to learn why survivors of sexual assault who experience tonic immobility are more likely to develop PTSD.

The responses they receive following the assault might play a part.

The stereotype of an “ideal victim” is someone who actively resists an assailant.

When someone doesn’t fit that stereotype, it can influence how they perceive themselves and how they are treated by others.

“Shame and guilt are some of the most common reactions among our patients,” Möller noted. “This is a very common reaction: ‘Why didn’t I say “No,” or push him off?’”

People who don’t actively resist may also be taken less seriously or granted less sympathy by other community members, including family, friends, law enforcement, and those involved in trying legal cases.

In turn, experiences of blame and skepticism can negatively impact survivors’ mental health.

“If you run into this paralyzed state and don’t resist, and you blame yourself and get blame from others, and maybe the legal system questions what really happened — that’s also important in the development of PTSD,” Möller suggested.

“It’s not just what you’ve experienced before and during the trauma itself, it’s also what happens afterwards. Did you get a lot of social support? Or are you being questioned? That’s going to affect your risk of developing PTSD.”

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Greater awareness needed

To promote better support for people who survive sexual assault, Möller emphasized the importance of raising awareness of tonic immobility.

“I think counsellors and health personnel who work with rape victims should know about this and talk about this with their patients, saying, ‘It’s not strange that you didn’t resist because it’s the most common way of reacting. It’s nothing wrong with you. You did what you could.’”

She also advocated for changes in how sexual assault is managed in legal systems.

For one thing, she argued that silence and passiveness should not be considered consent.

For another thing, she suggested that less emphasis should be placed on complainants’ injuries.

“In court proceedings, there’s still a lot of attention given to injuries — did the rape cause any injuries? But if the victim doesn’t resist, the perpetrator doesn’t have to use a lot of physical violence in order to continue with the rape, and there won’t be many injuries.”

As a next step in the research process, Möller plans to investigate the occurrence of tonic immobility among men who have survived sexual assault.