Freezing up during rape is a common biological response, study finds

"I couldn't move. My body just shut down."

"I froze."

When reporting rape, survivors often recall feeling paralyzed: Aware of what was happening but unable to stop it, because their limbs just wouldn't cooperate. A new study out of Sweden offers an explanation for that feeling of temporary paralysis: tonic immobility, or an involuntary self-defense mechanism cued during moments of extreme fear, which leaves the human body immobile and unresponsive until a threat has passed.

Tonic immobility is essentially the same thing that happens when a possum suddenly stiffens and rolls over, as if dead, or a deer freezes in the headlights. And according to the study, published in the journal Acta Obstetricia et Gynecologica Scandinavica Wednesday, it's also an eminently common response to sexual assault.

Researchers looked at the records for 298 women who had visited a rape crisis center within a one-month timeframe. The researchers found that 70% of the women involved experienced "significant tonic immobility" and 48% experienced "extreme tonic immobility." What's more, tonic immobility was associated with an increased risk of post-traumatic stress disorder and depression after the fact: When 198 of the women were evaluated six months later, 38% had developed PTSD and 22% were left with severe depression.

As psychologist James Hopper explained in the Washington Post, tonic immobility is the fight or flight instinct gone awry: One part of the brain perceives an attack and throws the brakes on bodily movement, sharpening the senses as the brain seeks out an escape route. But that freeze comes with a chemical flood that keeps the brain rational, which helps explain why survivors often remember a number of details from an attack, even as they seemed to be catatonic.

Unfortunately, if tonic immobility is a common response to sexual assault, it's also a commonly accepted defense against rape accusations. Among law enforcement officers, lawmakers and members of the judiciary, "she didn't fight back" has often been enough to nullify rape allegations. Their reasoning is: If something terrible is happening to you, then human instinct is to struggle against the threat. Or, as disgraced Missouri Rep. Todd Akin infamously put it, "If it’s a legitimate rape, the female body has ways to try to shut the whole thing down."

Turns out, sometimes the body just shuts down, full stop. Which is why lead study author Anna Möller told Broadly that tonic immobility "should be routinely assessed in all sexual assault victims."

"The courts may be inclined to dismiss the notion of rape [if] the victim didn't appear to resist," Möller said. "Instead, what might be interpreted as passive consent is very likely to represent normal and expected biological reactions to an overwhelming threat."