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.”

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.