Consent is the core issue in accusations of sexual assault. If both partners acquiesce to the sex, it’s consensual. If not, it’s rape.

But quite often it’s difficult to distinguish between consent and victimization. How can we tell the difference? Most people—psychologists, police, jurors, journalists, and the general public—judge consent to a large extent based on the “fight or flight” response, the mind-body reaction that kicks in automatically when people feel threatened, especially by predators. Those who feel endangered either combat the peril or flee from it. Consequently, if a victim doesn’t scream, resist, or try to escape, many people would judge the encounter as more or less consensual and not sexual assault.

However, many sexual assault survivors say they neither fought nor fled. Instead, they froze. They felt unable to scream, fight back, or run. They recall feeling paralyzed.

They also report feeling re-abused when family and friends challenged them saying, “What? You didn't scream? You didn’t fight back or run?”

Experts in sexual assault say our either-or concept of fight-or-flight response needs to be updated. People’s psychological responses to threats of violence actually involve three possible reactions: fight, flight, or freezing.

Tonic Immobility

Singer and actor Lady Gaga was sexually assaulted: “I froze.”

Actor Brooke Shields was raped: “I absolutely froze.”

Model Natassia Malthe was sexually assaulted: “I was like a dead person.”

Writer Jackie Hong was raped: “When he pulled down my underwear, my body froze.”

Serial rapist Harvey Weinstein assaulted Jessica Mann who said: “I froze.”

Their reactions were by no means unusual. When attacked by predators, many species freeze or play dead. The scientific term for this is “tonic immobility.” It has been identified in dozens of species of insects, fish, reptiles, birds, and mammals. Since the 19th century, this reaction has been colloquially known as “playing possum,” from the fact that when threatened by predators, opossums play dead. Evolutionary biologists say this behavior evolved in preyed-upon species because many predators lose interest in dead prey.

Humans also experience tonic immobility. One study at Boston City Hospital analyzed 92 women who had been sexually assaulted. One-third of them said they froze, went limp, or dissociated—that is, their minds disconnected from what was happening. Several studies of sexual assault show that only a minority of victims scream, fight, or flee. Meanwhile, in many states, rape laws specify that to prove non-consent, victims must show that they screamed at or fought their attackers or tried to flee from them.

In addition, most women—who are victims of sexual assault at higher rates than men—are socialized to be agreeable, to be nice to others, especially men, and to avoid offending them, which might invite verbal or physical retaliation. Combine that life training with tonic immobility, and it’s easy to see why many women in particular freeze when sexually assaulted.

A psychological defense mechanism related to tonic immobility is dissociation, common among victims of child sexual abuse. Quite often, children can neither fight nor escape adult molesters, so they mentally disconnect from what’s happening to them.

Jim Hopper, a clinical psychologist who teaches at Harvard Medical School, says that in the context of rape cases, the phrase fight-or-flight is harmful in two ways. Most sexual assault victims don’t fight or flee. And legal and cultural assumptions that someone threatened with rape must fight or flee leave those who freeze feeling confused about their behavior, ashamed of themselves, blaming themselves for the assault, and often questioning their sanity. Freezing also raises doubts among rape survivors’ families and friends.

Legal Blind Spot

Psychology researchers have documented rape-related tonic immobility since the 1970s. It’s been an undercurrent in sexual assault research for 40 years. But the concept has largely not emerged from the academic literature into everyday consciousness, and the legal system.

In 2012, Michigan State psychologist Rebecca Campbell analyzed 12 years of data that tracked how six Michigan jurisdictions dealt with sexual assaults. Few cases were prosecuted. More than three-quarters (86 percent) went no further than initial police reports. Why? Because the victims did not kick, scream, flail, or flee, so the police did not believe they’d been raped. Even when there was clear evidence the victims had been forced into sex, almost all first-responding police officers believed that without clear resistance, the sex was either consensual or insufficiently nonconsensual to hold up in court.

The situation is changing, but slowly. Today, some police departments train officers who respond to sexual assault calls to understand the possibility of tonic immobility, and not dismiss the allegations of sexual assault victims who react that way.

But the either–or fight-or-flight paradigm is still so deeply embedded in our culture that it’s going to take years for police, prosecutors, jurors, sexual assault survivors, and their friends and families to understand that when many people face sexual assault, it’s perfectly natural for them to go limp, dissociate, or freeze.