There's a scientific reason rape victims don't 'just fight back'.

Content warning: This post deals with issues related to sexual assault and rape which may be triggering for some readers.

One of the most common questions used to undermine victims of sexual violence is “why didn’t you fight back?”

It’s imbued with the sneering inference that a woman who does not actively resist her attacker has in some way resigned herself to being raped.

Not only does the question reflect victim-blaming of the highest order, it also ignores the simple fact that many women – a majority according to a new study – actually become paralysed during unwanted sexual contact.

“The courts may be inclined to dismiss the notion of rape [if] the victim didn’t appear to resist,” lead author of the study Dr. Anna Möller explained to Broadly.

"Instead, what might be interpreted as passive consent is very likely to represent normal and expected biological reactions to an overwhelming threat."

According to the study by the researchers from Sweden's Karolinska Institute, while the 'normal' reaction during sexual assault might logically be to try and break free, similar to animals, humans facing extreme threats may find themselves in a state of "tonic immobility" or, more simply put, paralysis.

It considered the experiences of 298 women who visited the Emergency Clinic for Rape Victims in Stockholm within one month of their sexual assaults.

Of these, as many as 70 per cent reported experience significant tonic immobility and 48 per cent reported extreme tonic immobility during the assault.

LISTEN: When the lines of consent are blurred (post continues)...

A link was also established between tonic immobility and the development of post-traumatic stress disorder and depression.

Of 189 women who agreed to be monitored for six months after being assaulted, 38.1 per cent developed PTSD and 22.2 per cent developed severe depression.

Those who had experienced tonic immobility were found to have be 2.75-times more likely to affected by PTSD and had a 3.42-times increased likelihood of becoming severely depressed.

Women who had previously suffered trauma were also more likely to become involuntarily immobile.

"The present study shows that tonic immobility is more common than earlier described," Dr. Möller said in the report.

"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."

If you or someone you know is impacted by sexual assault, domestic or family violence, call 1800RESPECT on 1800 737 732 or visit 1800RESPECT.org.au. In an emergency, call 000.