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A crowded dance floor at a club in Northampton, England, earlier this month.ANDREW TESTA/The New York Times

In stark contrast to established statistics that show the majority of sexual assaults occur in the victim's home by a known perpetrator who did not employ drugs, new research from the University of Ottawa shows that, in mass gatherings such as Canada Day celebrations or university frosh weeks, sexual assault is more likely to be committed by a stranger and with the use of some kind of drug.

The research is the first to look at the link between sexual assault and crowd events. It's too early in the research to say if the overall risk of sexual assault increases in these settings, says Dr. Kari Sampsel, study leader and director of the Sexual Assault and Partner Abuse Care Program at The Ottawa Hospital.

Still, Sampsel says she noticed she saw more patients after big events and parties. Upcoming July 1 festivities are of particular concern at the Ottawa clinic. "Canada Day was one of the big, big times that we have a lot of assault," she says.

Struck by the pattern, Sampsel and her team analyzed the 204 sexual-assault cases they saw in 2013, with funding from the $15K Challenge (an initiative of the Women's Xchange, which describes itself as a health-knowledge translation and exchange centre at Women's College Hospital in Toronto).

They found that 25 per cent of the sexual assaults were associated with a mass gathering. Only 33 per cent of those cases involved a known assailant. And 40 per cent of patients said they thought they had been drugged. "That's basically double what we saw in the control group," she says.

Although the drug Rohypnol is most often associated with date rape, previous research from Women's College found that it isn't always used in drug-facilitated sexual assaults. Alcohol, street drugs and even over-the-counter allergy medication are other common choices.

Sampsel says that patients who came to the clinic after a mass gathering were more likely to get a rape kit done, but were less likely to release the information to police. That tendency not to report is backed up by police information.

"We do not have statistics on this, but investigators have not seen increases after large events," said Constable Marc Soucy of the Ottawa Police Service.

Crowd mentality has been studied in association with a long list of violent behaviour. "When you put people together in a crowd, it unrestrains people's behaviour," says Tracy Vaillancourt, a psychology professor at the University of Ottawa.

Members of a crowd experience physiological arousal, such as increased heart rate, and feelings of anonymity that may lead them to act differently. "A larger crowd means you're going to get a lot of people acting less appropriate," says Vaillancourt. "I can imagine that sexual assaults would increase."

Crowds also attract people who might be more likely to commit a sexual assault, including those with frotteurism, a condition that the Diagnostic and Statistical Manual of Mental Disorders classifies as the recurring urge to touch or rub someone who is unconsenting.

Victims of sexual assault at a mass gathering may not be as anonymous as their assailants. Iggy Azalea, an Australian rapper, made headlines in April when she told a New York radio station that fans were reaching inside her clothing to assault her while she crowd-surfed at her own concerts.

Sampsel will soon share the results of her study and discuss possible prevention strategies with the City of Ottawa, local universities and the Alcohol and Gaming Commission of Ontario.

"With so many women affected by sexual assault in their lifetime, it's important the health-care community do what we can to better understand how to improve the health and well-being of women," said Robin Mason, scientific lead of the Women's Xchange.

Funding for the 15K Challenge comes from the Ministry of Health and Long-Term Care.