“This is exhibitionism, an extreme form of it,” he said. “They rarely escalate. The exhibition, the actual displaying of the genitals is the goal.”

For exhibitionism alone, the behavior can include leaving dirty notes behind for someone to see, flashing someone with and without an erection, or masturbating in front of others. The last, Cantor said, can be very frightening for the victim, because they don’t know what’s going to happen next.

“The rules have gone out of the window, and she doesn’t know if this will escalate into violence or rape,” he said. “But for these perpetrators, showing off their genitals is the goal, and they are more interested in that than in hands-on sex.”