Often, victims are afraid of escalating the violence against them, said James Alan Fox, a criminologist at Northeastern University. He pointed to the murder of eight nursing students in Chicago in 1966 by just one man, Richard Speck. Speck tied the women up in their apartment, promising that all he wanted was money, and then killed them.

“People just oftentimes trust that he wants the money . . . that taking my life is not going to get him anything,” Fox said. “They reason. They believe that, logically, the best move is then not to get them angry.”