“We have a cultural blind spot about this,” said David Lisak, a clinical psychologist who has done research on interpersonal violence and sexual abuse and is a founding board member of 1in6, an organization that offers information and services to men who had unwanted or abusive sexual experiences as children.

“We recognize that male children are being abused,” Dr. Lisak said, “but then when boys cross some kind of threshold somewhere in adolescence and become what we perceive to be men, we no longer want to think about it in this way.”

Even when high-profile cases dominate the news, said Mai Fernandez, executive director of the National Center for Victims of Crime advocacy organization in Washington, “attention goes to the things we feel more comfortable talking about — such as whether Penn State had done enough, and what will happen to their football program — and not to the question, ‘What do we do to prevent boys from being sexually assaulted?’ ”

In an interview with The Washington Post this month, Joe Paterno, the Penn State football coach who was fired after the abuse scandal erupted and who died of lung cancer on Sunday, said that when an assistant had told him about witnessing an inappropriate encounter between a young boy and Jerry Sandusky, the former assistant coach who is facing charges of sexual abuse, he had been confused and unsure how to proceed. Mr. Paterno said the assistant “didn’t want to get specific. And to be frank with you, I don’t know that it would have done any good, because I never heard of, of rape and a man.”

Much of the research on the sexual assault of men has focused on prisons. But men are also raped outside of prison, usually by people they know, including acquaintances and intimate partners, but occasionally by complete strangers. They are raped as part of violent, drunken or drug-induced assaults; war crimes; interrogations; antigay bias crimes; and hazing rites for male clubs and organizations, like fraternities, and in the military.