Experts say there is no shortage of factors contributing to the abuse, its cover up and the lack of action inside the Vatican.

Karlijn Demasure, the former executive director of the church’s Center for Child Protection at the Pontifical Gregorian University, where she is a professor and expert in the sexual abuse of minors and vulnerable adults, said there is no data on how widespread the problem is. But, she added, anecdotal evidence suggests “it’s not exceptional.”

Many members of the church, experts said, suffer from a medieval mind-set and consider the priests who commit abuse against nuns to be the victims of seductive temptresses. Since the victims in these cases are adults, the experts say, there is also a reflexive tendency to blame them. The reductive public image of the nun as existing to serve the priest and to pray quietly also undercuts those who speak up.

Often, the abuse occurs in a relationship of spiritual guidance, Professor Demasure said, with the priest grooming the victim over time, as is often the case in cases of child sexual abuse.

The apparent preponderance of such abuse in Africa and India has led some in the church to chalk the abuse up to cultural differences.

In many cases, sexual favors have been required of nuns who are financially dependent on priests, and traditions of subservience by women make them vulnerable to abuse.

Ms. Scaraffia said she subscribed to the pope’s critique of abuse, that it is rooted in a rot in clerical culture that leads priests to believe they are a higher authority, and thus entitled to do what they want with their parishioners. In developing countries, where the abuse of nuns seems more prevalent, priests tend to be put on even higher pedestals.