How many men are paedophiles?
The Pope was recently reported to have said that about 2% of Catholic clergy are paedophiles. But how does this compare with society as a whole - is it more or less than average?
As soon as you give this question a moment's thought, you realise that it's not going to be an easy one to answer. Paedophiles are not easy to identify.
"Because paedophilia is so secretive and so few people are willing to admit it, there is no meaningful way to get a reliable estimate," says Dr James Cantor, a psychologist and sexual behaviour scientist at the University of Toronto.
"There's no meaningfully ethical way of taking 200 men, hooking them up to detectors, showing them pictures of adults and children and seeing how many respond most to children."
One person who has attempted an estimate is Dr Michael Seto, a clinical and forensic psychologist at the Royal Ottawa Healthcare group.
In 2008 he wrote a book in which he put the prevalence of paedophilia in the general population at 5%.
The figure was based on surveys conducted in Germany, Norway and Finland in which men were asked whether they had ever had sexual thoughts or fantasies about children or engaged in sexual activity with children.
But Seto stresses that 5% was an upper estimate, and that the studies were limited in what they revealed.
"What those surveys don't include are questions on the intensity of those thoughts and fantasies, whether they were repeated or not. Someone might say 'Yes' because they once had a fantasy but our understanding of paedophilia would be that that person recurringly had sexual thoughts and fantasies about children."
Now, with more data and better methodology, he has revised his figure down to about 1% of the population, though he makes clear this is still only an educated guess.
One problem is that the term "paedophile" means different things to different people.
"It's very common for regular men to be attracted to 18-year-olds or 20-year-olds. It's not unusual for a typical 16-year-old to be attractive to many men and the younger we go the fewer and fewer men are attracted to that age group," says Cantor.
He thinks that if we say that a paedophile is someone attracted to children aged 14 or less, then he estimates that you could reach the 2% figure.
"If we use a very strict definition and say paedophilia refers only to the attraction to pre-pubescent children [then it] is probably much lower than 1%," he says.
The term is often applied to a person who sexually abuses someone below the age of 16, but given that in some countries - and even some US states - you can marry below the age of 16 this definition would clearly not be universally accepted.
There is consensus on the clinical definition. Michael Seto and his colleagues agree that a paedophile is someone who has a sexual interest in pre-pubescent children, so typically those under the ages of 11 or 12.
But whether the prevalence using this definition is 0.5%, as James Cantor says or 1%, as Michael Seto says, you can be assured than in any large group of people - whether they be politicians, entertainers, or Catholic clergy - you are likely to find some paedophiles.
Paedophilia is not restricted to men - some women also sexually abuse children, although research suggests this is much less common.
But back to the Pope. How would he define "paedophile"? We don't know, but there is a clue.
There is one well-known study of child abuse among Catholic clergy, carried out by John Jay College of Criminal Justice in New York. Its researchers went to each diocese in the US and found all the plausible accounts of abuse involving clergy who served between 1950 and 2002 - and they found that 4.2% of had been plausibly accused of abuse.
That included allegations of abuse of adolescents as well as pre-pubescent children.
But if you consider just pre-pubescent children, the figure drops to between 1-2% according to Prof Philip Jenkins from the Institute of Studies of Religion at Baylor University in Texas. This corresponds, more or less, with the figure attributed to the Pope.
"If he was using a different word like 'abusive clergy' then I think he would be going for a higher figure," says Jenkins.
The John Jay College study is not perfect, though. For some reason, 40% of the allegations referred to abuse said to have been carried out in a six-year period between 1975-1980.
It seems unlikely that cases of child abuse in the clergy would have been so heavily concentrated in one period. Furthermore, even if there was a peak in the 1970s, a lot of the perpetrators are probably no longer active in the church.
All we can confidently say is that the figures are imperfect - both for the number of active paedophiles among the Catholic clergy and the number of paedophiles in the general population and they are very difficult to compare.
More from the Magazine
In the late 1970s and early 1980s, the Paedophile Information Exchange publicly campaigned for the age of consent to be axed or lowered - the group was well known and was affiliated to the National Council for Civil Liberties - now Liberty. How did the network operate so openly?
Subscribe to the BBC News Magazine's email newsletter to get articles sent to your inbox.
Listen to More or Less on BBC Radio 4 and the World Service or download the free podcast.