The NSPCC last week called for action to drastically reduce the number of sexual assaults on children (their split infinitive) after new figures revealed there were more than 400 offences reported to police every week last year (2010-11) with fewer than one in ten resulting in a conviction.

The report led to predictable stories – “Child sex attack every 20 minutes” in the Daily Mail, and a claim in The Sun by campaigner Sara Payne that Britain is “overrun with paedophiles”. A column in The People by Carole Malone asserted that “Britain is now in the grip of a paedophile epidemic with 23,097 children abused in England and Wales last year.”

I have no argument with the NSPCC’s figures. They came from all 43 police forces in England and Wales, as a result of Freedom of Information requests. Much the same data is available already, in the police-recorded crime statistics from 2003 to 2011. There are some differences, because in the published figures some offences are not broken down by age. That probably accounts for the difference between the NSPCC’s figure of 23,097 reported offences against children and my count, of just over 19,000 offences against under-16s, from the published figures.

These are large totals, so let’s look at how they are assembled, using the official data which, unlike the NSPCC’s, are broken down by age and offence type. There are five offences that relate to children under 13: rape and sexual assault (both sexes) and sexual activity (which is not divided by sex). Together, these five categories added up to 10,105 reported crimes in 2010-11, (see Table for breakdown) or rather more than half of the 19,219 offences recorded against all those under 16.

At first sight, this might suggest an epidemic of paedophilia. But in reality offences in under-13s are defined rather peculiarly. As a result of the 2003 Sexual Offences Act, they are offences even if they are consensual. So, to use the two examples of advice given in the Home Office’s official counting rules, if a 15 year-old boy has consensual sex with a girl under 13, it is a rape. If a 12 year-old girl willingly performs oral sex on her boyfriend, it is rape. (Of her, not him. But what if he, too, was under 13? Please ask the Home Office, not me.)

So while sexual offences in adults and in those aged 13-15 are counted only if they are non-consensual, any sexual contact with under-13s is counted, even if it is by another child under 13 and is consensual. To categorise such activities as offences is relatively new and it’s hardly surprising there are a lot of them. Adolescents are curious about their bodies and what used to be categorised as innocent exploration may now go down as sexual assault or even rape. It’s equally unsurprising that so few result in a conviction.

Some assaults on children under 13 are, of course, by paedophiles. But these figures give us little idea how many. And most “assaults” are possibly not assaults at all, except that the law defines them as such.

The biggest increases in the figures are those relating to rape in under-13s, (see Table) which in girls have risen from 970 in 2004-05 to 2,235 in 2010-11.  Rapes in both females and males under 16 (which means, in effect, those involving 13, 14 and 15 year-olds, since under-13s are counted separately) have shown falls over the same period.

So we must conclude either that rapists have targeted a younger age group, or that the increase in offences against the under-13s results from the change in the counting rules gradually taking effect. The latter is far more probable.

None of this is to argue against the NSPCC’s attempts to protect children. But it does call into question its claim that more than one in three sex crimes are committed against children. While literally true, that claim is misleading because of the different way crimes in under-13s are defined. What is a sex crime in this age group may not be a crime at all in any other age group.

Whether it was wise to change the rules in 2003 is open to question. Perhaps it’s right that any sexual contact with under-13s should be deemed criminal behaviour but it casts the net so wide that it opens the statistics to the abuse of campaigners and well-meaning but poorly-informed journalists.