"While those in the care system account for just one per cent of children, a quarter of those in prison were in care as children."
David Cameron, 22 October 2012
The Prime Minister began the week with a high profile speech on criminal justice, where he promised a "tough but intelligent" approach to tackling crime.
Setting out some of the challenges that new Justice Secretary Chris Grayling would face as he tries to cut reoffending rates, Mr Cameron highlighted the disproportionate number of offenders that used to be 'looked after' children - those who were cared for by foster parents or their local authority when they were young.
Responding to the speech, Shadow Justice Secretary Sadiq Khan accused Mr Cameron of proposing "ill-thought out and evidence-free policies." So is there evidence to back up the Prime Minister's claim about the upbringings of those behind bars?
It isn't difficult to see where the PM's figure comes from, as the Centre for Social Justice (CSJ) - who hosted this morning's speech - have themselves made a similar claim.
"27% of the prison population, and half of all prisoners under 25, were in care."
As a source for its claim, the think tank references a report published over a decade ago by the now-defunct Social Exclusion Unit called 'Reducing Reoffending by ex-prisoners'.
Sure enough, this did find that just over a quarter of prisoners had backgrounds in care:
However this is where the trail goes cold - the Social Exclusion Unit references unpublished work undertaken by the British Market Research Bureau titled 'Estimating Criminality', conducted in 2001. Given its age and the fact that it is unpublished we're not able to know exactly how this figure was arrived at.
However there is more recent evidence that suggests that the Prime Minister's claim still holds true.
In March this year the Ministry of Justice (MoJ) published research on 'Prisoners' Childhood and Family Backgrounds.' This found that:
"Twenty-four per cent [of surveyed prisoners] stated that they had been in care at some point during their childhood."
The MoJ report - which was based on survey data gathered in 2005/6 by the polling organisation Ipsos Mori - went into further detail, considering the proportion of prisoners who spent all or most of their childhoods in care. 5% reported that they had grown up in an institution and a further 2% saying that they had been looked after by foster parents.
It is worth noting, however, that the MoJ study considers a specific type of prisoner: those serving sentences of between one month and four years. While only 10% of prisoners spend longer behind bars, this does mean that we can't use this research to form conclusions about the backgrounds of those found guilty of some of more serious crimes.
What about the general population?
As we saw earlier, Mr Cameron's claim that 1% of all children are in care is slightly different to the comparator used in the Social Exclusion Unit report, which found that 2% of people had been taken into care as children.
The figures are different because they consider different things: the first looks at the proportion of children currently in care, while the latter looks at the proportion of adults that were in care at some point during their childhoods.
According to the Department for Education, there were 67,050 looked after children in England at 31 March 2012, while the Welsh Assembly Government records a further 5,725 in care west of the Severn.
This means that approximately half a percent of the 12.7 million 0-18 year olds in England and Wales are in care at the moment.
The Social Exclusion Unit's assertion that 2% of the general population had been in care at some point during their formative years is taken from the 1991 National Prison Survey. While this research is now two decades old, it was also mentioned in the Ministry of Justice report in March this year.
Given that the Prime Minister was looking at the proportion of adult prisoners who had ever been in care, it might have been more instructive to compare this to the proportion of the adult population who had grown up similarly, rather than the percentage of children who are currently in care.
The studies we have seen on the upbringings of offenders in custody have consistently shown over a number of years that around a quarter of prisoners found themselves in care at some point during their childhoods, suggesting that the Prime Minister's claim is well supported by the evidence.
While Mr Cameron does also seem to be in the correct ballpark when he cites the proportion of children currently in care, a better comparison might be with the percentage of adults who had been looked after by the authorities at some point in their youths. While the data for this is slightly dated, it does suggest a slightly higher proportion (2%) of the group we're looking at have experienced care as children.