The Human Genetics Commission Chair, Professor Jonathan Montgomery, has asked a simple question: "It is not clear how far holding DNA profiles on a central database improves police investigations?". The HGC has just published a report containing the damaging allegation that police were arresting individuals in order to populate the DNA database.
Well I think I have found the answer to the Professor's question with an analysis of the Government’s own crime statistics. My answer undermines arguments for long DNA retention periods based on improving criminal detection in general, and suggests that the much vaunted Scottish retention period for DNA data is also questionable.
The first statistic I am using reads as follows “Research recently carried out on men born in 1953 revealed that one in three had a conviction before they were 46 years old. The same research shows that of the male offenders born in 1953, around half of them had been convicted on only one occasion”.
The second set of statistics is from a table “Time to reconviction: by gender, 1995-97”. This includes details of under-21 offenders who are released from prison; the commentary adds “Overall there was little difference in the reconviction rates for those released from prison and those who had served community sentences” (references at bottom of blog). The statistics relate to male young offenders under 21 serving custodial sentences are as follows:
16% of males re-offend within 3 months
35% of males re-offend within 6 months
50% of males re-offend within 9 months
60% of males re-offend within 12 months
67% of males re-offend within 15 months
71% of males re-offend within 18 months
74% of males re-offend within 21 months
77% of males re-offend within 24 months
The table shows that within 2 years (24 months) from release from prison, 77% of young male offenders who have been in custody will re-offend. This is a rather shocking statistic in itself (how useful is prison then?) but it appears that 60% of those who will re-offend do that re-offending within 1 year (12 months). This means 17% will re-offend in year 2. The table shows a tailing off towards the end of the two year period (months 18-24) which is significant and pronounced.
We can extrapolate the figures in the above table on a forward yearly basis by using the ratio (17/60=0.28) as a multiplier for each year of DNA retention (i.e. multiply the increase in offending by 0.28 for each year and add it to obtain the total of offending for the next year). We are thereby assuming that the rate of increase in re-offending in year 1 can be used to estimate the total offending in year 2 and so on to all subsequent years of the Government’s 6 year DNA retention period.
Such an assumption as tabulated below reproduces the tailing off displayed in the raw data and it is reassuring to note that the assumption does not permit 100% of offenders to re-offend (which of course is a nonsense). In other words, the extrapolation method does not provide rubbish and looks reasonable. The other tables in the survey (Male adults over 21; female offenders) show exactly the same.
Applying this to the extrapolation we make we find that:
60% re-offend in 1 year
77% re-offend in 2 years
82% re-offend in 3 years
83% re-offend in 4 years
84% re-offend in 5 years
84.5% re-offend in 6 years
So in relation to those with a criminal record, a three year retention period for DNA appears opitimal in that it would allow most reoffending (82%) to be caught (assuming that the DNA was the only means of identifying the offender). The extra three years retention add about an additional 2.5%.
However, if you factor in the first statistic that “Research recently ... shows that of the male offenders born in 1953, around half of them had been convicted on only one occasion”, and include arguments such as “if someone has not re-offended in three years, then there is a very good prospect that they are not going to re-offend at all” then it looks as if the benefit of DNA retention is around three years and that further retention does very little with respect to ordinary crime. This is the answer to Professor Montgomery's question.
As readers know there is not three years retention but indefinite DNA data retention on criminals which is the current policy in all parts of the UK - but the police would get 97% of the benefit needs only a three year retention.
In other words, a 6 year retention period borders on the excessive for criminals and obviously excessive in relation to non-criminals (i.e. those arrested but not convicted). Put simply, if a 6 year retention period is hard to justify in terms of active criminals, it won't be justified in terms of prospective criminal activity of people who have no criminal record.
It also follows that the reason why the police want to retain DNA on the maximum number of people for the maximum time has nothing to do with ordinary crime as a three year retention criteria would do that.
It could be that the real reason is to get assistance in relation to serious random crime (e.g. rape and violence). But if that is the case, then the Government should be honest enough to focus public policy debate on this issue.
References: 1953 statistic from https://www.statistics.gov.uk/STATBASE/ssdataset.asp?vlnk=4480. Raw data table of offenders from https://www.statistics.gov.uk/STATBASE/xsdataset.asp?More=Y&vlnk=405&All=Y&B2.x=16&B2.y=7.
More details on the extrapolation: the increase in offending in 1 year is 60%; so an additional 0.28*60% = 17% re-offend in year 2. 60%+17% = 77% gives us the number who re-offend in the second year. In the third year, there is an additional 0.28*17% = 4.76% (5%) re-offend in year 3 so at the end of year 3, 77%+5% = 82% have re-offended.