Impact of DNA testing
Date: 12 March 2009
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New research suggests that clear up and charge rates for a number of serious offences improved after the enactment of laws permitting mandatory DNA testing of NSW prison inmates. No improvement was observed, however, in the conviction rate at court.
Mandatory DNA testing of NSW prison inmates began on January 1st, 2001.
In 2008, the School of Mathematics and Statistics at the University of New South Wales and the NSW Bureau of Crime Statistics and Research conducted a joint evaluation of the new laws.
The specific questions addressed in their study were as follows:
- Has the introduction of DNA testing increased the crime clear up rate and, if so, which crime categories clear-up rates have increased?
- Has the introduction of DNA testing increased the ratio of charges laid to crimes reported and, if so, for which categories of crime?
- Has DNA testing increased the proportion of charges laid that result in convictions and, if so, for which categories of crime?
To answer these questions the researchers examined trends in (1) and (2) over the period January 1995 to March 2007 and trends in (3) over the period January 1995 to June 2007 inclusive.
Trends in eight different offence categories were examined: assault, sexual assault, robbery with a firearm, robbery without a weapon and with a non-firearm weapon (combined), break and enter dwelling, break and enter non-dwelling, motor vehicle theft and stealing from a motor vehicles.
After some time delay following the introduction of mandatory DNA testing, there was consistent evidence of an improvement in the clear up, charge and charge to clear-up rates for four of the eight crime categories (robbery with a firearm, robbery without a firearm, break and enter dwelling and break and enter non-dwelling). For sexual assault historical downward trends ceased approximately 18 months after the introduction of mandatory DNA testing.
For assault, however, the results were mixed. The clear up rate for this offence fell after the introduction of DNA testing but charge rate and charge to clear-up rates improved.
There was no consistent pattern in the clear up and charge rates for motor vehicle theft or stealing from a motor vehicle. Nor was there any evidence that the advent of DNA testing improved the conviction rate for any offence at court.
Commenting on the findings, the Director of the Bureau, Dr Don Weatherburn, said they should be interpreted with caution.
"The fact that clear up and charge rates improved for some offences and declined for others after the introduction of mandatory DNA testing does not by itself prove that the advent of DNA testing caused the change."
"Other factors not measured in our research may have swamped or accentuated the change in crime clear-up, charge and conviction rates following the advent of DNA testing."
"Follow up research examining individual cases is needed to see whether there are better outcomes for cases where DNA evidence is available than for comparable cases where it is not."
Further enquiries: Dr Don Weatherburn 9231-9190 or 0419-494-408