DNA courts doubt

The fate of countless defendants may depend on the way in which DNA evidence is presented to a jury, says Jonathan Koehler, associate professor of behavioural decision-making at the University of Texas at Austin.

He gave the same DNA evidence to two groups of mock jurors, in one case presenting the DNA statistics using probabilities (0.1 or 0.01 per cent) to highlight a suspect's chance of matching by coincidence. In the other case the statistics were presented as frequencies (one in 1,000 and one in a billion) in a way that brought out the chance that others would match by coincidence. In the first case 56 per cent of jurors thought the defendant was 99 per cent likely to be source of the DNA. This fell to 10 per cent in the second group.