The Effect of Evidentiary Rules on Conviction Rates

In a criminal trial, a jury may observe characteristics about the defendant (or victim) and use them to form a belief on the likelihood of guilt. Many evidentiary rules attempt to limit this inference. If jury beliefs are rational, such rules may be counterproductive. Any prohibition on the use of defendant characteristics as a form of evidence will never simultaneously reduce conviction rates for the innocent and increase conviction rates for the guilty. In fact, the opposite will occur under plausible conditions. However, if juries are sufficiently biased in their beliefs about guilt rates within groups of defendants, an evidentiary rule may simultaneously reduce wrongful convictions and increase rightful convictions.