Likelihood and Uncertainty: Understanding Probabilities
The jury was facing a difficult decision in the case of People v. Collins, 1968 (cited in Arkes & Hammond, 1986). The robbery victim could not identify his assailant. All he could recall was that the robber was a woman with a blonde pony tail who, after the robbery, rode off in a yellow convertible driven by a Black man with a moustache and a beard. The suspect fit this description, but could the jury be certain "beyond a reasonable doubt" that the woman who was on trial was the robber? She was blonde and often wore her hair in a pony tail. Her codefendant "friend" was a Black man with a moustache, beard, and yellow convertible. If you were the attorney for the defense, you would stress the fact that the victim could not identify this woman as the robber. What strategy would you use if you were the attorney for the prosecution?
The prosecutor produced an expert in probability theory who testified that the probability of these conditions "co-occurring (being blonde plus having a pony tail plus having a Black male friend plus his owning a yellow convertible and so on, when these characteristics are independent) was 1 in 12 million. The expert testified that this combination of characteristics was so unusual that the jury could be certain "beyond a reasonable doubt" that she was the robber.
The jury returned a verdict of "guilty."
The theory of probabilities is nothing but common sense confirmed by calculation.
-- La Place ( 1749-1827)
As seen in the preceding example, the legal system recognizes that we can never have absolute certainty in legal matters. Instead, we operate with various degrees of uncertainty. Juries are instructed to decide that someone is guilty of a crime when they are certain "beyond a reasonable doubt." This standard was adopted because there is always some small amount of doubt that the accused may be innocent. Jurors are instructed to operate under a different level of doubt when they are deciding about guilt or innocence in a civil case. In civil cases, they are told to deliver a verdict of guilty when the "preponderance of evidence" supports this decision. Thus, jurors are instructed to operate under two different levels of uncertainty when the case before them is either criminal or civil. They need to be more certain when deciding that an accused party is guilty in a criminal case than in a civil case.