Dateline: March, 2007, Issue 1
The definition of reasonable doubt provided to jurors can influence their verdicts.
Horowitz and Kirkpatrick (1996) examined five definitions of reasonable doubt when the evidence against the defendant was either strong or weak. The definitions of reasonable doubt studied were:
When the case against the defendant was strong, the definition of reasonable doubt did not matter and no significant differences in the conviction rate of the defendant occurred.
However, when the case against the defendant was weak, the definition of reasonable doubt affected the conviction rate. Significantly fewer convictions occurred when reasonable doubt was defined as "firmly convinced" than when it was defined in any of the other four ways when the case against the defendant was weak.
The "firmly convinced" definition produced the highest proportion of time spent discussing the evidence, and the "undefined" definition resulted in the most discussion of non-trial scenarios and personal experiences.
In sum, it is important to define reasonable doubt for jurors, and jurors needing to be "firmly convinced" convict less often when the evidence is weak.
Source Horowitz, I. A. & Kirkpatrick, L. C. (1996). A concept in search of a definition: The effects of reasonable doubt instructions, certainty of guilt standards, and jury verdicts. Law and Human Behavior, 20, 655-670.