Blackstone's Commentaries on the Laws of England is comprised of four books, written in two volumes, running well over 1500 pages in length. Within this enormous work there may be no more well-known or more memorable line than that which has come to be known as the Blackstone Ratio: "the law holds that it is better that ten guilty persons escape than that one innocent suffer." (1)
The Blackstone Ratio clearly acknowledges that there are two kinds of errors that can be made within the criminal justice system, and that one, the false conviction, is far worse than the other, the false acquittal. Two other points are also implicit within the Blackstone Ratio, specifically that there is a trade-off relationship between false convictions and false acquittals, and the criminal justice system has some control over that trade-off in terms of the kinds of errors, false acquittals or false convictions, it will allow. The implication of the Blackstone Ratio is that the criminal justice system could reduce the number of false convictions, but in the process would lose some correct convictions. Alternatively, the criminal justice system could increase the number of correct convictions, but would likely convict more innocent people as well.
Without these two assumptions, regarding the trade-off between false convictions and false acquittals, and regarding the criminal justice system's role in determining that trade-off, the Blackstone Ratio would be meaningless. There would be little point in considering the proper relationship between false convictions and false acquittals to the extent that it was possible to reduce both errors simultaneously, or if the criminal justice system was powerless to control the relationship between the two. Of course, much has happened since Blackstone wrote those famous words. Technological advances in forensic science in just the last thirty years do make it possible to reduce both errors simultaneously, DNA evidence being the obvious example. For other forms of evidence, however, the trade-off implied by the Blackstone Ratio may operate today much as it did almost 250 years ago.
The focus here is on eyewitness identification evidence for three reasons. First, the connection between eyewitness identification and wrongful conviction is well-established. In approximately 75% of the 261 cases in which innocent people were convicted and later exonerated through DNA evidence, the original conviction was obtained in whole or in part through mistaken eyewitness identification. (2) These and other archival analyses of false convictions have led to a consensus among legal scholars that mistaken eyewitness identification is one of the primary causes of wrongful convictions in the United States. (3) This dubious distinction begs the question, why does eyewitness identification contribute to so many wrongful convictions? An answer to this question brings us to the second reason for focusing on eyewitness evidence: it is extremely malleable, both in terms of the underlying information in memory and in the decision processes that operate on that information. This malleability not only affects the outcome of the identification procedure, in terms of whether a witness fails to identify the guilty or falsely identifies the innocent, but also influences the way eyewitness evidence flows through the criminal justice process. Third, this malleability of information and decision processes connects eyewitness identification to the two components of the Blackstone Ratio--the trade-off between false convictions and false acquittals, and the criminal justice system's control over that trade-off.
The relationship between mistaken identification and wrongful conviction is often described retrospectively by starting with the wrongful conviction and tracing the path backwards to the cause of that wrongful conviction. The statistic linking wrongful convictions to mistaken identifications is such a retrospective analysis. …