The Supreme Court decision in Mapp v. Ohio (1961) extended the Exclusionary Rule which bars the admission of evidence obtained in violation of the Constitution to state courts, where most criminal defendants are tried. Its more honest defenders admit that this will sometimes lead demonstrably guilty defendants to be acquitted. What is less widely appreciated is that it also leads innocent defendants to be convicted.
It is useful to consider how a generally well-informed and rational finder of fact (in criminal cases, ordinarily a jury) would react to the existence of an Exclusionary Rule. This is not to assert that juries are universally well-informed and rational; clearly, they are not. Rather this is a useful base case to think about as one can reason about the behavior of rational actors, while reasoning about the irrational is either difficult or impossible. Moreover, it is often, but not universally, observed that even those who are not fully rational or well-informed will often over time unconsciously gravitate towards the behavior of their well-informed and rational peers.
So how would such a rational and well-informed juror, earnestly trying to apply the beyond-a-reasonable-doubt standard of proof to convict the guilty and acquit the innocent or merely likely guilty, respond to the existence of the Exclusionary Rule? The juror would know that the Exclusionary Rule exists and how frequently it was applied. But the juror would be completely ignorant of whether any evidence was excluded in the present case.
The math of this can get a little fiddly, but it turns out that under a broad array of assumptions about juror utility, the juror would slightly lower his standard for what is considered evidence beyond a reasonable doubt. The approximately 90% chance of factual guilt standard as equivalent to beyond-a-reasonable-doubt corresponds mathematically to legal maxim that it is better for ten guilty men to go free than one innocent to be convicted.See here for an excellent and funny survey on the N Guilty Men
standard by the redoubtable Prof. Sasha Volokh, written and published before he even went to law school. But if a juror knew that there was an Exclusionary Rule and some evidence might not have been presented, a rational juror would respond by slightly lowering that to perhaps 89% or 88%, depending on the prevalence and significance of excluded evidence.
The general result is that a rational juror would lower the standard of proof just enough to balance the Exclusionary Rule’s harms of acquitting the guilty and convicting the innocent. In other words, rational juries applying the 10-guilty-men standard would on average convict one innocent man for every ten guilty men the Exclusionary Rule set free. Thus, in so far as juries are rational, we should expect to see one innocent in prison for every ten cases in which a guilty person escaped justice thanks to the Exclusionary Rule.
This is not the only or even the most powerful argument against the Exclusionary Rule. Another is that it is exclusively useful to the guilty—those who would have been convicted if all probative evidence would have been presented, but went free because some probative evidence was excluded because of the misconduct of some well-meaning, but over-eager investigator. The Exclusionary Rule is also completely useless in the paradigmatic case justifying it: A malevolent cop who uses his power to harass the innocent has nothing to fear from the Exclusionary Rule; an innocent person of course will never have the opportunity to exclude probative evidence at trial.
This is not to say that the Exclusionary Rule is not aimed at an admirable goal—the protection of Constitutional rights—or that there ought to be no remedy for those whose rights were violated. But other remedies, perhaps beyond the power of courts to order, would serve that purpose far, far better than the Exclusionary Rule and without its drawbacks. A relaxation or abolition of the rule of Qualified Immunity for official misconduct combined with strict liability for police misconduct would be a vastly superior option.The related idea of the anti-ticket was explored here and here.
A civil remedy for police violations of Constitutional rights is also preferable because it uncouples the possibly quite disparate issues of the magnitude of the Constitutional violation and the enormity of the crime at issue. A cop who unconstitutionally enters a home through an unlocked door and immediately observes strong evidence of a serial killer at work should probably face a small fine, even as the killer is convicted. Conversely, cops who unconstitutionally trash a home causing extensive property damage in pursuit of a jay walker, should pay large compensation, even as the jay walker pays a small one for his offense, if proven.
Even if one is principally concerned about the dignitary insult of police misconduct, one should favor this remedy. Nothing convinces of the sincerity of an apology as readily as accompanying it with cold, hard cash.
