Against a backdrop of accelerating developments in the science of lie detection certain to reopen the debate on the reliability and therefore admissibility of lie detector evidence in the federal courts, this Article examines whether the prohibition on hearsay evidence (or other evidentiary objections) will preclude admissibility of even scientifically reliable lie detector evidence. The Article concludes that the hearsay prohibition, which has been largely ignored by courts and commentators, is the primary obstacle to the future admission of scientifically valid lie detector evidence. The Article also suggests a potential solution to the hearsay problem that may allow admission of lie detector evidence in narrowly defined circumstances.
Cross-examination is often described as the “‘greatest legal engine ever invented for the discovery of truth,”’ but few would deny that it has achieved this exalted status largely by default. Science has simply failed to produce any valid alternative, leaving the criminal justice system to dutifully rely on this age-old practice in the hope that it will enable juries to distinguish lies from truth.
Recent scientific advances herald the arrival of a more modern “engine” for the discovery of truth–scientific lie detectors based on modern medical technologies such as magnetic resonance imaging, i.e., brain scans. These advances may constitute the first signs of a revolution in the criminal trial system. If permitted by the courts, evidence developed through scientifically valid lie detector examinations could become a ubiquitous legal tool that would aid juries to evaluate witness credibility and permit defendants to conclusively demonstrate their innocence (or guilt).
It is not clear, however, how courts will react to a scientifically valid lie detector. For decades, so-called “lie detector” evidence has been barred in federal and state courts on the ground that traditional lie detector technology is unreliable, amounting to little more than junk science. Given the availability of this facile response to any effort to place lie detector evidence before a jury, courts have rarely gone beyond the reliability determination to evaluate whether, once the science of lie detection improves, lie detector evidence will be admissible. This Article attempts to undertake that analysis with respect to the admissibility of scientifically valid lie detector evidence in the federal courts. In doing so, the Article notes a potentially devastating objection to lie detector evidence that has been largely unaddressed by courts and commentators–that lie detector evidence is inextricably intertwined with inadmissible hearsay. This objection is particularly significant because if courts conclude that traditional evidentiary principles, such as the prohibition of hearsay, preclude admissibility of lie detector evidence, advances in lie detector technology will prove to be largely irrelevant to the courts of law–ironically, the very setting where such technology could have the greatest impact.
Part I of the Article examines the threshold “scientific validity” requirements for the admission of expert testimony such as lie detector evidence, set out in the Supreme Court’s decision in Daubert v. Merrell Dow Pharmaceuticals, Inc. and Federal Rule of Evidence 702. It then discusses recent scientific advances that signal that lie detector evidence may soon satisfy these threshold requirements, eliminating the traditional barrier to the admission of lie detector evidence in federal courts.
Part II examines three commonly cited evidentiary objections to lie detector evidence that courts have relied on in addition to, or in concert with, an objection to the underlying validity of lie detector science: (1) lie detector evidence impermissibly invades the traditional province of the jury to evaluate witness credibility, (2) it is barred under Federal Rule of Evidence 403 because it is likely to “mislead” the jury, and (3) it violates Federal Rule of Evidence 704’s prohibition of expert testimony regarding the “ultimate issue.” Part II concludes that these objections are unlikely to constitute a continuing obstacle to the admission of scientifically valid lie detector evidence.
Part III analyzes the key remaining obstacle to the admission of scientifically valid lie detector evidence–the prohibition against hearsay. The hearsay problem arises because lie detector evidence consists of expert analysis of out-of-court statements offered for their truth (i.e., hearsay) and is consequently inadmissible under Federal Rule of Evidence 801 absent an applicable hearsay exception. Part III concludes that in almost all cases, the only potentially applicable exception will be the residual hearsay exception found in Federal Rule of Evidence 807, which although rarely utilized presents a potentially viable (and perhaps the only viable) legal basis for admission of scientifically valid lie detector evidence. Consequently, in Part IV, the Article concludes that if modern advances in lie detector science are to have any impact in the federal courts (or in the courts of the numerous states that have adopted evidentiary rules analogous to the federal rules), that impact will be funneled through the narrow and relatively obscure gateway of Rule 807, with significant consequences for future proponents of lie detector evidence.