From a practical perspective, the challenge judges face in being gatekeepers of expert evidence is deciding on which side of a critical line a proffer stands. Is the proffered testimony “shaky but admissible” (and open to attack at trial for weight and credibility), or does it stand on ice so thin that it is likely to be more misleading to fact finders than helpful?
A number of forensic “sciences” have been especially difficult for judges to evaluate. At the root of the difficulty is something of a perfect storm: faith in these areas is unusually strong, while their empirical foundations (that is, the evidence that answers the question of how sound they are or aren’t) are unusually weak. Hollywood and popular fiction have persuaded generations that, when a “forensic scientist” asserts a conclusion, it will be flawlessly correct in the extreme. And yet, as a committee of the National Academy of Sciences (NAS), formed pursuant to a congressional mandate to study the state of the forensic sciences, concluded: “The bottom line is simple: In a number of forensic science disciplines, forensic science professionals have yet to establish either the validity of their approach or the accuracy of their conclusions, and the courts have been utterly ineffective in addressing this problem.” COMM. ON IDENTIFYING THE NEEDS OF THE FORENSIC SCI. CMTY., NAT'L RESEARCH COUNCIL, NAT'L ACAD. OF SCIS., STRENGTHENING FORENSIC SCIENCE IN THE UNITED STATES: A PATH FORWARD 53 (2009) (“NAS report”), available at http://www.ncjrs.gov/pdffiles1/nij/grants/228091.pdf.