Amicus Brief: Tyler Scott Johnston
United STates v. Tyler Scott Johnston
Filing Date
November 26, 2025
Charge
Rape
Jurisdiction
United States Court of Appeals for the Second Circuit
Partners
40 other scholars of evidence and forensic science
Summary
Kate Judson of CIFS and 40 other scholars of evidence and forensic science filed this amicus brief in the United States Court of Appeals for the Second Circuit in Tyler Scott Johnston’s case to ask the court to affirm the District Court’s grant of Johnston’s motion to preclude the DNA analysis in this case. In our brief, we argue that the government’s laboratory has not done validation testing on samples of the degree of complexity at issue and, thus, the analysis of the samples at issue cannot be demonstrated to meet the requirements of admissibility under Federal Rule of Evidence 702.
PRELIMINARY STATEMENT
Rule 702(d) requires scientific evidence, like the DNA evidence at issue here, to be reliably applied as a precondition of admissibility. Fed. R. Evid. 702(d). “To warrant admissibility, ... an expert’s analysis [must] be reliable at every step.” Amorgianos v. Amtrak, 303 F.3d 256, 267 (2d Cir. 2002). The proponent—here, Appellant—carries the burden of proving reliability by a preponderance of the evidence. Fed. R. Evid. 702.
Whether a method has been tested is a key factor in assessing scientific reliability. Daubert, 509 U.S. at 593; see Paul C. Giannelli, Forensic Science: Daubert’s Failure, 59 Case W. Res. L. Rev. 869, 872 (2018) (explaining “the first and most important factor is empirical testing,” because other Daubert factors are themselves dependent on testing) (emphasis in original). The mere existence of testing, however, is insufficient. To establish reliability, test data must be representative of the facts in a case. See Daubert, 509 U.S. at 591 (“[S]cientific validity for one purpose is not necessarily scientific validity for other, unrelated purposes”); see also Gen. Elec. Co. v. Joiner, 522 U.S. 136, 146 (1997) (exclusion is warranted where there is “too great an analytical gap” between studies relied upon and an expert’s testimony).
Internal validation plays a critical role in ensuring reliability as applied and, accordingly, admissibility of forensic evidence. Internal validation is the process by which a method is tested to ensure that it performs reliably as implemented by a particular laboratory. Sci. Working Grp. on DNA Analysis Methods, Guidelines for the Validation of Probabilistic Genotyping Systems § 4 (2015) [hereinafter SWGDAM]. Internal validation identifies the conditions under which a method works as intended and, crucially, when a method no longer works reliably. Id.; NIST Mixture, supra, at 95. Where DNA analysis goes beyond the limits established through internal validation, the results of such analysis cannot be deemed reliable. See United States v. Ortiz, 736 F.Supp.3d 895, 908 (S.D. Cal. 2024) (excluding DNA evidence based on insufficient internal validation); United States v. Williams, 382 F.Supp.3d 928, 938 (N.D. Cal. 2019) (same); United States v. McCluskey, 954 F.Supp.2d 1224, 1280 (D. N.M. 2013) (same).