Once upon a time, categories of experts were excluded from testifying in criminal cases on the antiquated and inaccurate claim that they “usurped the function of the jury” to determine credibility. The wall came down in 2014 with the seminal decision in Commonwealth v. Walker allowing experts on the vagaries of eyewitness perception and memory.

The reasoning was simple and persuasive—the expert would not comment on whether a particular witness was accurate in making an identification, and thus the jury function remained intact. Rather, the expert would be a “teaching” witness, one necessary because other means—in particular cross-examination and closing argument “are insufficient to convey factors regarding the fallibility of eyewitness identification to the jury.” See Commonwealth v. Walker, 625 Pa. 450, 486 (2014).