FBI Agent Cannot Testify As Human Lie Detector, Opining On Defendant’s Truthfulness

Sometimes, a court opinion reveals the prosecutorial tactic under scrutiny, the lower court’s endorsement of the same and defense attorney’s failure to object to the same, to be a real head-scratcher, a kind of “what in the world could they have been thinking?” experience. The Tenth Circuit recently had such an experience involving a federal prosecutor’s decision to call an FBI agent as an expert witness on truth detection and to have the agent opine as to the deceptiveness of a defendant during a custodial interview; a defense lawyer’s inexplicable failure to object contemporaneously to any portion of that remarkable testimony; and the trial court’s allowance of clearly impermissible opinion testimony in contravention of its Federal Rule of Evidence 702 gateway function.