Consider the familiar expert witness scene from the movie My Cousin Vinny. Vinny, played by Joe Pesci, calls as a witness Mona Lisa Vito, played by Marisa Tomei. Her experience as an auto mechanic is held to qualify her as an expert in general automotive knowledge. The court allows her to testify that the set of tire tracks made by the fleeing felons’ car could not have been made by the defendant’s car because only a car with positraction could have left those tracks. Her testimony leads to the dismissal of all charges. But is she an expert? Should she be given the latitude accorded an expert to base her testimony on information not personally known to her and to express an opinion without clarifying its basis for the jury? Or is she a lay witness with an unusual experience base and therefore limited to conclusions rationally derived from her base of knowledge?
Take an example of typical testimony offered as expert opinion in a prosecution for a narcotics violation. The defendant is charged with illegal distribution of cocaine, but claims he is not a drug dealer. At trial, the prosecution calls a law enforcement agent to testify that certain aspects of the defendant’s conduct were consistent with the behavior of an experienced drug dealer. Specifically, the agent opines that when the defendant circled the parking lot before meeting with the undercover agents, he was engaging in counter-surveillance, that the defendant’s use of a rental car is the mark of an experienced drug dealer, and that, when the defendant spoke with the informant to set up the sale, he used the coded language of a drug dealer. The law enforcement witness bases these opinions on experience investigating drug cases. Is the opinion testimony admissible? Should the court evaluate the evidence as expert or as lay opinion?