Downing (D) was indicted for mail fraud because of his scheme to defraud vendors in 1978 and 1979. D used the name, Reverend Claymore, claiming to be in a group called the Universal League of Clergy (ULC). He visited trade shows with his partners, Silva and Piazza, and received large orders from vendors. They bought the goods on credit by using false references, they got rid of the goods without paying the vendors. The Government's (P) only evidence against D was the testimony of 12 eyewitnesses who had met with 'Reverend Claymore' and identified D as the same person. D attempted to introduce testimony by an expert in the field of human memory and perception about these eyewitness' identification. The trial court did not admit this testimony because it held that the admission would usurp the function of the jury. D was convicted and appealed.