D was charged with aggravated sexual abuse (forcible rape) in violation of 18 U.S.C. §§ 1153, 2241(a), and with sexual abuse of a minor (statutory rape) in violation of 18 U.S.C. §§ 1153, 2243. The incident occurred one month before D's twenty-first birthday and not quite six months before the sixteenth birthday of the minor. D admitted that he had sexual intercourse with the minor. His defense was that the intercourse was consensual. The jurors acquitted D of forcible rape but convicted him of statutory rape. D's sole defense to the statutory rape charge was an affirmative defense permitted under 18 U.S.C. § 2243(c): that at the time of the incident, he reasonably believed that the minor, who was then fifteen-and-a-half years old, was at least sixteen. D testified that the minor smoked cigarettes drove a car, used makeup, and looked 'mature' enough to be at least sixteen. He testified that he knew the minor because he had previously dated her older sister, but that the minor never told him her age. D called several witnesses who offered to testify that as of the date of the alleged sexual abuse, their observations caused them to believe the minor to be between sixteen and twenty years old. The court ruled that defense witnesses were permitted to testify to their perceptions of the minor's physical appearance and behavior at the time but were barred from stating their opinion that the minor was at least sixteen years of age. It held that a witness' belief as to the minor's age was 'subjective and has nothing to do with what D might have believed.' Three of D's witnesses confirmed his claim that the minor smoked cigarettes on the night in question; two testified that they had seen her drive a car before the alleged sexual abuse took place; and two testified that she wore makeup at the time of the incident. Three witnesses testified that the minor appeared sexually mature at the time of the alleged sexual abuse. One stated that the minor 'was tall and [] appeared to be a lady a lady like she was full, how do you say, she was fully developed,' and that 'she was mainly filled out, she was very tall.' Another testified that the minor's body shape made her look 'like an older person.' A third stated that the minor 'was well into her womanhood, well developed [and] had her curves and was into her maturity.' In addition, one witness testified that the minor drank beer on the night of the incident. The minor testified and did not deny that she occasionally drank beer and smoked cigarettes; indeed, she testified that she did both on the night in question and did not tell D her age because she thought he knew it. P introduced evidence that D lived down the street from the minor; that he had dated her sister for several months, and during the time he dated her sister he went on walks with the minor and helped her with her math homework; and that on the night of the incident, the minor did not wear sophisticated clothing but rather high-top athletic shoes, tube socks, jean shorts over bicycle tights, and a tee-shirt. D denied attending the minor's fifteenth birthday party, which had been held during the time that D was dating the minor's sister, he denied that he had done so. The prosecution also introduced substantial evidence that people under the age of sixteen commonly drove on the reservation. D was convicted and appealed.