For example, 20 years from now for all we know bell bottom pants, "wife beater" shirts, and oiled hair might be all the rage. If you tried to portray that in a movie filmed today it would be laughed out of the theatre, so to speak.
William Ware Theiss said exactly - well, almost exactly - this on the one occasion that I spoke with him. His example was that if he'd been asked to design clothing for the 1960s back in the 1950s, the last thing he'd have come up with would have been blue jeans, buckskin jackets, boots and long hair/beards for young men - all of those clearly signified the American West in the 19th century, yet you'd see all of them pretty regularly on city streets in 1967. Designing for the future, he said, was deciding what believable lies you were going to tell the audience.