The government's introduction of a film ratings system in 1988 had a certainly unintended effect on subsequent trends. The "Category III" (adults only) rating became an umbrella for the rapid growth of pornographic and generally outré films; however, while considered graphic by Chinese standards, these films would be more on par with movies rated "R" or "NC-17" in the United States, and not "XXX". By the height of the boom in the early 1990s, roughly half of the theatrical features produced were Category III-rated softcore erotica descended from the fengyue movies of the 1970s. (Yang, 2003) A definitive example of a mainstream Category III hit was Michael Mak's "Sex and Zen" (1991), a period comedy inspired by "The Carnal Prayer Mat", the seventeenth century classic of comic-erotic literature by Li Yu (Dannen and Long, 1997). "Naked Killer" (1992) also became an international cult classic.