The earlier flat line has not returned subsequently (see, for example, Jackwerth 2000); indeed, the skew continued to grow at least until 1992 (Rubinstein 1994, pp. 772-774). The direction of the skew has been stable: puts with strike prices well below current stock index levels have higher implied volatilities are relatively more expensive than puts with higher strike prices, and the putcall parity relation discussed in chapter 6 means that the same is true of calls.

Donald MacKenzie, An Engine, Not a Camera, loc. 901