By 2000, however, a crucial aspect of the BlackScholes-Merton model was almost never present. As we saw in chapter 6, in that model the relationship between the strike price and the implied volatility of options on the same underlying asset with the same time to expiration is a flat line. In the fall of 1987, however, the flat-line relationship, empirically manifest in the late 1970s and the early 1980s, disappeared, and was replaced by a distinct "skew" Figure 7.

Donald MacKenzie, An Engine, Not a Camera, loc. 900