Last night I had some serious talk to a systems programmer who works with neural networks for >15years. He did research on the markets for the first time in his life over the last 6 months and I have to admit he came up with great ideas & results. He presented me this phenomenon: The trading behaviour of the S&P 500, (when analyzed with his algorithms) from 1960 - today shows some significant change in 1991 - 1993. Trading before these years was different from today's. The system seems very revolutionary and solid to me. Has anyone some idea, what could have triggered this change during this time ? It appears it is NOT related to the large -scale introduction of information technology into trading. A more likely possibility is, it seems to have something to do with options & derivatives. Anyone remembering what happened then ?