Theories predict that initiation of index derivatives could affect the informativeness of the underlying stocksf prices. We test this hypothesis by exploring stock price behavior around the introduction of the SandP 100 options on the CBOE in March 1983. Applying two alternative statistical methods to both daily and weekly data, we find that, following the listing of the index options, the underlying stocksf returns become significantly more random and, thus, less predictable, net of contemporary marketwide efficiency shifts. That is, the underlying stocksf prices tend to be more informative following the commencement of the index options, consistent with the hypothesis.
