Why do we smile? On the determinants of the implied volatility function

We report simple regressions and Granger causality tests in order to unders tand the pattern of implied volatilities across exercise prices. We employ all calls and puts transacted between 16:00 and 16:45 on the Spanish IBEX-3 5 index from January 1994 to April 1996. Transaction costs, proxied by the bid-ask spread, seem to be a key determinant of the curvature of the volati lity smile. Moreover, time to expiration, the uncertainty associated with t he market and the relative market momentum are also important variables in explaining the smile. (C) 1999 Elsevier Science B.V. All rights reserved.