Do Bonds Span the Fixed Income Markets? Theory and Evidence for Unspanned Stochastic Volatility

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Abstract
Most models of the term structure are restrictive in that they assume the bond market forms a complete market. That is, they assume all sources of risk affecting fixed income derivatives can be completely hedged by a portfolio consisting solely of bonds. Below, we present empirical evidence which suggests this prediction fails in practice. In particular, we find that changes in swap rates have very limited explanatory power for returns on at-the-money straddles - portfolios mainly exposed to volatility risk. We term this empirical feature 'unspanned' stochastic volatility (USV). We demonstrate that bivariate Markov models (e.g., Fong and Vasicek (1991), Longstaff and Schwartz (1992)) cannot exhibit USV. Then, we determine necessary (and apparently sufficient) parameter restrictions for trivariate Markov affine systems to exhibit USV. Finally, USV is shown to occur naturally within the Heath-Jarrow-Morton framework.