Shared appreciation mortgages (SAMs) feature mortgage payments that adjust with house prices. They are designed to stave off borrower default by providing payment relief when house prices fall. Some argue that SAMs may help prevent the next foreclosure crisis. However, home owners' gains from payment relief are mortgage lenders' losses. A general equilibrium model in which financial intermediaries channel savings from saver to borrower households shows that indexation of mortgage payments to aggregate house prices increases financial fragility, reduces risk‐sharing, and leads to expensive financial sector bailouts. In contrast, indexation to local house prices reduces financial fragility and improves risk‐sharing.

We present a simple, linear asset pricing model of the cross section of Mortgage‐Backed Security (MBS) returns. MBS earn risk premia as compensation for their exposure to prepayment risk. We measure prepayment risk and estimate risk loadings using prepayment forecasts versus realizations. Estimated loadings on prepayment risk decrease monotonically in securities' coupons relative to the par coupon, consistent with the predicted effect of prepayment on bond value. Prepayment risk appears to be priced by specialized MBS investors. The price of prepayment risk changes sign over time with the sign of a representative MBS investor's exposure to prepayment shocks.