The volume of residential MBS in the market fell again in the fourth quarter of 2011, sustaining a nearly constant decline that’s been underway since the midway point in 2009. A total of $6.437 trillion single-family MBS were outstanding as of the end of last year, down 1.7 percent from the third quarter. The last time there was growth in the supply of MBS was in the second quarter of last year. The major factor is the ongoing decline in the supply of home mortgages, which fell 0.5 percent in the fourth quarter and 2.2 percent over the full year in 2011. Mortgage collateral has... (Includes one data chart)