The FOMC has also acted to improve market functioning and to push longer-term interest rates lower through its large-scale purchases of agency debt, agency mortgage-backed securities (MBS), and longer-term Treasury securities, of which the Federal Reserve currently holds more than $2 trillion. The channels through which the Fed's purchases affect longer-term interest rates and financial conditions more generally have been subject to debate. I see the evidence as most favorable to the view that such purchases work primarily through the so-called portfolio balance channel, which holds that once short-term interest rates have reached zero, the Federal Reserve's purchases of longer-term securities affect financial conditions by changing the quantity and mix of financial assets held by the public. Specifically, the Fed's strategy relies on the presumption that different financial assets are not perfect substitutes in investors' portfolios, so that changes in the net supply of an asset available to investors affect its yield and those of broadly similar assets. Thus, our purchases of Treasury, agency debt, and agency MBS likely both reduced the yields on those securities and also pushed investors into holding other assets with similar characteristics, such as credit risk and duration. For example, some investors who sold MBS to the Fed may have replaced them in their portfolios with longer-term, high-quality corporate bonds, depressing the yields on those assets as well.
The logic of the portfolio balance channel implies that the degree of accommodation delivered by the Federal Reserve's securities purchase program is determined primarily by the quantity and mix of securities the central bank holds or is anticipated to hold at a point in time (the "stock view"), rather than by the current pace of new purchases (the "flow view"). In support of the stock view, the cessation of the Federal Reserve's purchases of agency securities at the end of the first quarter of this year seems to have had only negligible effects on longer-term rates and spreads.
The Federal Reserve did not hold the size of its securities portfolio precisely constant after it ended its agency purchase program earlier this year. Instead, consistent with the Committee's goal of ultimately returning the portfolio to one consisting primarily of Treasury securities, we adopted a policy of re-investing maturing Treasuries in similar securities while allowing agency securities to run off as payments of principal were received. To date, we have realized about $140 billion of repayments of principal on our holdings of agency debt and MBS, most of it prior to the end of the purchase program. Continued repayments at this pace, together with the policy of not re-investing the proceeds, were expected to lead to a slight reduction in policy accommodation over time.
However, more recently, as the pace of economic growth has slowed somewhat, longer-term interest rates have fallen and mortgage refinancing activity has picked up. Increased refinancing has in turn led the Fed's holding of agency MBS to run off more quickly than previously anticipated. Although mortgage prepayment rates are difficult to predict, under the assumption that mortgage rates remain near current levels, we estimated that an additional $400 billion or so of MBS and agency debt currently in the Fed's portfolio could be repaid by the end of 2011.
At their most recent meeting, FOMC participants observed that allowing the Federal Reserve's balance sheet to shrink in this way at a time when the outlook had weakened somewhat was inconsistent with the Committee's intention to provide the monetary accommodation necessary to support the recovery. Moreover, a bad dynamic could come into at play: Any further weakening of the economy that resulted in lower longer-term interest rates and a still-faster pace of mortgage refinancing would likely lead in turn to an even more-rapid runoff of MBS from the Fed's balance sheet. Thus, a weakening of the economy might act indirectly to increase the pace of passive policy tightening--a perverse outcome. In response to these concerns, the FOMC agreed to stabilize the quantity of securities held by the Federal Reserve by re-investing payments of principal on agency securities into longer-term Treasury securities. We decided to reinvest in Treasury securities rather than agency securities because the Federal Reserve already owns a very large share of available agency securities, suggesting that reinvestment in Treasury securities might be more effective in reducing longer-term interest rates and improving financial conditions with less chance of adverse effects on market functioning. Also, as I already noted, reinvestment in Treasury securities is more consistent with the Committee's longer-term objective of a portfolio made up principally of Treasury securities. We do not rule out changing the reinvestment strategy if circumstances warrant, however.
By agreeing to keep constant the size of the Federal Reserve's securities portfolio, the Committee avoided an undesirable passive tightening of policy that might otherwise have occurred. The decision also underscored the Committee's intent to maintain accommodative financial conditions as needed to support the recovery. We will continue to monitor economic developments closely and to evaluate whether additional monetary easing would be beneficial. In particular, the Committee is prepared to provide additional monetary accommodation through unconventional measures if it proves necessary, especially if the outlook were to deteriorate significantly. The issue at this stage is not whether we have the tools to help support economic activity and guard against disinflation. We do. As I will discuss next, the issue is instead whether, at any given juncture, the benefits of each tool, in terms of additional stimulus, outweigh the associated costs or risks of using the tool.
Notwithstanding the fact that the policy rate is near its zero lower bound, the Federal Reserve retains a number of tools and strategies for providing additional stimulus. I will focus here on three that have been part of recent staff analyses and discussion at FOMC meetings: (1) conducting additional purchases of longer-term securities, (2) modifying the Committee's communication, and (3) reducing the interest paid on excess reserves. I will also comment on a fourth strategy, proposed by several economists--namely, that the FOMC increase its inflation goals.
