Some investors blame the stock market’s volatility on the Federal Reserve shrinking its bond portfolio. But the critique puzzles Fed officials and some economists because there is little evidence of turmoil in the two markets where the central bank actively intervened: Treasurys and mortgage debt.

The Fed is shrinking its $4tn portfolio by allowing Treasury and mortgage securities to mature without replacing them, the Wall Street Journal reported. Up to $50bn worth is allowed to expire every month under the plan, though the actual amounts have been closer to $40bn in recent months.