The gulf between bulls and bears has never widened so quickly in the $12 trillion market for U.S. government bonds.

The proportion of investors betting on gains or losses in Treasuries, rather than holding a neutral view, almost doubled this year to 51 percent, a client survey by JPMorgan Chase & Co. showed. The 23 percentage-point jump is the most for the period since the top-ranked firm for U.S. fixed-income research by Institutional Investor magazine began its weekly survey in 2003.

With Treasury yields at historically low levels, the stakes are rising as the Federal Reserve cuts back the bond buying that's held down borrowing costs on trillions of dollars of debt from governments to companies and individuals. The debate pits Pacific Investment Management Co.'s Bill Gross, who says yields can stay low because growth and interest rates won't return to pre-crisis levels, against market-rules theorist Robert Farrell, who contends asset prices always revert toward the average.