The biggest bond gains in almost a decade have pushed returns on Treasuries above stocks for the first time since before the Civil War, Bloomberg reported.

Fixed-income investments advanced 6.25 percent this year, almost triple the 2.18 percent rise in the Standard & Poor’s 500 index through last week, according to Bank of America Merrill Lynch indexes. Debt markets are on track to return 7.63 percent this year, the most since 2002, the data show.

Long-term government bonds have gained 11.5 percent a year on average over the past three decades, beating the 10.8 percent increase in the S&P 500, said Jim Bianco, president of Bianco Research in Chicago.

The combination of a core U.S. inflation rate that has averaged 1.5 percent this year, the Federal Reserve’s decision to keep its target interest rate for overnight loans between banks near zero through 2013, slower economic growth and the highest savings rate since the global credit crisis have made bonds the best assets to own this year, Bloomberg said.

“The generation-long outperformance of bonds over stocks has been the biggest investment theme that everyone has just gotten plain wrong,” Bianco told Bloomberg. “It’s such an ingrained idea in everyone’s head that such low yields should be shunned in favor of stocks, that no one wants to disrupt the idea, never mind the fact that it has been off.”

Stocks had risen more than bonds over every 30-year period from 1861 until the period ending Sept. 30, said Jeremy Siegel, a finance professor at the University of Pennsylvania’s Wharton School in Philadelphia.