WASHINGTON — It has been a truism of the American economy for decades: When oil prices rise, the economy suffers; when they fall, growth improves.

But the decline of oil prices over the last two years has failed to deliver the usual economic benefits.

As oil prices have fallen to levels not seen since 2003 — sagging below $27 a barrel on Wednesday before rebounding to about $30 on Thursday — many experts now say they do not expect lower prices to bolster the domestic economy significantly in 2016.

“We got this wrong,” John C. Williams, president of the Federal Reserve Bank of San Francisco, told an audience in Santa Barbara, Calif., this month.