The election of President Donald Trump is eclipsed only by the 9/11 terrorist attack and the battle over the fiscal cliff in 2011 in terms of generating doubt about future economic policy, CNBC reported, citing the Economic Policy Uncertainty Index.

Increases "in policy uncertainty foreshadow declines in investment, output, and employment in the United States," the three professors from Stanford, the University of Chicago, and Northwestern who created the index wrote in a 2016 paper, CNBC reported.

They do not say recession necessarily follows, only that there is a measurable economic effect from uncertainty, and they urge lawmakers to keep that in mind.

From the standpoint of creating economic uncertainty, the election of Trump has been more tumultuous than the 1987 stock market crash and the 2008 financial crisis.

Judged by the index, Trump's election stands as the third-biggest source of uncertainty in the index's 30-plus-year history. It is eclipsed only by the 9/11 terrorist attack and the battle over the fiscal cliff in 2011 in terms of generating doubt about future economic policy.

Meanwhile, U.S. companies reportedly have put the brakes on cash spending as Trump’s tax reform promises have yet to lead to a rise in investment and overall uncertainty surrounds the actual details of his plan.

“The data, in a survey of corporate treasurers, suggests the new administration’s promises of tax reform, deregulation and infrastructure spending have not yet led US executives to increase investment,” the Financial Times reported.

“In January, more companies said they were planning to draw down their cash reserves than to add to them — the strongest signal in two years of expansive intentions, from the quarterly survey by the Association for Financial Professionals survey,” the FT reported.