A recession is coming and "we are already slowing," said Campbell Harvey, who developed the yield-curve indicator.

A contraction may begin this month and last two or three quarters, he told the Wall Street Journal.

The Fed's decision to keep hiking rates means a recession is a "self-inflicted wound," he added.

A recession is coming and it could even start this month, according to the developer of the yield-curve indicator.

In January, Duke University professor Campbell Harvey expressed some uncertainty on whether the inverted curve was sending a reliable signal about a contraction.

But since then, that doubt has evaporated.

"We are already slowing," he told The Wall Street Journal. "The question is how deep the recession will be."