Existing home sales declined for much of 2018. In December, according to the National Association of Realtors, sales of single-family homes, condos, and co-ops fell a larger-than-expected 6.4% from the previous month. A Reuters survey of economists projected home sales to decline by a modest 1% in December.

On a year-over-year basis, the decline was even starker. In December 2017, 5.56 million homes sold. Last month’s figure was more than 10% lower at 4.99 million homes. That marked the lowest level in three years.

For economists, the decline in home sales last month seemed as hard to explain as it was surprising, with many offering different explanations for the slowdown. Lawrence Yun, the chief economist at the National Association of Realtors, credited higher interest rates late last year. With mortgage rates somewhat lower now, he predicted “some revival in home sales” in coming months.

Other economists blamed everything from rising home prices—the median existing-home price rose 2.9% last month despite lower sales—to turmoil in the stock market for the decline. Given that the pace of home sales is where it was before Trump became president, MUFG Chief Economist Chris Rupkey said to Reuters that the decline signaled that “the initial confidence boost from the new ideas and new legislation is falling flat.”

The report added to stock market declines, with the Dow Jones Industrial Average falling 1.2% Tuesday to 24,404.48. The drop may indicate a vicious cycle of falling markets that could make home buyers more cautious—which in turn makes stock prices decline.

“We’re in a mental recession,” Sam Khater, chief economist at Freddie Mac, told the Wall Street Journal. “It’s a constant stream of negative headlines for a couple of months… it wears on you.”