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(CNN Business) —  

The leaders of Corporate America are cashing in their chips as doubts grow about the sustainability of the longest bull market in American history.

Corporate insiders have sold an average of $600 million of stock per day in August, according to TrimTabs Investment Research, which tracks stock market liquidity.

August is on track to be the fifth month of the year in which insider selling tops $10 billion. The only other times that has happened was 2006 and 2007, the period before the last bear market in stocks, TrimTabs said.

Investors often view insider buying and selling — transactions performed by top executives, leading shareholders and directors — as a signal of confidence. Even though the stock market is much larger than it was in 2007, so the $10 billion mark may not mean as much now as it did then, the acceleration of insiders heading for the exits could indicate concern about the challenges ahead, especially as the US-China trade war threatens to set off a recession.

“It signals a lack of confidence,” said Winston Chua, an analyst at TrimTabs. “When insiders sell, it’s a sign they believe valuations are high and it’s a good time to be outside the market.”

Recession fears have ignited a burst of market volatility over the past year, punctuated by the worst December since the Great Depression. Although the S&P 500 remains up 14% in 2019, markets have tumbled in August as the trade war escalated. The Dow dropped 623 points, or 2.4%, on Friday. It regained about a third of those losses Monday.

Heavy insider selling is often considered an ominous signal about a given company because execs presumably have a better idea about where the stock is going than the average investor. The thinking is that if they thought the stock was going straight up, they wouldn’t leave cash on the table by selling.

But Nicholas Colas, co-founder of DataTrek Research, noted insider selling is not always a helpful indicator at a high level. Rather than reflecting a lack of confidence, he said, the selling may simply be the result of insiders bracing for leaner compensation.