New York (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."