Data from TrimTabs Investment Research shows that corporate insiders have sold an average of $600 million of stock per day in August “as doubts grow about the sustainability of the longest bull market in American history.” This according to a recent CNN article.

“August is on track to be the fifth month of the year in which insider selling tops $10 billion,” the article reports, adding that the only other time this happened was in the period before the 2008 financial crisis. While the $10 billion mark may not mean as much as it did in 2007 (when the market was much smaller), TrimTabs analyst Winston Chua says the acceleration of selling by insiders “signals a lack of confidence. When insiders sell, it’s a sign they believe valuations are high and it’s a good time to be outside the market.”

The article explains that 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.” But according to DataTrek Research founder Nicholas Colas, such activity may merely be triggered by their expectations for lower compensation and may not necessarily be a helpful indicator.  Colas explains, “Most managers get paid on earnings growth. If they anticipate bonuses will be slower, they will sell stock to make up the gap. It’s one more sign that managements know this will be a tough year for growing earnings.”