Palantir and Tesla insiders have collectively sold nearly $13 billion in stock over the last three years.

Palantir Technologies ( PLTR -0.19% ) and Tesla ( TSLA 3.54% ) rank among the 10 most widely held stocks on Robinhood , a brokerage platform built for young investors. The companies both have opinionated and somewhat cavalier CEOs, qualities some retail traders find refreshing. But it is their capacity to profit from artificial intelligence (AI) that makes them truly popular.

However, Palantir and Tesla are two of the three most expensive stocks in the S&P 500 ( ^GSPC 0.47% ) , and a preponderance of insider selling may be a warning sign. Form 4 filings show board members and executives at Palantir and Tesla sold a net total of $12.7 billion in stock in the

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