More executives at Standard & Poor’s 500 Index companies are buying their stock than any time since the depths of the credit crisis after valuations plunged 25 percent below their five-decade average.

Sixty-six insiders at 50 companies bought shares between Aug. 3 and Aug. 9, the most since the five days ended March 9, 2009, when the benchmark index for U.S. equities reached a 12- year low, according to data compiled by Bloomberg. Morgan Stanley (MS) Chief Executive Officer James Gorman and two other managers purchased 175,000 shares of the New York-based bank as the shares fell to the lowest level since March 2009, according to filings with the U.S. Securities and Exchange Commission.

Almost $3 trillion has been erased from U.S. equity values in the last three weeks as signs the economy is slowing and S&P’s downgrade of the government’s AAA credit rating left the benchmark gauge for U.S. shares within 30 points of a bear market. Some analysts say insider buying is bullish because executives have the best information about their prospects.

“Nobody knows a company better than the people running it,” Shawn Price, who manages $2.4 billion at Navellier & Associates Inc. in Reno, Nevada, said in a telephone interview. “It’s a positive sign that they are committing their personal capital.”....