When bulls stampede on Wall Street, brokerage stocks are usually the first to jump. So it goes in the current equity bull run: Since late last year, Standard & Poor's index of brokerage shares has climbed 38%, more than double the market's gain. The trend isn't completely incidental. Brokerages are sort of the groundhog of Wall Street: Historically, they have started to sputter four to six weeks before the broader market heads south. For now, there's no sign of weakness.

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