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But most stocks weren’t that over­heated. Many ma­jor com­pa­nies traded at 14 to 19 times earn­ings around the mar­ket’s peak in Sep­tember 1929. Prof­its were grow­ing far faster than stock prices. In­dus­trial stocks be­gan 1929 priced at about 15 times earn­ings; by Sep­tember, they traded at slightly above 13 times, the econ­omist Irv­ing Fisher pointed out in his 1930 book “The Stock Mar­ket Crash—and Af­ter.”

The hottest stock of 1929, Ra­dio Corp. of Amer­ica, peaked at 73 times earn­ings and more than 16 times book value, ac­cord­ing to Mr. Wig­more’s re­con­struc­tion. How does that com­pare to to­day’s tech­nol­ogy dar­lings? Ama­zon.­com Inc. traded ear­lier this week at 73 times earn­ings and more than 16 times book value, ac­cord­ing to Fact­Set.

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