Fact-checking question from Freakonomics / Shaw quote

[Shapiro, Fred]

I have been asked by a fact-checker for the forthcoming Freakonomics book about the following quote:

"Few people think more than two or three times a year; I have made an international reputation for myself by thinking once or twice a week."

Can anyone help me trace this quote back to its earliest findable source, and help me ascertain the plausibility of its being attributed to George Bernard Shaw?

Fred Shapiro

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