Describing himself as “a teacher for the majority my life,” Mr. Jewett noted the strangeness of a declaration of his giving intentions, “as I have never sought to gather the kind of wealth required to feel like saying such a thing would have particular meaning.” Mr. Jewett teaches at the Lakeside School, which counts Mr. Gates among its prominent alumni.

Mr. Soskis, of the Urban Institute, said that in the past the events that have shaped giving decisions tended to come late in life, centering on retirement and death, in the form of bequests. Leading philanthropists are now much younger, raising a new set of issues. Priscilla Chan and her husband, the Facebook founder Mark Zuckerberg, cited the birth of their daughter as a motivating factor in their giving.

“The fact that giving is now happening in the prime of life means that giving decisions and giving narratives are being shaped by different life cycle events like divorce and marriage and birth in a way that hadn’t really been the case 30 or 40 years ago,” Mr. Soskis said.

Jack Beggcontributed research.

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