WHAT makes some men miserly and others generous? What motivated Bill Gates, for example, to make more than $28 billion in philanthropic gifts while many of his billionaire peers kept relatively tightfisted control over their personal fortunes? New evidence reveals a surprising answer. The mere presence of female family members — even infants — can be enough to nudge men in the generous direction. In a provocative new study, the researchers Michael Dahl, Cristian Dezso and David Gaddis Ross examined generosity and what inspires it in wealthy men.