  Reflective of income and wealth distributions, philanthropic gifting appears
to follow an approximate power-law size distribution as measured by the size of
gifts received by individual institutions. We explore the ecology of gifting by
analysing data sets of individual gifts for a diverse group of institutions
dedicated to education, medicine, art, public support, and religion. We find
that the detailed forms of gift-size distributions differ across but are
relatively constant within charity categories. We construct a model for how a
donor's income affects their giving preferences in different charity
categories, offering a mechanistic explanation for variations in institutional
gift-size distributions. We discuss how knowledge of gift-sized distributions
may be used to assess an institution's gift-giving profile, to help set
fundraising goals, and to design an institution-specific giving pyramid.
