One weekend several years ago, as I was arriving at my neighborhood farmers’ market, a college student approached me with a clipboard and an impassioned, if easily forgettable pitch for Greenpeace. If I signed the papers he had in his hand, a $20 charge would appear on my credit card statement each month and a small but vital fraction of the world’s marine life could be saved. Confronted by an actual person and standing in the vicinity of cheese whose per-pound price was equal to the monthly cost of potentially saving a monk seal, I was too embarrassed to say, “Thanks, but I’ll pass,” only then to move on to the artisanal cheddars of Pennsylvania.

Need confronts us all the time, especially in New York, where density and dependence on mass transit place those on various sides of the economic divide into more routine contact than might occur in Houston or Denver or Seattle. Need, of course, has been escalating in recent years, and liberals fear that it would soar far more aggressively under a Republican leadership likely to make significant cuts to federal programs that benefit the poor. Could charitable inclination, further aroused by new tax breaks, really assume the burdens relinquished by government? Though it may be the case, as some would contend, that decentralized giving is the most efficient means of reaching the poor, the argument rests on the assumption that human beings give efficiently.

It is not entirely clear that they do. Americans give generously but they give polymorphously. Charitable contribution is motivated by compassion, affinity, history, fealty, social contest and, not infrequently, shame. The bulk of charitable donations in this country are made by individuals, and the passions of individuals do not typically align with the broader exigencies of a particular social moment. Nationally, 32 percent of the $298 billion given away last year went to religious institutions, 13 percent to cultural organizations and 12 percent to social services, according to a report issued annually by the American Association of Fundraising Counsel. But if giving were conducted with the greatest consideration paid to the most urgent needs of the society, then Yale, a private institution with a $19.2 billion endowment, would arguably never receive another 50 cents.