Alice and Bob live in the forest. To sustain themselves, they collect fruits and berries and snare an occasional animal. The nights get cold, but Alice is a skillful fire-builder. Bob has never mastered this art. His fires fizzle and he never seems to collect the right kind of wood. Alice divides her time between collecting food and gathering wood. She does this in such a way that her marginal benefit from time spent collecting food is the same as that from gathering wood. Bob does not attempt to build fires. He spends all of his time gathering food, and every night slinks up and huddles beside Alice’s fire. Bob appreciates the fire’s warmth, but wishes it were larger. Bob has learned to leave morsels of food by the fire for Alice. Warmth and food are both “normal goods ” for Alice. The extra food that Bob leaves induces her to increase her total food consumption, but not by the total amount that Bob leaves for her. She uses some of the time saved by Bob’s gifts to gather more firewood. 1.1 Equilibrium with Unilateral Gifts–An Example Alice’s utility function is U(cA, y) = cAy where cA is the amount of food that she eats and y is the amount of wood on the fire. She has T hours to allocate between collecting food and wood. In an hour, she can collect either one unit of wood or πA units of food. If Bob leaves g units of food by the fire, she maximizes her utility by choosing y = 1

How should benefit-cost analysis account for the value that benevolent individuals place on others' enjoyment of public goods? When adding up the benefits to be compared with costs, should we sum the private valuations, the altruistic valuations, or something else? This paper argues that private valuations are appropriate if concern for the well-being of others respects their private preferences. The discussion has implications for family decision-making, welfare economics, and the design of applied contingent valuation studies.

This paper explores the evolutionary foundations of altruism among siblings and extends the biologists' kin-selection theory to a richer class of games between relatives. It shows that a population will resist invasion by dominant mutant genes if individuals maximize a "semi-Kantian" utility function in games with their siblings. It is shown that a population that resists invasion by dominant mutants may be invaded by recessive mutants. Conditions are found under which a population resists invasion by dominant and also by recessive mutants.

Stem cell transplants save lives of many patients with blood diseases. Donation is painful, but rarely has lasting adverse effects. Patients can accept transplants only from donors with compatible immune systems. Those lacking a sibling match must seek donations from the general population. The probability that two unrelated persons are compatible is less than 1/10,000. Health authorities maintain a registry of several million genetically tested potential donors who agree to donate if asked. We find that the benefits of adding registrants of every race exceed costs. We also explore the peculiar structure of voluntary public good provision that faces potential donors.