Final answer:
Altruism will spread within a population when the benefits to the recipient are high, costs to the actor are low, and when the participants are closely related or engaged in reciprocal relationships that are maintained through social structures.
Step-by-step explanation:
The student's question is about the conditions under which altruism will spread within a population. Based on the principles of social behavior in biology, altruism will spread when the benefits to the recipient are high, the cost to the actor is low, and the participants are closely related. This is often observed in the animal kingdom and can be explained by the concept of inclusive fitness, where an individual's genetic success is believed to be derived from cooperation and altruistic behavior, not just by personal reproduction but by also helping close relatives who share many of the same genes.
Social exchange theory further elucidates why humans exhibit altruistic behavior by suggesting that individuals aim to maximize the benefits and minimize the costs of social exchanges. This principle applies to various relationships, ranging from romantic partnerships to interactions among unrelated individuals, such as reciprocal altruism observed in many monkey species. Reciprocal altruism requires repeated encounters and a social structure that punishes cheaters to function effectively.