All expenditures involve an opportunity cost. This is true in economics: Money spent on one activity is unavailable for other activities. But it is also true in evolutionary biology: Time, resources, or energy expended on one fitness-enhancing activity is unavailable for other fitness-enhancing activities. Organisms, like consumers, are faced by trade-offs. Beyond a certain level of reproductive expenditure on any particular offspring, a parent's resources are better allocated to other uses, say to fighting disease or laying down fat to survive the next winter. And these other uses, either directly or indirectly, translate into less investment in other offspring. An organism usually maximizes its expected number of surviving offspring, not by investing everything in a single offspring, but by spreading its reproductive effort across multiple offspring (1).

Trivers (2) recognized that the reproductive trade-off between continued investment in one particular offspring and investment in other offspring implied the existence of an evolutionary conflict between parents and offspring. A parent is equally related to all its offspring (a gene in the parent has a 50% chance of being transmitted to each offspring), but an offspring is more closely related to itself than to its sibs (a gene in an offspring is definitely present in that offspring but has only a probability of being present in the other offspring that compete for parental resources). Therefore, offspring will have evolved to extract more investment from parents than parents have evolved to supply. It should be emphasized that this is not a conflict in which every gain to the offspring is a loss to the parent, but one in which conflict arises over how much the parent supplies.