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Trivers treated a parent animal as if it were a rational agent calculating what economists call the \u2018opportunity cost\u2019 of an action. A parent has to pay costs of rearing each offspring. Among these costs might be food, including time and effort spent gathering it, time spent protecting the child from predators, and the risks incurred by the parent in doing so. Trivers wrapped them all up in one metric which he called Parental Investment or PI. Trivers\u2019 key insight was that PI must be an opportunity cost: the investment in any one child is measured as lost opportunities to invest in other children. Trivers used the notion to develop a penetrating theory of \u2018parent – offspring conflict\u2019. The decision on the best time to wean a child, for example, is subject to a \u2018disagreement\u2019 between the child and its mother, both behaving as rational economists whose \u2018utility function\u2019 is the long term survival of their own genes. The mother \u2018wants\u2019 to terminate suckling earlier than the child does, she place greater \u2018value\u2019 then he does on her future offspring, who will benefit from early weaning of the present child. The present child also \u2018values\u2019 his future siblings, but only half as highly as his mother does because of the way Hamilton\u2019s Rule pans out. Therefore there is a period of \u2018weaning conflict\u2019, an uneasy phase of transition between the early time when both parties \u2018agree\u2019 that suckling should continue and the later time when both parties \u2018agree\u2019 that it should end. During this phase, when the mother \u2018wants\u2019 weaning but the child doesn\u2019t, observers of animal behaviour should see the symptoms of a subtle battle between mother and child. In passing I should add that, long after The Selfish Gene<\/em> was published, the Australian biologist David Haig cleverly showed how many of the ailments of pregnancy can be explained in terms of the same Triversian conflict going on inside the womb – not about weaning in this case, obviously, but about other aspects of the allocation of necessarily scarce resources.<\/p>\n\n\n\n