Definition
Parent-offspring conflict is the evolutionarily expected divergence of interest between a parent and its offspring over the amount of parental investment the child should receive. Introduced by Robert Trivers in 1974, the theory explains weaning conflicts, sibling rivalry, manipulative begging signals, and the cuckoo's exploit of host parental circuitry.
The mathematics is short: a parent is equally related (r = ½) to each of its offspring, so the parent's gene-level optimum is to distribute investment so as to maximize the total reproductive output across all children. Each offspring, by contrast, is related to itself by r = 1 and to its siblings by only r = ½ — so each child values its own investment twice as much as its parent does, relative to investment in siblings.
The two optimums do not coincide. Conflict follows.
Why it matters
How it works
The parent's gene pool benefits when investment is distributed evenly enough that total offspring survival is maximized. The offspring's gene pool benefits when that particular offspring gets more than its even share — even at some cost to siblings.
Both parties have evolutionary pressure to behave according to their own optimum. The parent pulls toward equal distribution and weaning at age T. The offspring pulls toward more investment and weaning at age T + ΔT. The outcome — actual age of weaning, actual distribution of food, actual response to begging — is the equilibrium of this conflict, shaped by who can hold out longer.
Children have only their persistence and their evolved appeal: crying, cuteness, begging displays. These are signals selected because they extract investment from parental nervous systems. The cuckoo chick exploits the same circuitry — the cuckoo's exaggerated gape is the parasite's extended phenotype reaching into the host's parental-care system.