Definition
Parental investment is, in Robert Trivers's 1972 formulation, any investment by a parent in an individual offspring that increases the offspring's chance of survival (and hence the parent's reproductive success) at the cost of the parent's ability to invest in other current or future offspring.
The investment can be material (energy, food, protection) or temporal (gestation time, brooding, child-rearing). The defining feature is that it is finite and traded off — every unit given to one offspring is a unit unavailable to others.
Why it matters
How it works
A parent has a finite reproductive budget (in calories, time, energy). She must distribute that budget among current and potential future offspring. The gene-level optimum is the distribution that maximizes total surviving descendants — not the distribution that maximizes any single offspring's prospects.
Parental investment theory predicts trade-offs that are observed across species:
- Clutch size: more eggs per clutch means less investment per egg. The optimum is David Lack's Lack number — the clutch size that maximizes total surviving young.
- Sex-role asymmetry: in species where one sex (usually the female) invests substantially more per offspring, that sex is the limiting reproductive resource. The other sex competes for mating access.
- Mating-system stability: monogamy is stable when both sexes' optimum investment per offspring is high; polygamy is stable when one sex's investment is much higher than the other's.
- Parent-offspring conflict: the parent's optimum investment per offspring is less than the offspring's optimum, by a factor of two for siblings sharing r = ½.