The Selfish Gene

Dawkins addresses issues dividing generations in genetic terms, rather than philosophically or psychologically: “Should a mother have favourites, or should she be equally altruistic towards all her children?” (94). Mothers invest food, time, and risk for their offspring. They allot these resources among children. Ecologists measure investment in caloric energy. Evolutionists measures using gene survival. R.L. Trivers developed the Parental Investment model:

Parental Investment (P.I.) is defined as 'any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring.' The beauty of Trivers's parental investment is that it is measured in units very close to the units that really matter. When a child uses up some of its mother's milk, the amount of milk consumed is measured not in pints, not in calories, but in units of detriment to other children of the same mother (94).

Parental investment in one child decreases the life expectancy of siblings, whether already or not yet born. Dawkins generalizes the concept to any altruistic investment. People have finite resources to invest. If a mother has too many children, she cannot feed them all and they fail to survive.