1930

The book invokes 1930 as the year when R. A. Fisher used the term “parental expenditure,” anticipating a central problem in evolutionary accounts of care: how to measure what a parent gives one offspring in terms of what that investment costs others. Its importance lies in this conceptual link, rather than in a narrated historical episode: the date anchors a comparison between Fisher’s phrasing and R. L. Trivers’s later concept of parental investment.

The date and parental expenditure

In discussing how a mother might distribute resources unequally among her children, the book identifies food, the effort required to gather it, protection from predators, nest or home maintenance, shelter from the elements, and, in some species, teaching as forms of parental investment. These costs are difficult to reduce to a single measure. Energy, such as calories, offers one possible accounting unit, but it does not directly capture the evolutionary consequence that matters: the survival of genes.

Against this background, 1930 marks Fisher’s use of “parental expenditure.” The book suggests that Fisher meant much the same thing as Trivers’s later concept, while carefully presenting this as a comparison of ideas rather than claiming that the two formulations are identical in every respect.

From expenditure to investment

The book credits Trivers with neatly solving the measurement problem in 1972 by defining parental investment as a parent’s investment in an individual offspring that improves that offspring’s survival and reproductive prospects at a cost to the parent’s ability to invest in other offspring. The definition shifts attention away from the physical amount of a resource and toward its opportunity cost.

Milk makes the logic concrete. If a mother has two babies, X and Y, the significance of a pint of milk consumed by X is measured partly by the increased probability that Y will die without it. Parental investment is therefore expressed in terms of the reduced life expectancy of other children, already born or yet to be born, not simply in pints or calories.

The limits of the measure

The book also qualifies parental investment as an imperfect measure because it privileges parentage over other genetic relationships. An ideal measure would account for the appropriate relatedness between the person providing help and every potential recipient, including other children, nieces, nephews, and the helper herself. On that broader account, investment means improving another individual’s chance of survival at a cost to the capacity to invest elsewhere, with costs weighted by genetic relatedness.

That qualification links the 1930 reference to the book’s larger treatment of kin altruism. Its discussion distinguishes genetic relatedness from the ways people identify and organize kinship: names, marriage customs, written records, family obligations, rituals, and ancestor worship shape human understandings of who counts as family. Animal rules of behavior need not involve conscious recognition of kin; a practical rule can produce benefits for relatives on average, even when it sometimes misfires. The date thus belongs to a broader effort to make care and its costs comparable without treating parental bonds as the only relevant form of genetic relationship.