1972

1972 is identified as the year R. L. Trivers introduced the concept of Parental Investment, a measure of parental care in terms of its effects on an offspring’s survival and on the parent’s capacity to invest in others. In the book, the date sits within a broader argument: evolutionary behavior can be analyzed through costs to reproductive success, while human and animal systems of kinship, care, and population control must not be mistaken for one another.

Parental Investment and its measure

The discussion begins with the practical difficulty of comparing what a parent gives to a young animal. Food is only one resource: investment can also involve effort spent obtaining it, risk taken to protect offspring, time devoted to maintaining a nest or home, protection from the elements, and, in some species, teaching. Calories might appear to provide a common measure, but energy costs do not translate neatly into the outcome that matters to an evolutionary account: the survival of genes.

Trivers’s concept offers a more relevant unit. Parental Investment is defined as investment in an individual offspring that raises its prospects of survival and reproductive success while reducing the parent’s ability to invest in other offspring. The measure therefore captures a cost not simply as a quantity of food or energy, but as a possible reduction in the prospects of another young animal. In the book’s example, milk consumed by one of two babies counts partly through the increased chance that the other will die without it.

The concept also makes unequal parental care analyzable without treating a parent’s “favourite” as a conscious preference. A mother can allocate food, protection, effort, and time differently among her offspring; the evolutionary question is how that allocation affects the prospects of each. The date 1972 thus anchors a framework for comparing forms of care in units tied to competing reproductive outcomes.

Kinship beyond parent and child

The account places parental care within the wider logic of kin altruism. A parent and child have a relatedness of one-half, but so do two full siblings. On that basis, an adult is genetically as closely related to an orphaned baby brother as to its own child; a gene favoring care for a younger sibling could spread on the same broad terms as one favoring parental care. The text stresses that this is a simplified comparison, and that sibling care is much less common in nature, but rejects the idea that parenthood is genetically unique.

Relatedness is not always directly known. The book proposes estimating it from shared ancestors and the number of generational steps between two individuals, while noting that actual shared genes vary by chance. Where kinship is uncertain, an average estimate may be used: two individuals who could be either half-brothers or full brothers do not have a single certain relatedness value unless further information is available.

Human kinship, however, is not only genetic. People recognize relations through names, marriage, written records, memory, customs, and rituals. Family obligations, ancestor worship, blood-feuds, and inter-clan conflict can be interpreted in relation to kinship, but cultural ideas of family should not simply be equated with genetic relatedness. The discussion also distinguishes the rationale for an incest taboo from kin altruism: it presents the taboo as presumably connected to harmful recessive genes associated with inbreeding, rather than as a means of promoting altruism.

Behavioral rules and imperfect recognition

Wild animals need not consciously identify relatives or calculate genetic advantage. The book asks instead what simple inherited rules might tend, under ordinary conditions, to benefit kin. A tendency to favor individuals who resemble oneself could sometimes produce that effect, but only statistically. If circumstances change—for example, if a species begins living in larger groups—the rule may lead to mistaken choices. Resemblance-based behavior is therefore presented as a rough mechanism, not reliable knowledge of family ties.

Adoption supplies another example of imperfect recognition. An orphan in a herd or troop may be taken in by a female who has lost her own young. Observers may call her an “aunt,” although there is often no evidence that she is related. From the book’s gene-centered perspective, adoption may be a misfiring of a built-in rule: the female spends time and energy on an unrelated orphan that could otherwise go toward her own kin or future offspring. Some orphans are not adopted and are left to die.

The more extreme case described is a bereaved monkey mother stealing and caring for another female’s baby. The account treats this as a particularly puzzling case because the adopter may both invest in an unrelated infant and relieve its real mother of the costs of child-rearing. Rather than resolving the example, the book calls for research into how often it occurs, the likely relatedness between adopter and infant, and the real mother’s response. It raises, without answering, whether mothers might deceive inexperienced females into adopting their young.

Population regulation and individual interests

The year also falls within a discussion of population regulation and the disagreement associated with Wynne-Edwards. His proposal, as presented in the book, is that animals deliberately and altruistically limit births for the benefit of the group. The hypothesis is attractive as a response to human fears about overpopulation, but the book contrasts conscious human concern for future welfare with its account of animal behavior: genes cannot be assumed to foresee the consequences of population growth or to act for the welfare of a species as a whole.

The text distinguishes population growth from the timing of births. For the world population, births and deaths are the relevant changes because immigration and emigration cancel at that scale. If couples average more than two children who survive to reproduce, growth compounds rather than increasing by a fixed number; lengthening the interval between generations can slow the annual rate. Its illustration of Latin America imagines the consequences of continued growth over centuries, but explicitly marks the calculation as hypothetical and notes that famine, plague, war, or birth control would prevent that trajectory.

Animal populations likewise do not grow indefinitely, but their patterns vary. Some remain relatively stable, while others, including lemmings, undergo surges followed by crashes and near-extinction; the Canadian lynx population is described as oscillating in estimates based on pelts sold by the Hudson’s Bay Company. The book agrees that wild populations are regulated, but disputes the explanation that animals restrain reproduction altruistically for the group.

Instead, the discussion turns to the costs faced by parents themselves. Rearing birds requires investment in eggs, nests, incubation, and repeated food gathering; a parent Great tit brings an average of one item of food to the nest every 30 seconds of daylight. Under the Lack theory, natural selection adjusts clutch or litter size to the resources parents can provide. Parents who produce more young than they can rear leave fewer descendants because fewer of those young survive—not because the entire population is threatened.

The contrast with modern human society is the role of public support. Where a welfare state keeps children alive even when their parents lack the resources to support them, the family’s immediate resource limit no longer operates in the same way. The book argues that this makes the human case different from the natural conditions under which excessive reproduction is penalized through reduced survival of offspring. Across these examples, 1972 serves as a precise date for a concept of parental cost, while the surrounding argument insists that care and reproductive restraint be explained through their actual effects rather than assumed to serve a larger collective purpose.