Chapter 8: Battle of the Generations

Battle of the Generations utilizes R.L. Trivers' concept of Parental Investment (P.I.) to quantify the resources a mother allocates to a child—such as food, energy, and risk protection—measured specifically by the detriment caused to her ability to invest in future offspring. The discussion challenges the notion of maternal benevolence, explaining that while a mother is equally related to all her children, she may genetically favor those with better life expectancy, potentially ignoring or sacrificing runts to maximize her total reproductive success. A significant portion of the chapter is dedicated to the timing of weaning, illustrating a biological tug-of-war where the mother wishes to cease investment when the cost exceeds the benefit, while the child is programmed to demand resources until the cost to their siblings is double the benefit to themselves. The text also offers an adaptive explanation for menopause, positing that it evolved because genes for investing in grandchildren became more successful than genes for risky, less efficient late-life reproduction. Furthermore, the chapter explores aggressive selfishness in nature, contrasting human sibling rivalry with the ruthless behaviors of brood parasites like cuckoos, which may kill foster siblings to monopolize food. Finally, the author refutes R.D. Alexander’s theory of parental manipulation—which suggests parents always win evolutionary conflicts—by arguing that genes are selected to maximize advantage at every life stage, meaning children effectively evolve methods to deceive and exploit parents just as parents evolve to detect such cheating. The chapter concludes with the sobering insight that altruism is not innate in biology and must therefore be taught.