Data from: Parents face quantity-quality tradeoffs between reproduction and investment in offspring in Iceland

How to optimally allocate time, energy, and investment in an effort to maximize one’s reproductive success is a fundamental problem faced by all organisms. This effort is complicated when the production of each additional offspring dilutes the resources that parents have available to invest. Although a quantity-quality tradeoff between producing and investing in offspring has long been assumed in evolutionary biology, testing it directly in humans is difficult, partly due to the long generation time of our species. Using an Icelandic genealogy, which spans two centuries, I am able to address this issue and analyze the quantity-quality tradeoff in humans. I demonstrate that a parent’s impact on offspring’s fitness most likely results from parental investment rather than genetic contribution. In addition, this effect changes significantly across time in response to environmental conditions: parents invest more in reproduction when mortality is low and invest more in offspring when mortality is high. Overall, increasing reproduction has negative fitness consequences on offspring, such that each additional sibling reduces an individual’s average lifespan and lifetime reproductive success. This analysis provides insight into the evolutionary conflict between producing and investing in children while also shedding light on some of the causes of the demographic transition.