Parental Dilemma: More Kids or Better Outcomes?
A couple who decides to have children faces a choice—whether consciously or not—to either have fewer children and invest their resources in the health and well-being of those children, or to have more children with the realization that there will be fewer parental resources to invest in each child. Robert Lynch, a postdoctoral student in anthropology, says this is the well-known biological concept of a quality–quantity trade-off. However, Lynch says that while this trade-off has long been an assumption in evolutionary biology, testing this assumption in humans has been difficult because of the long generation time of our species—until he discovered an Icelandic genealogy that spans more than two centuries.
“This huge database is the best part of the story,” Lynch says. “It’s probably the best record of human reproduction on Earth, ever. The database has 600,000 individuals, and it’s got good data going back to 1700, so we can see historic trends over time.”
What those historical trends revealed is that fertility rates precede mortality rates—as people have fewer children, the average lifespan of those offspring begins to increase, which runs counter to the conclusions of previous studies.
“People have fewer kids because of environmental conditions—there’s some signal in the environment, though we don’t know what it is—and around the same time these parents begin investing more in their children,” Lynch says. “So they are lowering reproduction while increasing investment per child, and in about 20 years that seems to reduce mortality rates, and you see that pattern continuing for a couple hundred years.“
Lynch admits it’s impossible to measure the “investment” Icelandic parents made in their children 200 years ago, but he can approximate that investment by comparing how much full siblings reproduce like each other. Because siblings in a nuclear family will have similar genetic profiles, Lynch says the fact that siblings tend to reproduce alike suggests an environmental factor—siblings share a common household.
“When people ask me what my paper is about, I tell them that if they want to know how long they are going to live or how many children they are going to have, just look at their siblings,” Lynch says. “Parental investment is the key. When you have fewer kids, you can invest more in each kid, and when you invest more, they live. Your siblings cost you—you lose a year of your life for each sibling, and you also lose .07 (future) children per sibling.”
Lynch notes that following Iceland’s Laki volcanic eruption in 1783 that killed a third of the population, fertility rates exploded, but lifespans shortened. In other words, people had more children following the natural disaster but had fewer resources to invest in each child, increasing the likelihood those children would die at a younger age. In new a new paper, he tries to pinpoint the precise reason for that correlation as parental investment. He expands the range of familial relationships to include half siblings and cousins to determine if shared parents is the main reason siblings reproduce at similar rates.
His research is being published this month by the Royal Society Open Science Journal.