by Molly Peterson

When predicting future global population growth, sometimes scientists look to the past. Using a database with historical records that began in 871 A.D., an anthropologist at the University of Missouri was able to show reproductive patterns and shed new light on the “quantity-quality” trade-off, a biological concept used to describe a parent’s unconscious decisions to balance between producing and the time and investing in offspring. Results from this study could help to predict future population growth and could help explain how parents allocate their time and financial resources in raising their children.

“The database is probably the best record of human reproduction on earth, with centuries’ worth of data,” said Robert Lynch, a post-doctoral fellow in anthropology in the MU College of Arts and Science. “Using this incredible resource, we evaluated the relationships and trade-offs among fertility, mortality and parental investment, or the amount of time parents spend in child-rearing. We wanted to see how parental investment — including the amount of time and financial resources parents spend with each of their children — impacted the lifespans and reproductive success of offspring.”

Lynch, in partnership with deCode Genetics, analyzed data from individuals born between 1700 and 1919. These dates were chosen to ensure the data were reliable and that all individuals in the study had complete life histories. The analyses examined the relationship between mortality rates and lifetime reproduction.

The study found that parents and offspring do not have similar lifespans or reproductive patterns. However, siblings who share the same mother and father (referred to as “full siblings”) were found to have similar lifespans and reproductive success. These similarities among full siblings suggests that parental investment impacts how successful their children are when they decide to reproduce. The study found that parents who had more full siblings had shorter lifespans and reproduced less. In other words each subsequent full sibling imposed a fitness cost on all the previous ones.

“It is important for parents to maintain a balance between investment, which can be hard to measure, and reproduction,” Lynch said. “It is important to recognize that, as a result of the investment balance that is struck by the parents, there is a cost to children when they have more siblings. For each additional sibling, the cost is one year of life less for their brothers or sisters and siblings may have fewer children. So, when people ask me how long they will live or how many kids they will have, I tell them to look to their siblings.”

With the results from this study, Lynch is expanding familial relationships to include half siblings and first cousins to further establish the impact of parental investment on reproductive success of offspring. Half siblings and first cousins share genetic traits; yet they lack many key environmental factors, like a shared household, Lynch said.

The study, “Parents Face Quantity-Quality Trade-Offs Between Reproduction and Investment in Offspring in Iceland,” recently was published by the Royal Society Open Science.