In order to predict the human population of the future, scientists often view data from the past. With the help of a database containing historical records that date as far back as 871 A.D., an anthropologist at the University of Missouri was able to show reproductive patterns that shed light on the trade-off of quantity and quality.

This is a biological concept that has been used to describe a parent’s decisions they make on a subconscious level that balance between producing and how much time they choose to invest in offspring. Results from this particular study may be used in order to predict the growth of our population and could also help to explain how parents choose to spend their time and financial resources when it comes to raising their children.

Robert Lynch, a post-doctoral fellow in anthropology in the MU College of Arts and Science says the database is probably the best record of human reproduction on planet earth with centuries of data included in the collection. With the use of this incredible resource, the team was able to evaluate both the relationships and trade-offs among fertility, mortality and parental investment, as well as the amount of time parents spend during child-rearing. One of the main goals of the research was to see how parental investment (including both time, and money) impact the lifespan and reproductive success of offspring.

Lynch analyzed data with the help of a partnership with deCode Genetics from people who were born during the years 1700 and 1919. The dates were chosen to make sure the data was reliable and that all people involved in the study had complete life histories. The relationship between mortality rates and lifetime reproduction were examined.

The study concluded that parents and their offspring do not have comparable lifespans or reproductive patterns, but siblings who share the same mother and father will have similar lives as well as reproductive patterns. This suggests that the success of children is directly related to parental investment. The study also found that parents who have more full siblings had shorter lifespans and reproduced less often. Meaning, each subsequent full sibling reduced a fitness cost on the previous siblings.

Lynch has said that it is important for parents to find a way to maintain a balance between how much they invest as well as recognizing that for each child who is added to the family, a cost is placed on the lifestyle of all previous children. When people ask how long they have to live or how many children they will have, they can take a direct look at their siblings to get a pretty good idea.

Based on the results from the study, Lynch is working on expanding familial relationships to include half siblings and even first cousins to establish research in the future. Half siblings and first cousins share genetic traits; however they lack many key environmental factors, like sharing the same household and specific upbringing.

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