Marrying your cousin: Exploring the economic and health effects of kinship marriage

Earlier this year, The Economist published an article titled “Cousin marriage is probably fine in most cases.” The article cited the updated National Society of Genetic Counsellors guidelines for consanguineous couples (meaning couples who share a bloodline) which noted that in couples with no known genetic disorders, there is an associated 1.7 to 2.8 per cent risk increase in birth defects in comparison to the general population.

Despite half the US states banning marriage between first-cousins, The Economist argued that these types of marriages were still “pretty low risk.”

Recent research from UBC economists Dr. Sam Hwang and Dr. Munir Squires says otherwise. They use historical US census data to explore the intersection of kinship marriages with economic and health factors, finding significant health effects on offspring.

Historical economic effects on close-kin marriage

Last year, Hwang and Squires, along with Duke University’s Dr. Arkadev Ghosh, published research exploring the economics of close-kin marriages.

“The amazing thing about this historical US data is that you can kind of follow people and follow families over a very long period of time with good data on people for 150 years,” said Squires.

Using census data from 1850-1940, they first established that married cousin couples were more common among lower-paying occupations.

Then, they turned to an analysis of US state legislation against cousin marriages to show that bans led individuals from families with high rates of cousin marriages to move into urban areas, which resulted in income increases. They hypothesize that with declining rates of cousin marriages came weaker family ties and more dispersal and migration.

In addition to weaker attachments to locations, they also argue that the bans led to weaker family ties. As a result, the data shows the household size shifting toward the nuclear model partially due to having fewer children and fewer cohabiting relatives. Their research also shows a “dark side” to this cultural transition. This includes more people living in institutional settings, including elderly homes and infirmaries, because their families were increasingly dispersed.

Cousin marriages can lead to decreased life expectancies

In Squires and Hwang’s recent research, as well as a forthcoming publication alongside pre-doctoral student Deaglan Jakob, they shift their focus from economic consequences to health consequences.

“In our first paper, we say that we think genetic effects on health because of marriage are not very big,” said Squires, in reference to the paper on economic consequences. At the time, most of the literature agreed with this consensus, in that there was some health effect, albeit still quite small.

Their research provides new evidence that the lifespan effects may actually be quite large.

Specifically, their sample showed that marrying between cousins can lead to more than a three-year reduction in offspring life expectancy. Interestingly, Squires noted that the gap between children of cousins and non-cousins stays the same for nearly the entire 150 years they studied.

“We also find these big health effects in adulthood, which is something that people hadn't looked at before,” he said.

Similarly to their previous piece, their methodology uses historical US genealogical records. Instead, this time the data came from FamilySearch, a crowdsourced database.

Hwang and Squires research sheds new light on the effects of kinship marriage structures.

“We're not the first paper to look at the health effects of cousin marriage. There's probably hundreds of papers on that,” said Squires.

“But ours is the first paper that looks at the effects throughout the lifetime,” he said. Squires explained that studies like this commonly ask the parents to report the number of children they have had, how many died, and at what age. However, this fails to account for offspring that might die at later ages of 60 or 70. By examining the entire lifespan and comparing it across the offspring’s cousins, they can make more comprehensive conclusions about long-term health effects.

Hwang echoed these sentiments by noting their research reaches novel conclusions that prior research has not. Contributing to the vast stock of existing literature is what drives his passion for research.

“Pushing the boundaries of knowledge, that’s important.”