Should you marry for genes or money?

Charles Darwin and his son might have a few ideas

In 1883, George Darwin was appointed Professor of Astronomy and Experimental Philosophy at the University of Cambridge. His research field was the ocean’s tides, a topic he would become a leading expert on, but he was also interested in questions about the moon’s origins.

However, Darwin also had an interest in genetics. His father was, of course, Charles Darwin, the renowned biologist who developed the theory of evolution. But father and son’s interest in genetics was not just due to scientific curiosity. The Darwin family was not a happy one. Of Charles’s ten children, three died young. The others, including George, were often ill. And they hoped science had an answer.

Darwin Sr. was one of the first to demonstrate the adverse effects of inbreeding experimentally. He observed that self-fertilisation in plants often led to weaker offspring. This had Charles wondering about his own marriage. See, Charles Darwin and his wife, Emma Wedgwood, were cousins, as were his parents. In Victorian England, such marriages were not uncommon, a way to keep wealth and status within the family. However, because of his experiments, Darwin suspected that his children’s poor health resulted from his and his parents’ inbreeding.

His son, George, was not convinced. He was, after all, a professor at Cambridge. While busy with his studies of the moon and tides, he also wrote an article – Marriages Between First Cousins in England and Their Effects – which found evidence that children from consanguineous marriages are not necessarily worse off. How did he do it? He calculated the probability that people with the same last name marry each other – about 4% – and then calculated that no more than 4% of people in insane asylums are the children of consanguineous marriages:

Taking into account the uncertainty of my methods of finding the proportion of such marriages in the general population, the percentage of such offspring in asylums is not greater than that in the general population, to such an extent as to enable one to say positively, that the marriage of first cousins has any effect in the production of insanity or idiocy.

But George was wrong, and Charles was right. In 2014, researchers studied 30 marriages within the Darwin-Wedgwood family tree, including Darwin’s own children. They found that men in the Darwin lineage who were the product of inbreeding had fewer children and experienced shorter periods in which they fathered children. This was the first time that a study showed that human inbreeding had a negative effect on male fertility.

A new Quarterly Journal of Economics paper by Arkadev Ghosh, Sam Il Myoung Hwang and Munir Squires confirms these results for America, but instead of just 30 marriages, they use 12 million. They find that cousin marriages decreased from 7% in the eighteenth century to 1.5% in the early twentieth century. They then do something interesting: they investigate the effects of a ban on cousin marriages in certain American states. They find that such legislation forced men to move from the countryside (where they were typically more inclined to marry their cousins) to the cities. There, they found better-paying jobs, which led to upward social mobility, with better outcomes for their children. However, the authors emphasize that this social mobility was due to cultural and not genetic changes.

To what extent your genes determine your success in life is a question not only the Darwins were interested in. But the topic comes with a big warning sign: Charles Darwin’s half-cousin, Francis Galton, was one of the leading proponents of eugenics, the study of the selective reproduction of humans to improve the genetic quality of the population, a field that would lead to cruel consequences in the 20th century.

But one does not have to go to this extreme to wonder what our children really ‘inherit’ from us: is it our behaviour that they learn or simply our genes that they receive at birth? Some would say it’s an unanswerable question: that our genes and environment are too closely intertwined to distinguish the influence of the two. But, equipped with new methods and especially new (Big Data) sources, academics are once again trying to solve this ancient riddle.

The publication of two recent articles has caught the attention of many academics (and made some hairs stand on end). Economic historian Gregory Clark argues, in a new PNAS paper, that most of our social status, and even our level of education, depends on our genes.

He studies the family trees of 422,374 people in England between 1600 and 2022 and finds remarkably high correlations in status between people who are five generations apart. He also finds that these correlations do not change over time; despite substantial changes in the structure of the English economy due to the Industrial Revolution and social institutions, such as voting rights and education for everyone, these correlations remain the same. His third and most controversial conclusion is that a simple genetic model can explain all these correlations.

A second article, by Dolores Collado, Ignacio Ortuño-Ortín and Jan Stuhler in The Review of Economic Studies, uses detailed Swedish data and finds strong multigenerational correlations but attributes it to cultural factors rather than genetics. They look at the status of people's in-laws (with no genetic relationship) and also find a high correlation between the status of different generations. They attribute this to assortative mating, people’s preference to marry people of the same status group as themselves.

This is precisely the conundrum: Clark would argue that people marry based on their genetic fit – equally intelligent people, beautiful people, or athletic people are attracted to each other. The Swedish authors would argue that we match on cultural preferences – people who are equally wealthy, equally educated, or equally status-conscious. (Razib Khan recently hosted a podcast that discussed both papers.) ‘He is a chip off the old block’, we say. But is that ‘block’ due to nature or nurture?

The good news is that with better data and methods, we are getting closer to an answer. A new (unpublished) study by researchers affiliated with the Cape of Good Hope Panel project uses 18th-century tax censuses and the South African family register of Afrikaner families to examine consanguineous marriages and their consequences in the early Cape Colony. They show that marriages between cousins were a known phenomenon in early Cape Town. Interestingly, it was wealthier families who entered into such marriages – evidence of a cultural preference to keep people’s status and wealth ‘within the family’ – but the outcome was that the children from these marriages were not better off than the rest of the population. Despite these wealthy parents’ best efforts to preserve status and wealth within the family, their genetic inbreeding meant the opposite.

Charles Darwin would have found some comfort in these findings, empirical proof that supported his theories. His Cambridge astronomer son, however, would have been less pleased.

An edited version of this post appeared (in Afrikaans) in Rapport on 10 September 2023. The image was created using Midjourney v5.2.