Is divorce hereditary? To some extent, according to new sociological research – and more so for women than for men.
Used DNA to study divorce
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– For women, genes collectively explain 9 per cent of the variation in the risk of relationship breakdown. This means that there are many other factors that explain why couples separate, says Ruth Eva Jørgensen. She has written a doctoral dissertation on the relationship between genetics and relationship breakdown. Photo: Amund Aasbrenn
Could our genes help explain why some relationships end in separation?
Sociologist Ruth Eva Jørgensen believes they could. In her doctoral research project, she explored the connection between genetics and relationship breakdown, and the extent to which the likelihood of separation is something that "runs in the family":
– Genes and genetically informed methods can help us understand processes of transmission between generations, from parents to children, says Jørgensen, who recently defended her doctoral dissertation at the University of Oslo.
Common explanations
There are many different reasons why relationships end in separation. In sociology and demography, researchers have focused on differences between groups and factors such as age, socioeconomic background, and education.
International and Norwegian research has shown that:
- People with higher levels of education are less likely to get divorced.
- Women are more likely to initiate a separation.
Other studies have examined the consequences of divorce for women and men.
– A German study, for example, found that men experience more severe short-term effects on their quality of life, while women are more likely to face lasting negative financial consequences.
Our genes cannot act on their own
Explains 9 per cent among women
Genetics has long been something of a "black box" in research on relationship breakdown. Although twin studies have established that genetic factors influence the likelihood of separation, we still do not know what this genetic influence consists of or which traits are important.
Jørgensen has used data from Norwegian registries and the Norwegian Mother, Father and Child Cohort Study (MoBa) to better understand the role of genetics.
– The registry data provides information about relationship breakdowns, among other things, while MoBa contains genetic data, she explains.
In the first analysis, the researchers attempted to estimate the overall influence of genetics.
They found a small gender difference.
– Overall, genetics appears to explain slightly more of the variation in relationship breakdown among women (9 per cent) than among men (3 per cent).
Jørgensen explains:
– For women, genes collectively explain 9 per cent of the variation in the risk of relationship breakdown. This means that there are many other factors that explain why couples separate.
More about the research method
How did Jørgensen arrive at these figures?
The researchers combined genetic data from the Norwegian Mother, Father and Child Cohort Study (MoBa) with Norwegian registry data.
- The registry data showed which individuals had experienced a relationship breakdown.
- The MoBa data provided information about each person’s genetics.
This allowed researchers to compare genetic differences between people who had and had not experienced a relationship breakup.
The analysis was complex and involved several steps:
- The analyses included genetic variants across the entire genome, which encompasses all of a person's genetic material. They then calculated how much of the variation in relationship breakups could be collectively explained by these variants. The analysis included more than 120,000 people.
- The analyses were conducted separately for women and men. This allowed the researchers to investigate whether the genetic associations differed by sex.
- The researchers found that the measured genetic variants explained around 9 per cent of the variation among women and 3.5 per cent among men.
Four predispositions were associated with a lower probability
In addition to estimating the heritability of an outcome, there is another tool frequently used by researchers: polygenic scores (see fact box). Using MoBa data, the researchers calculated several such scores for a range of traits. A polygenic score is a measure of how many of the numerous small genetic variants that previous research (GWAS studies) has linked to a particular trait a person possesses. The more of these variants a person has, the higher their score.
Among other things, they found that higher polygenic scores (PGS) for education, subjective well-being, neuroticism, and older age at first sexual intercourse were all associated with a lower likelihood of experiencing a relationship breakdown.
This means, for example, that people with a genetic predisposition towards higher education had, on average, experienced fewer relationship breakdowns. Whether people actually had a higher education was therefore not taken into account.
The finding concerning neuroticism was perhaps particularly surprising.
– If we were to speculate about what this means, it could be that people with higher levels of neuroticism also have a greater need for the security that a partner can provide. But that is a possible explanation of the mechanism, not something we can actually demonstrate.
Jørgensen adds that the researchers found no significant differences between women and men in these associations.
Polygenic scores
Polygenic scores originate from large studies that investigate which genetic variants are statistically associated with particular traits (as Jørgensen and colleagues themselves did in the first part of the study). A polygenic score is the sum of a very large number of small genetic contributions: some people have slightly more of the variants associated with longer education, while others have slightly fewer.
Using the genetic data from the Norwegian Mother, Father and Child Cohort Study, the researchers calculated such scores for a range of traits, including depression, ADHD, education, and quality of life. They then analysed the association between each individual polygenic score and the risk of experiencing a relationship breakup.
Genes operate through the environment
Jørgensen emphasises that the findings apply to people living under specific social conditions in Norway today. The significance of different predispositions can vary between societies and historical periods.
– You write that genes "operate through the environment" – what do you mean by that?
– Our genes cannot operate on their own. They depend on other actors and on opportunities in the surrounding environment. If we go back a hundred years, there were far fewer relationship breakups. Not because we humans were genetically different, but because social structures and legislation were different. In a society where ending a relationship is illegal and nobody separates, there is no variation in breakups for genetics to help explain.
We can also imagine that different genetic predispositions may have more or less room to manifest themselves depending on the social context in which a person lives.
– So the findings do not show that relationship breakdowns are "in our genes"?
– No, that would be a deterministic interpretation, says Jørgensen.
Sociology + genetics = true
Jørgensen believes that genetically informed methods can offer insights that previous sociological research has overlooked.
– When studying what is passed down ”socially” from one generation to the next, much of the earlier research has focused primarily on the family environment.
But children inherit 50 per cent of their genes from each biological parent. This means that factors beyond family circumstances and upbringing may help explain why parents and children are similar or experience similar outcomes.
– This makes it harder to determine how much of children's outcomes can actually be attributed to their family environment and, in our case, to their parents' separation. It also makes it difficult to distinguish these influences from characteristics that parents and children share through biological inheritance.
Sociologists are generally most concerned with environmental factors, Jørgensen adds.
– But if we want to understand the relationship we're studying, it helps to account for other factors that might influence it. Genetics is an obvious factor to consider.
A certain degree of heritability
To better distinguish between these explanations, Jørgensen and her colleagues have used two different approaches: an adoption study and a children-of-twins study.
– In the adoption study, we have adopted children who do not share genes with their adoptive parents. We then match the adoptive families as closely as possible with other families in the registry data, so that the families are as comparable as possible.
In the children-of-twins design, researchers analyse the children of twins. Children of identical twins share 25 per cent of their genetic material, while children of fraternal twins share an average of 12.5 per cent. Among other things, the researchers examine differences within families where one twin has experienced a relationship breakdown and the other has not. These within-family comparisons help control for shared genetic factors and childhood environments.
In families with biological children, they find a strong association between parents' and children's relationship breakdowns. In adoptive families, however, this association disappears. The children-of-twins study shows a similar pattern.
– This suggests that genetics is part of the explanation for why relationship breakdowns are passed down through generations. Much of the association may be explained by the fact that parents and children resemble one another in several ways, including genetically.
Genetic methods are both common and useful, including in research on social outcomes
Many people choose genetically similar partners
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– Genetic methods are both common and useful, including in research on social outcomes, writes Hans Frederik Sunde, a researcher at the Norwegian Institute of Public Health (NIPH), in an email to Kilden magazine.
Sunde has studied the effects of what researchers refer to as partner similarity.
– In other words, whether similar people are attracted to one another, whether partners become more alike over time, and what consequences these similarities may have.
He has also used genetic methods in his research. Together with colleagues, he has shown that partners share genetic predispositions for traits such as education, height, BMI and, to a lesser extent, personality.
– This suggests that some of the similarity is due to similar people finding each other, rather than simply becoming more alike over time, Sunde explains.
Fant også små kjønnsforskjeller
– And what does your research say about the significance of gender?
– My primary focus is not on gender, but we examine differences between women and men when relevant. For example, in our divorce study, we find, as Jørgensen and her colleagues do, that women's predisposition to divorce appears to have a slightly greater influence on whether a couple divorces than men's predisposition. But the difference is not very large.
More about Sunde's research
Sunde and his colleagues have shown how similarity between partners can contribute to greater genetic and social differences between families. Some children get the best of both worlds.
In a more recent study, they also found that people from families with a high risk of divorce are more likely to marry others who also come from families with a high risk of divorce.
