Schizophrenia and major depressive disorder are both severe psychiatric conditions, estimated to affect up to 1% of the population, in the case of schizophrenia, and up to 20% of people for severe depression.
We know from studies of families and twins that both can be influenced by both genetic and environmental factors – probably by some combination of the two. In other words, both conditions may be partly inherited, but may also be influenced by environmental factors, such as socio-economic status, level of education, and whether they’re in a stable relationship.
We wanted to investigate gene-environment correlation in adults – that is, whether exposure to these environmental risks might be determined by people’s genes, and if so to what extent.
What we know
Existing research suggests that schizophrenia may be inherited in 64 – 81% of cases, and major depressive disorder in 34-39%. Various studies have tried to identify the specific genetic variants responsible, but only few have been found yet. It seems likely that both disorders are explained by hundreds or thousands of different genetic variants, each one having a small effect on its own, but by combining them into polygenic risk scores we can find out if they can increase the likelihood of suffering from one of these illnesses.
This picture is further complicated by the fact that schizophrenia and major depressive disorder are treated and thought of as being separate, but depressive episodes are common in people who have psychotic episodes, suggesting some overlap between them.
Environmental risks
Environmental risk factors which have been linked to schizophrenia and major depressive disorder in adults include:
- substance abuse
- drinking and smoking
- unemployment
- low socio-economic status
- lower levels of education.
In both cases, relationship status can have a two-way effect. In other words, people who are separated or divorced are more likely to have severe depression, but also people with severe depression have an increased risk of a disrupted marriage. Similarly, people with schizophrenia are less likely to be married, and early onset schizophrenia is associated with a greater risk of a marriage failing.
Gene-environment interplay
Research has shown that the polygenic risk score for schizophrenia in adults correlates with a number of behavioural traits, such as higher odds for self-reported risk taking and smoking. The risk score for major depressive disorder is higher for people in urban areas than rural areas. It’s important to understand these relationships in order to prevent and treat the conditions. In cases where genetic causes could be ruled out, for example, environmental risks could be a focus for prevention and treatment.
The data
We used data of adult participants from the 1958 National Child Development Study (NCDS) and Understanding Society. NCDS is a sample of people born in March 1958, surveyed throughout their lives. It included a bio-medical survey in the late 1990s, and collected DNA in the early 2000s. We used Understanding Society from Wave 1 (2009-10) to Wave 9 (2017-18), which included a health assessment from a registered nurse and DNA samples in Waves 2 and 3 (2010-12).
We looked at three kinds of environmental risk:
- economic situation, including: unemployment, financial difficulties, and socio-economic status, income, number of bedrooms in the house, and whether they rented or owned their home
- substance abuse: alcohol consumption, and smoking
- psychosocial factors: educational attainment, and marital status
We took into account whether people had reported having psychiatric treatment, and checked for differences in each case between the main study, and the sub-sample who had allowed their DNA to be collected.
Results
In NCDS, we found no link between genetic susceptibility for schizophrenia and the environmental risk factors we investigated– but there was correlation between people’s genetic risk for depression and living in rented accommodation and having a small number of bedrooms. There was no statistically significant link between other environmental factors and severe depression.
In Understanding Society, the genetic propensity for schizophrenia was associated with being single or divorced, and that for depression linked with unemployment, low number of bedrooms, finance issues and low income.
Overall, we were able to see that the genetic risk of schizophrenia seems to make it less likely that someone will be married. However, we can’t see whether this is simply because having a propensity for schizophrenia puts off potential partners. We also couldn’t rule out the idea that symptoms of schizophrenia, such as psychotic episodes and hallucinations, have detrimental consequences for work and family.
We did find links between genetic risk for depression and several indicators of low socio-economic status, including unemployment, low income, financial difficulties, rented accommodation, and low number of bedrooms. These results were able to withstand a technique known as correction for multiple testing, which is designed to weed out false positives.
Putting things in context
It’s possible that people with a genetic susceptibility for severe depression are more likely to show behaviours that may prevent them finding work or advancing in their careers. This could then contribute to being unable to buy a home. It’s also possible that people may be choosing adverse social environments because of their genetic propensity for severe depression – and that living in a poor environment could influence how a disease progresses.
We can’t fully explain the interplay between genes and environments, so we can’t rule out the idea that targeting environmental risk factors could help people with a genetic predisposition to suffer from one of these conditions. Environmental risk factors, such as financial problems, are complex, involve genetic and non-genetic factors, and need more research.
Conclusion
However, we can say that there are some environmental risk factors which were more associated with one disease than another. Schizophrenia was more strongly linked to psychosocial factors such as being divorced or separated, while major depressive disorder was more correlated with indicators of low socio-economic status. We’ve also provided more evidence that these environmental risks are influenced by genetics.
Authors
Sandra Machlitt-Northen
Sandra Machlitt-Northen is a PhD student in the School of Biological and Behavioural Sciences at Queen Mary University of London
Michael Pluess
Michael Pluess is Professor in Developmental Psychology at Queen Mary University of London
