The effect of genetic and environmental factors on mental illness

Congratulations to Lorcan, who achieved an excellence award in the final Global round of the 2024 Minds Underground Psychology Olympiad competition with this essay on genetic and environment contributions to mental illness…

Although genetic factors can sometimes play a role in the development of mental illness, environmental factors are by far the most important. To what extent do you agree with this statement?

Introduction

Mental illness is a significant challenge in many people’s lives with 1 in 4 of us (P.1) experiencing some form of mental health illness each year in England, and many more indirectly affected because someone they know experiences mental illness. This highlights the importance for society and for those affected of understanding how and why mental illness develops. The causes of mental illness and whether genetic or environmental factors are more important are debated hotly by psychologists. In this essay I will examine the case and weight of evidence for both arguments and attempt to answer the question of whether environmental factors are the most important in the development of mental illness.

Argument for the importance of environmental factors

One argument advanced for environmental factors is that exposure to environmental toxins have a significant impact on the brain and that millions of adults now entering middle age were exposed to high levels of lead as children (P.2). According to the World Health Organisation, lead is a dangerous neurotoxin that can seriously alter the brain and the central nervous system (P.10). For example, a multi-decade longitudinal study published in JAMA Psychiatry (P.2) examined whether childhood lead exposure was associated with greater psychopathology in adulthood. Measuring blood levels at 11 years of age and following up through clinical interviews for mental health disorder symptoms at 18, 21, 26, 32 and 38, the researchers concluded that higher exposure to lead as a child was directly associated with greater psychopathology and difficult adult personality traits. Harmful toxins like lead can clearly interfere with our brain chemistry, providing a plausible explanation for the effect of environment on mental illnesses.

Another argument that illustrates the importance of environment is the evidence from traumatic events. Depressive disorder is a relatively common but severe mental illness defined by the Mayo Clinic as “a persistent feeling of sadness and loss of interest” and can be related somewhat to environmental influence (P11). Brown and Haris (P3) investigated the impact of stressful life events on the development of depression. Using cross-lagged panel analysis they examined the influences of stressful life events on depression and on anxiety five years apart, with a (p=0.1) result that stressful life events cause depression. It is now widely accepted that stressful events paly a formative role in the development of mental illnesses and in particular depression. There are many practical examples of psychologically traumatic events such as the Covid-19 pandemic affecting mental health – the percentage of young people with suicidal thoughts rose significantly during the pandemic and has continued long after (P.5). The correlation between traumatic events and depressive symptoms highlights the underlying importance of environmental factors in the development of mental illness.

Limitations of the environmental factors argument

Although the environmental factors argument has many strengths, it also has limitations. Environmental toxins are linked to genetic susceptibility, and individuals are often exposed to an abundance of toxins simultaneously. Despite the evidence from the (P.2) study about lead referenced earlier, the underlying causation remains unclear, because how these air pollutants affect us could be determined by our genetics. This suggests we cannot definitively establish environmental toxins like lead as the prime cause of mental disorders.

Looking at the argument for the importance of environmental factors from traumatic events, it is important to recognise the role of resilience and protective factors in each individual’s outcomes. Whilst evidence suggests that traumatic events are directly linked to higher rates of depression, resilience and protective factors can mitigate this risk for an individual. Examples of protective and resilience factors could include a person’s problem-solving skills or intellect. These are arguably not purely environmental but frequently play a significant role in whether a person develops depression after experiencing trauma. Support for this argument comes from a longitudinal investigation by Karestan Koenen et al (P5). In an attempt to measure the impact of childhood intelligence on mental disorders they used an IQ test across multiple ages and a mental disorder assessment. The researchers concluded that those with a lower level of intelligence as a child had a far higher likelihood of developing a mental disorder as an adult. A striking statistic was that for every standard deviation increase in childhood IQ participants had a 37% reduction in the odds of a depression diagnosis. While environmental factors and events probably do play a significant role in the development of mental illness, an individual’s own make-up, and resilience and protective factors specific to them, must also be credited as important.

Argument for the role of genetic factors

Turning to the other side of the nature vs. nurture debate, it could also be argued that genetic factors are by far the most important in developing mental illnesses. The high concordance rate of mental illnesses within families makes it very hard to deny that genetic factors must be very important. One of the most famous studies of all time, Sullivan’s (P6) meta-analysis of existing twin and family studies, provides a stark example of the contribution of genetic factors to the development of schizophrenia. Using a multigroup twin model, they found the point estimate of heritability in liability to schizophrenia was 81%. Furthermore, Nestadt (P7) reviewed previous twin studies and found that 68% of identical twins shared OCD as opposed to 31% of non-identical twins. This identified link between mental illnesses and family members highlights the role inheritance and genes play in our susceptibility to disorders. These strong links suggest that genetic components may be more influential than environmental factors.

A further argument supporting the importance of genetic factors is the development of an understanding of the bio-chemcial mechanisms by which genes can contribute to dysfunction of the brain. For example, researchers have identified various candidate genes linked to vulnerability for OCD (P12). Some of these genes are involved in regulating the development of the serotonin system. For example, the gene 5TH1-D beta is implicated in the efficiency of transport of serotonin across synapses. Antidepressants act as a form of evidence for the role of the 5TH1-D beta gene. Most antidepressants work purely on the serotonin system, increasing available levels of the neurotransmitter. Such drugs are effective in reducing OCD symptoms – therefore implying that serotonin is implicated in OCD and that a change in the 5HT1-D beta gene would contribute to the development of OCD. Furthermore, OCD symptoms form part of other conditions like Parkinson’s disease (P7) that are known to be biological in origin. This strongly suggests that biological processes controlled by genetics are directly implicated in disorders such as OCD.

Limitations of the genetic argument

The role of inheritance and genetic components might be overstated. Twin studies are a standard source of evidence for genetic influence. However, they make the assumption that identical twins are only more similar in terms of their genes, but overlook the fact that identical twins may also be more similar in terms of shared environment. For example, parents are more likely to dress identical twins alike, and they are more likely than non-identical twins to share the same toys and bedroom and school environment given they are of the same sex. This greater similarity in environment could also account for some of the greater likelihood of sharing OCD than non-identical twins.

Reviewing the argument from candidate genes, although the effectiveness of antidepressants appears to explain the role of biological components like neurotransmitters, we should not assume genetic components cause mental illnesses. Whilst there is evidence to suggest that various neurotransmitters and genetic structures of the bran don’t function normally in patients with OCD, this does not necessarily imply that the abnormal functioning causes the OCD – it could instead be that OCD triggers the abnormalities. So whilst genetic factors do appear to play a role in the development of mental illnesses it is important to recognise the role of other factors and the uncertainty of their influence.

Integration and synthesis of genetic and environmental factors

The environmental and genetic arguments are at the opposite ends of the spectrum, each with their own strong evidence to support their claims. However, genes do not exist in a vacuum and neither do environments. Each of us represents a unique interaction between our genetic makeup and our environment. A theory that displays the close interaction between genetics and environment is range of reaction. Range of reaction is the idea that a person’s genetic makeup establishes a range of potential characteristics and traits, but which trait we display is determined by environmental factors. Malaspina (P8) published an article attempting to understand how schizophrenia is maintained in the population. She concluded that there is a strong genetic component for schizophrenia risk but that we need to expand our viewpoint to include the effects of environmental exposure. Perhaps each person’s genetic factor establishes a range of mental illnesses that they are susceptible to. However, it is the environment that determines where the individual sits in that range.

In addition to range of reaction the multitude of gene-environment interaction studies supports the integration of genetic and environmental factors. An example is Van O’s (P9) study on the interaction between genetic risk for psychosis and urban upbringings. Measuring for the COMT genotype (associated with an increased vulnerability to psychosis) they then assessed participants for psychotic experiences. They found individuals with vulnerability to the COMT gene had a higher likelihood of experiencing psychotic symptoms if they grew up in an urban environment. They concluded that this correlation was an example of gene-environment interplay highlighting the importance of both factors as causing psychosis. The evidence from gene-environment studies suggests that mental illnesses are frequently caused by the interplay between genetic factors and environments, rather than being directly caused by just one type of factor.

Conclusion

Both genetic and environmental factors play a vital and necessary role in the development of mental illness. Environmental factors such as environmental toxins and traumatic events can have a substantial influence on mental health. Despite this, the complexity of environmental pollutants and the role of resilience and protective factors imply that environment alone cannot be responsible for mental illnesses. The role of genetic factors is supported by evidence from twin studies and research on the connections between OCD and candidate genes. Although these examples create a compelling argument, twin studies could serve as flawed evidence and the uncertainty surrounding the effects of candidate genes mean we cannot credit mental illness to pure genetics either.

I believe a more holistic understanding of mental illness is needed. An approach that considers both genetic and environmental factors including their interactions is essential for addressing mental health issues effectively. Neglecting either side of the nature vs. nurture debate will only make the vast issue of mental illness even worse. Instead, further research into the interplay of genetic and environmental influences is needed to understand the development of mental illnesses.

Lorcan Farrell

Bibliography

(P1) Mental health resource-.Facts and figures | Mental Health Resource

(P2) Reuben et al JAMA Psychiatry 2019- Association of Childhood Lead Exposure With Adult Personality Traits and Lifelong Mental Health – PubMed (nih.gov)

(P3) Brown and Harris 1978- Social Origins of Depression: A Study of Psychiatric Disorder in Women. By George W. Brown and Tirril Harris. New York: Free Press, 1978. 399 pp. $19.95 | Social Work | Oxford Academic (oup.com)

(P4) Wise 2020- Covid-19: Suicidal thoughts increased in young adults during lockdown, UK study finds | The BMJ

(P5) Koenen et al.2009- Childhood IQ and Adult Mental Disorders: A Test of the Cognitive Reserve Hypothesis | American Journal of Psychiatry (psychiatryonline.org)

(P6) Sulivan et al.2000- Schizophrenia as a complex trait: evidence from a meta-analysis of twin studies – PubMed (nih.gov)

(P7) Nestadt et al.2010- Genetics of OCD – PMC (nih.gov)

(P8) Malaspina et al.1999 – Prenatal exposures in schizophrenia. (apa.org)

(P9) Van Os et al.2010- The environment and schizophrenia – PubMed (nih.gov)

(P10) Effects of lead- WHO guidance to reduce illness due to lead exposure