Ever since English polymath Francis Galton’s mid-1800 discussion about the influence of heredity and environment on social intelligence and heredity, the phrase “nature versus nurture” has sparked a long-time heated debate until now (Mick Serpell, 2013). Environmental factors include exposure to natural substances (Genome.gov, 2024), and social status, roles, and the ambient environment individuals live in (Avison, 2016). To disentangle the effects genetics and environment have on human behaviors, researchers conducted numerous quantitative studies, especially on the relationship between these two respective factors and their impact on the development of mental illness. Mental illness is defined as a health condition associated with distress and can cause changes in emotions, thinking, or behavior (Psychiatry.org, 2024). Most mental health professions follow the categorization of the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association in 2013 (DSM Library, 2024). In examining the root causes of such disorders, researchers commonly conclude a dual influence of both factors. While environmental factors play a significant role, genetic factors are identified as the initiating factor.

As the primary instigator, genetic factors play an important role in shaping an individual’s behavior and causing the development of mental illnesses. Researchers often conduct twin studies to examine how genetics affect mental development. Twin studies are based on the fact that identical monozygotic (MZ) twins share 100% common genetic materials whereas fraternal dizygotic (DZ) twins share only 50% in common. Within the Twin Design, MZ twins are assumed to be a perfect representation of genetic influence if both twins exhibit the same behavior. In contrast, DZ twins present only one-half the degree of genetic correlation between MZ twins. By comparing how frequently one trait may be seen in a pair of identical and fraternal twins, researchers can measure the genetic contribution to behavior (Prescott & Kendler, 1995).

For example, in 2007, Raine & Baker used the Twin Design to investigate the correlation between genetic and antisocial behaviors, a symptom of antisocial personality disorder (NHS Choices, 2024), in children. The study was conducted on 1,210 twins in California, USA. Researchers could perform statistical analysis and compare MZ and DZ twins by issuing questionnaires on the children’s personalities, behaviors, and social skills to the children themselves, their teachers, and a caregiver. Results have shown that the heritability of antisocial traits, including bullying, cruelty, stealing, and skipping school, is around 50% (Baker et al., 2007). However, the heritability varies according to different types of mental illness. Heritability for other psychiatric traits ranges from 30% for major depression (Tinca J C Polderman et al., 2015) to 87% for Autism Spectrum Disorder (ASD) collected from a sample of 37,570 twin pairs and  2,642,064 full sibling pairs (Sandin et al., 2014). The findings shed valuable insights into the role genetic factors play in the development of multiple mental illnesses; however, the systemic issues involved in the Twin Design might limit a firm conclusion. Because MZ twins are more identical, they are often treated more similarly than DZ twins, suggesting that MZ twins usually receive similar environmental influences and that the environment affects them in a more similar way than they do for the DZ twins. Moreover, as none of the studies presents a 100% heritability, genetics cannot always be identified as the sole indicator of mental illness, and its extent of involvement varies from case to case. However, it certainly is a key factor in disorder development.

In addition to genetic factors, environmental factors exacerbate the development of mental illness under some circumstances. Adverse environmental factors, such as poverty and family abuse, often contribute to the neurobiological alterations of individuals through stress (Davis et al., 2017). Children growing up in poverty are ten times more likely to die accidentally due to greater exposure to pollutants, toxins, and waste. Meanwhile, poor household financial stability makes children more likely to endure turmoil, strife, and violence at home. As a result, children growing up in chaotic environments have poorer cognitive development, presenting them with a greater risk of experiencing mental disorders due to immature brain functions and an inability to regulate emotions (Evans, Brooks-Gunn, & Klebanov, 2007). Chronic stress is one of the most common products created by external forces and pressures that may worsen people’s mental health. Robust evidence suggests that stress plays a notable role in impairing an individual’s psychiatric conditions.

Specifically, a longitudinal study conducted by Najman et al. aimed to discover the correlation between exposure to family poverty over a child’s early life course and their later symptoms of anxiety and depression as a teenager and young adult. 2,609 pregnant moms were recruited, and their children were followed up at ages 14 and 21. Results have shown that the more frequently a child is exposed to poverty, the more likely they will experience anxiety and depression both at age 14 and 21 years. Additionally, the longer a child is immersed in poverty during their critical developmental period, the higher the chance they would experience mental health problems (Najman et al., 2010). The deprivation of resources may explain this. Negligence of busy working parents and more domestic conflicts. In some neuroscience research that focused on how poverty changes the neuron pathways of particular brain areas, it is found that people with lower socioeconomic status have worse executive functioning (Lawson, Hook, and Farah, 2017). In research with a sample of 1,099 youth aged 3 to 20, Noble et al. found that poverty is associated with a reduced volume of dlPFC, a brain area responsible for emotion regulation and attention allocation (Noble et al., 2015). Therefore, youth living in poverty tend to be more vulnerable in situations where they should but are unable to direct attention away from negative stimuli and traumatic triggers (Palacios-Barrios & Hanson, 2019). The defective brain development makes them unable to cope with stressful events, and they experience more frequent mental breakdowns than youths growing up in healthy environments, leading to the induction of mental illness. This reveals that environmental factors can change certain brain structures through neuroplastic mechanisms, rendering individuals more susceptible to depression and anxiety when facing crisis. Nevertheless, given the nascent nature of neuroscience as a new discipline, the corresponding detection technologies may not have reached full maturity, implying that current findings may not indicate a comprehensive view.

Furthermore, given that both genetic and environmental factors play a role in fostering mental illnesses, it is worth noting that epigenetic changes, which can occur when environmental exposures alter one’s gene expression, sometimes impact behavioral formation (CDC, 2022). Under some circumstances, environmental factors may induce reversible epigenetic changes in gene expressions, leading to an increased likelihood of psychiatric disorders diagnosis. For example, a major psychiatric disorder, Alcohol Use Disorder (AUD), is commonly caused by epigenetic mechanisms rather than exclusively environmental or genetic mechanisms. Heavy alcohol consumption leads to changes in DNA methylation and histone modifications, resulting in the development of the disorder (Hamid Mostafavi-Abdolmaleky, Glatt, and Tsuang, 2011). Moreover, a study by Yehuda and colleagues found that increased methylation and histone acetylation are associated with an increased risk of PTSD. The study compared data between two groups: adult offspring with at least one Holocaust survivor parent, who has PTSD, and adult participants without parental Holocaust exposure (Yehuda et al., 2014). Unsurprisingly, participants with PTSD parents exhibited higher DNA methylation, indicating a higher risk of PTSD. This study explores how parental PTSD, as a type of environmental influence, influences epigenetic alterations in the offspring.

In conclusion, the influences of both genetic and environmental factors are acknowledged as important factors, though genetics plays an initiating role. The degree to which genetic or environmental factors exert their effects varies depending on the type of mental illness. However, in most cases, both factors contribute greatly. Despite the multitude of factors influencing the development of mental illness, there are some potential interventions and strategies for mitigating those impacts. By practicing mind-body interventions, as one of the most common mitigation strategies, individuals can alter their cognitive appraisal of events to become more objective through practice (Burnett-Zeigler, 2016). Being able to perceive and accept stimuli in a neural manner, individuals are less likely to ruminate about their past or future, decreasing the frequency of their experiencing anxiety and depression. Additionally, pharmacological interventions targeting epigenetic modifications are in the early stages of development, hinting at a promising future with direct and effective treatments.