The stress-diathesis model of mental illness is a prominent theory in psychology that seeks to explain how genetic and environmental factors interact to influence the development of mental disorders. This model proposes that individuals have a predisposition (diathesis) towards developing a particular mental illness due to genetic or biological factors, and that the manifestation of the disorder is triggered by stressful life events.

Genetic factors play a significant role in the development of mood disorders such as depression and bipolar disorder. Studies have shown that these disorders tend to run in families, suggesting a genetic predisposition. These genetic factors can influence neurotransmitter activity, brain structure, and other biological processes that are implicated in mood regulation.

Environmental factors also play a crucial role in the development of mood disorders. Stressful life events such as trauma, loss, abuse, or chronic stress can trigger the onset of a mental disorder in individuals who are genetically predisposed. A person with a genetic vulnerability to depression may experience a major life stressor such as the loss of a loved one or a job, which then triggers the onset of depressive symptoms.

The interaction between genetic and environmental factors in the stress-diathesis model can be seen in various mood disorders.

Major Depressive Disorder (MDD):

In the case of MDD, genetic factors influence an individual’s susceptibility to developing the disorder. For example, variations in the serotonin transporter gene (5-HTT) have been linked to an increased risk of depression.

The type of serotonin transporter with which you are born determines in part how likely you are to develop a mood disorder when exposed to stress, and how probable your depression is to respond to an SSRI/SNRI.

Those individuals with the ‘s’ genotype of SERT are are more likely to develop depression and other psychiatric conditions when exposed to chronic stress or trauma. In contrast, the ‘l’ genotype of SERT confers more resilience and less likelihood of an individual developing depression.

Bipolar Disorder:

The stress-diathesis model can also be applied to bipolar disorder. Genetic factors play a significant role in the development of the condition, with studies suggesting a strong hereditary component especially for the more severe type (Bipolar Type 1). Specific genes related to neurotransmitter function and circadian rhythms have also been implicated.

Environmental factors such as substance abuse, sleep disturbances, or life stressors interact with these genetic vulnerabilities to trigger manic or depressive episodes.

Post-Traumatic Stress Disorder (PTSD):

Genetic factors can influence an individual’s risk of developing PTSD after experiencing trauma. Variations in genes related to the stress response, such as the FKBP5 gene, have been associated with an increased susceptibility to PTSD. 

External conditions such as the severity of the trauma, lack of social support, or ongoing stress can interact with genetic vulnerabilities to increase the likelihood of developing PTSD.