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Depression has an elusive origin, making it a strange disorder.
Sometimes, a person experiences depression due to a negative life event, while at other times it may occur without any apparent reason. In the past, specialists classified depression into exogenous and endogenous categories. Exogenous depression resulted from adverse life events, while endogenous depression occurred due to biological malfunctions in the body.
Today, experts no longer consider this classification to be relevant. Instead, scientists and psychiatrists believe that depression arises from a combination of various reasons, including biological and socio-psychological factors. The following are the main contributors to depression.
REASON #1 – Genetic predisposition
If a person’s immediate family members, such as their parent, sibling, or child, have been diagnosed with depression, their likelihood of developing the illness increases by two to three times, up to 20-30%. This indicates that genetic factors play a significant role in the development of depression, as highlighted in the Harvard Review of Psychiatry. However, the contribution of the environment to this risk remains uncertain. For instance, a depressed parent’s inadequate attention towards their child could make the child vulnerable to mental health issues. Similarly, constant worry due to a sibling’s depression could also lead to depression.
To determine the influence of genetic factors on depression risk, scientists study adopted children, thereby eliminating the biological parent’s behavior from the equation. They also analyze twin data, finding a person with depression who has a twin and determining whether the twin suffers from the same disorder.
In 2006, scientists from the Virginia Institute of Psychiatry and Behavioral Genetics employed this strategy and concluded that depression’s heritability ranges from 31-42%. This value is considerable, although less than the heritability of schizophrenia, which is estimated at 70-80%.
REASON #2 – Stress and decreased brain neuroplasticity
Although genes can increase the risk of depression, it is still possible for individuals without a family history of depression to suffer from the disorder. Conversely, individuals with depressed family members may never experience depression themselves. This is because chronic stress can also trigger depression.
Depression can be triggered by seemingly ordinary events, such as job loss and prolonged unemployment. Despite its mundanity, such situations can cause chronic stress in individuals.
The biological effects of chronic stress can contribute to the development of depression. During chronic stress, the body produces a high amount of cortisol, which is responsible for the “fight or flight” response and helps to defend against threats. However, excessive cortisol levels can have negative side effects.
Excessive cortisol production resulting from chronic stress can reduce the level of the BDNF protein responsible for maintaining and growing brain cells. If stress persists for a prolonged period, the level of BDNF becomes too low, and the brain’s function is impaired. As a result, some scientists believe that depression develops.
Moreover, individuals experiencing chronic stress may adopt destructive coping mechanisms. For instance, after losing their job, they may resort to making purchases to calm themselves down, thereby worsening their financial problems. Alternatively, they may turn to alcohol, which can increase the risk of impulsive and self-destructive behavior, leading to the depletion of their bank account. Furthermore, alcohol is a depressant, increasing the likelihood that feelings of melancholy will progress into a full-fledged disorder.
REASON #3 – Perfectionism and other personality traits
Certain personality traits make a person particularly sensitive to stress and prone to depression. Researchers consider the following traits to be the most dangerous:
- Anxiety. People who frequently and excessively worry are less stress-resistant.
- Shyness. Shy people often do not feel safe when communicating with others. Social situations that seem safe or slightly challenging to others can be experienced by them as intense stress.
- Sensitivity to the judgments of others. If a person is constantly worried about the opinions of others, their words can hurt them – an awkward phrase can make them feel humiliated and rejected.
- Egocentrism. People who are fixated on their desires and needs easily destroy relationships, and loneliness makes them vulnerable.
- Perfectionism and self-criticism. As a rule, people who are strict with themselves cope poorly with stress – they have few tools to regulate emotions. At the same time, their demands on themselves are higher than average, which means they experience stress more often.
REASON #4 – Traumatic events
Sometimes, a single but severely painful event can lead to the development of depressive states, in addition to chronic and prolonged stress. These events are referred to as “psychological trauma,” and some scientists believe that they are one of the most common causes of depression and anxiety. For instance, after surveying 32,000 English individuals ranging from 18 to 85 years old, researchers from Liverpool University concluded that psychotraumatic events can lead to such conditions.
Psychological trauma can manifest in various forms, such as sexual, physical, and emotional violence, loss of a loved one, receiving news of an incurable disease, experiencing a car accident, natural disasters, and war. However, there is no universal list of traumas, as what may severely wound one person may only cause moderate pain for another. For instance, the death of a pet can affect some people as catastrophically as the death of a family member.
While trauma is commonly discussed as a cause of post-traumatic stress disorder, it can also be related to depression, albeit not as directly. PTSD typically arises as a delayed reaction to a traumatic event after it has occurred, where psychological trauma is the specific cause of the disorder.
In the case of depression, trauma acts as a risk factor rather than a direct cause. It renders a person more mentally vulnerable, and when combined with other factors such as genetics and perfectionism, can increase the likelihood of developing depression.
Trauma heightens a person’s tendency to worry. It disrupts the functioning of the amygdala, a critical area of the brain that plays a crucial role in regulating emotional responses to external stimuli. The amygdala is responsible for determining when to be scared and initiating a stress response. However, trauma causes the amygdala to become hypersensitive, leading it to perceive threats where there are none. As a result, a traumatized person is constantly under stress.
Moreover, trauma weakens the connection between the amygdala and the prefrontal cortex, the region of the brain responsible for controlling emotional reactions and adapting to society. Consequently, a traumatized individual may have difficulties regulating their emotions, adapting to societal norms, and becoming less resilient to frequent stressors that they may face.
REASON #5 – Negative childhood experiences
Children are particularly vulnerable to the harmful effects of chronic stress and traumatic events because their brains are still developing and highly sensitive to new experiences. According to researchers from Yale University, adverse events can cause changes in the brain’s architecture, resulting in a decrease in cell density in areas that regulate emotions. These changes can persist into adulthood.
It’s not surprising that difficult childhood experiences significantly increase the risk of various mental health issues in adulthood, including depression and addiction. Studies by New Zealand researchers have shown that childhood sexual abuse increases the risk of encountering mental health issues by 2.4 times, and physical abuse by 1.5 times.
Additionally, a study has revealed that women who experienced a traumatic childhood are more prone to depression in response to stress than those who had a carefree childhood. Furthermore, a meta-analysis of 16 studies with over 23,000 participants demonstrated that adults who experienced violence as children find it more challenging to recover from depression than those who have not undergone such experiences.
Childhood difficulties that can make an individual vulnerable to depression are not solely limited to abusive treatment. The long-term effects of poverty, parental alcoholism, bullying, or a lack of affection can also impact mental health.
Furthermore, vulnerability to these factors is not confined to young children alone. Older adolescents are also highly susceptible, given that the frontal lobe, responsible for emotional control, decision-making, planning, and social adaptation, is not fully developed until after the age of 20.
Some researchers suggest that the critical period of brain development extends up to the age of 30, making individuals susceptible to depression during their adolescence and young adulthood.
REASON #6 – Loneliness
Throughout much of human history, people have faced a direct threat to survival due to loneliness. Hunting for food, protecting oneself from predators, and inventing various things were much easier accomplished together with someone, rather than alone.
The brain has evolved to encourage people to maintain social connections. It has made loneliness literally painful, among other things: brain imaging studies show that this feeling activates the same brain areas as physical pain. Loneliness is extremely dangerous to health, as scientific observations show it is closely linked to depression. For example, a large-scale Dutch study conducted during the pandemic from March 2020 to January 2021 indicated this.
In this study, 75,000 people participated. They filled out several questionnaires to assess their level of loneliness and underwent an interview with a psychiatrist. It turned out that lonely participants more often met the criteria for diagnosing “major depressive disorder.” According to the interview authors, young men suffered particularly strongly. Those who felt the greatest loneliness were 14 times more likely to be depressed than those who were less lonely.
REASON #7 – Brain dysfunction
Depression can sometimes occur seemingly without any reason, even in individuals who lead a calm and happy life, do not display perfectionism or excessive anxiety, and had a carefree childhood. This is because depression is not merely a reaction to difficult life events; it is a disorder that is related to biological processes in the brain.
While it is not entirely clear which malfunctions in the body cause depression, scientists have put forth several theories. The most well-known is the serotonin theory, which suggests that a disorder in the brain leads to a deficiency of certain chemical substances called monoamines, such as serotonin, dopamine, and norepinephrine. These substances participate in regulating emotions, arousal, and memory by transmitting signals between neurons.
A shortage of monoamines can disrupt the connection between neurons, causing different areas of the brain to stop communicating with each other effectively. This results in asynchronous occurrence of critical processes such as memory, emotional regulation, and information processing. As a result, depression symptoms may appear, such as sadness, loss of interest in activities, insomnia, fatigue, decreased appetite, and problems with attention and concentration.
Antidepressants are commonly prescribed to restore the balance of monoamines, thereby returning the brain to normal functioning.
While this theory of depression was once widely accepted, it still lacks concrete evidence. New ideas about the biological causes of depression continue to emerge, leading to different interpretations.
One such theory is that depression may be a consequence of inflammation. For various reasons such as diseases, injuries, sleep problems, smoking, or obesity, a person’s immune system produces inflammatory molecules. These molecules may disrupt the brain’s functioning when they enter the nervous system, potentially leading to depression.
If this theory proves correct, depression treatment may shift towards using anti-inflammatory drugs. Although there is still insufficient evidence to support this approach, some scientists are already exploring it.
Another popular view of depression in scientific circles is that it is not one singular disorder, but rather a cluster of different disorders with similar symptoms. Each type of depression is believed to have its own unique biological causes and treatment options.
This idea is supported by the fact that antidepressants do not provide relief to everyone. In approximately 30% of patients diagnosed with “major depressive disorder” who are prescribed various antidepressants, none of the medications seem to alleviate their symptoms. Typically, these individuals exhibit elevated levels of inflammation, suggesting that their depression may be linked to neuroinflammation, while others may be related to a serotonin deficiency or some other brain malfunction.
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