There are various explanation of unipolar depression. Those are:

The Biological View

Research on genetic factors, biochemical factors, brain circuits, and the immune system suggests that unipolar depression often has biological causes.

1. Genetic Factors

Three types of research show that some people may inherit a predisposition to unipolar depression:

•          Family Pedigree Studies

Around 30% of close relatives of a person with depression also suffer from it, compared with less than 10% in the general population.

•          Twin Studies

If one identical twin has unipolar depression, there is a 38% chance the other twin has already had or will develop it.

•          Gene Studies

Using molecular biology, researchers can identify genes linked to depression. Studies suggest unipolar depression may be tied to genes on at least two-thirds of the body’s 23 chromosomes.

 

2. Biochemical factors

•          Neurotransmitters:

Low activity of two neurotransmitter chemicals, norepinephrine and serotonin, has been strongly linked to unipolar depression..

•          hypothalamic-pituitary-adrenal (HPA) pathway:

If one identical twin has unipolar depression, there is a 38% chance the other twin has already had or will develop it.

•          Hormones:

The hypothalamic-pituitary-adrenal (HPA) pathway is one of the brain’s stress pathways. In people with depression, pathway is often overly reactive.  As a result, their bodies release excessive amounts of cortisol and related stress hormones during stressful situations. Another hormone linked to depression is melatonin, often called the “Dracula hormone” because it is released only in the dark. High level of melatonin can lead to depression.

 

3. Brain Circuits

•          Brain circuits —networks of brain structures that work together, triggering each other into action and producing a particular kind of emotional or behavioral reaction. Dysfunction in brain circuit can contribute to unipolar depression.

•          Several brain structures are linked with depression, including the prefrontal cortex, hippocampus, amygdala, and subgenual cingulate (also called Brodmann Area 25)

•          Among depressed people, activity and blood flow are unusually low in certain parts and unusually high in other parts of the prefrontal cortex; the hippocampus is undersized and its production of new neurons is low; activity and blood flow are high in the amygdala; the subgenual cingulate is particularly small and active; and the communication, or interconnectivity, between these various structures is often problematic.

 

4. The Immune System

•          Immune system dysregulation produce depression.

•          It is the body’s network of activities and body cells that fight off bacteria, viruses, and other foreign invaders.

•          When people are under intense stress for a while, their immune systems may become dysregulated, leading to slower functioning of important white blood cells called lymphocytes and to increased production of pro-inflammatory cytokines, proteins that spread throughout the body and cause inflammation and various illnesses.

 

The Psychological View

The psychological models, such as psychodynamic and cognitive-behavioral models also explain unipolar depression.

1. Psychodynamic Model

Sigmund Freud (1917) and Karl Abraham (1911, 1916) were the first to give a psychodynamic explanation of depression. They noticed that depression looks very similar to grief after losing a loved one.

According to them, when someone loses a loved one, a set of unconscious processes begins.

At first, the mourner cannot accept the loss and mentally goes back to the oral stage of development—a time in infancy when the child is completely dependent on their parents and cannot separate their own identity from them.

By regressing to this stage, the mourner merges their identity with the lost loved one, almost as if to keep them alive inside. This process is called introjection.

During introjection, the mourner directs all feelings about the loved one—both love and anger—toward themselves. For most people, this is temporary and they eventually recover. But for some, especially those who did not have their dependency needs properly met in childhood, the grief deepens and turns into clinical depression.

Freud also explained that many people get depressed without losing someone close. He introduced the idea of symbolic (or imagined) loss, where a person treats other kinds of experiences—such as failure, rejection, or disappointment—as if they were the loss of a loved one.

 

2. The Cognitive-Behavioral Model

Cognitive-behavioral theories proposed that unipolar depression is a results from a combination of problematic behaviors and dysfunctional ways of thinking. These theories fall into three groups:

A. THE BEHAVIORAL DIMENSION

Psychologist Peter Lewinsohn suggest that depression is connected to changes in the number of rewards and punishments people experience in life.

He explained that when life’s positive rewards decrease, some people start doing fewer productive activities. As the rewards in life decrease, these people do even fewer constructive activities, which lead to depression.

Research supports this idea. Studies show that depressed people usually report receiving fewer positive rewards than non-depressed people. Also, when their rewards increase, their mood often improves

B. THE COGNITIVE DIMENSION

Psychologist Aaron Beck believed that depression is mainly caused by negative thinking. He explained that four things that create depression: maladaptive attitudes, the cognitive triad, errors in thinking, and automatic thoughts.

•          MALADAPTIVE ATTITUDES:

These are inaccurate attitudes that often start in childhood.

For example, “My worth depends on everything I do perfectly.”

•          COGNITIVE TRIAD:

They think negatively in three main areas:

1.        experiences

2.        the self

3.        the future

•          ERRORS IN THINKING:

Depressed people often make error in their thinking and logic. For example, they may jump to negative conclusions without enough evidence.

•          AUTOMATIC THOUGHTS:

They experience a constant stream of negative thoughts,

such as “I’m not good enough” or “My situation is hopeless.” Research supports Beck’s theory. Studies show that depressed people hold more maladaptive attitudes and make more errors in thinking than non-depressed people. They also tend to have stronger automatic thoughts. However, research also shows that while negative thinking is linked to depression, it may not always be the root cause. It’s possible that depression itself creates mood problems first, which then lead to negative thinking, making the condition worse.

C. THE COGNITIVE-BEHAVIORAL INTERPLAY

According to psychologist Martin Seligman (1975), feelings of helplessness are at the center of depression.

It holds that people become depressed when they think:

(1) that they no longer have control over the reinforcements (the rewards and punishments) in their lives, and (2) that they themselves are responsible for this helpless state. The learned helplessness explanation of depression has been revised somewhat over the past several decades.

THE ATTRIBUTION-HELPLESSNESS THEORY: when people view events as beyond their control, they ask themselves why this is so.

 

The Socio-cultural View

Sociocultural theorists propose that unipolar depression is influenced by the social context that surrounds people.  There are two kinds of sociocultural views — the family-social perspective and the multicultural perspective.

1. The Family-Social Perspective

A decline in social rewards plays an important role in depression. This link works in both directions.

•          Depressed people often have weaker social skills. They may speak slowly and quietly, pause too much, or take longer to respond in conversations. They may repeatedly ask others for reassurance. These behaviors can make others feel uncomfortable, causing them to avoid the depressed person. As a result, the depressed person’s social contacts and rewards decrease.

2. The Multicultural Perspective

Researchers also study depression from a multicultural angle, focusing on two areas:

a)        Gender and Depression

Women are twice as likely as men to be diagnosed with unipolar depression.

b)        Cultural Background and Depression

People across cultures experience depression with similar symptoms. However, the exact expression of depression differs by culture. In non-Western countries, physical symptoms are more common, such as fatigue, weakness, sleep problems, and weight loss. In contrast, cognitive symptoms like self-blame, low self-esteem, and guilt are less emphasized than in Western countries.