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Depression (major depressive disorder) is diagnosed to individuals experiencing two weeks of of either a depressed mood/loss of interest & pleasure.
Additional symptoms include: insomnia, appetite disturbances, loss of energy, feeling of worthlessness, suicidal thoughts, difficulty concentrating.

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Depression is perplexing illness that takes many form. it includes the feeling of sadness, depressed mood, loneliness and many more.

Arguments:
-Kendler: Finding that the twins
-Caspi:

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Depression is a major depressive disorder in moods that causes a persistent feeling of sadness and loss of interest which could interfere with your daily functioning
Symptoms: Affective (feelings), behavioral (passivity), cognitive (self-blame), somatic (insomnia)
Genetic Arguments (Kendler, Caspi)
Kendler: investigated the heritability of depression
Depression is somewhat heritable, but environment & stress plays a role
Caspi: investigated how genetic vulnerability and negative life experiences can interact to cause depression
The risk of depression is highest in people who have both a genetic vulnerability and multiple stressful life events
Neurotransmitters (Serotonin hypothesis)
Serotonin: Chemical messenger that allows signals to be passed from one neuron to the next. 
Low levels of serotonin in the brain could cause depression because Serotonin is believed to play an important role in regulating mood/sleep/emotion
Low levels of serotonin makes it difficult for signals to be passed from one neuron to the next, slowing down the flow of brain activity

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Depression is a mood disorder that usually causes persistent feelings of sadness. It's sometimes also associated with general loss of interest. However, the symptoms of depression can vary based on the severity: from mild to severe.

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= common and serious medical illness that negatively affects one’s feelings, thoughts, and behavior

Genetic Arguments
- Kendler et.al : genetic factors may predispose people to depression, and MZ twins are more genetically vulnerable
- Caspi et.al : genetic inheritance is insufficient to lead to depression, but its interaction with stressful environment increases vulnerability

Serotonin Hypothesis

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It is a mental illness that negatively affects on how people feel and behave. It usually decreases the ability of function at work.

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Has major negative affect on how you feel, think or act

--> involved in controlling mood, emotions, aggression, sleep, and anxiety

- Mostly causes feelings of sadness
- Interest loss in the activities(loss of energy)
- Can cause emotional and physical problems
-  Can cause difficulties while thinking, concentrating or making decisions

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Definition: Emotional status that is marked by feeling of low self-worth, guilt and a reduced ability to enjoy life.

Kendler et al: Heritability of depression and whether there is a significant gender difference in the inheritance

Caspi et al: Investigate the significance of a short allele (5-HTT gene) in combination with stress increases the risk of depression

Serotonin hypothesis:

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Also known as Major depressive disorder, depression influences one's attitudes and behaviors through a persistent feeling of sadness and a loss of interest in usually compelling activities. Symptoms of depression can range from physical to psychological effects. However, this varies for each individual.

Kendler et al Study:

Aim:
To determine the role of genetics in the depression

Procedure: (Summarized)
Multiple twin pairs participating in the experiment participated in phone interviews.

Results:
Researchers concluded that:

Heritability of depression is higher in women than in men

Some genetic risk factors of depression were gender-specific

Caspi et al. Study

Aim:
To investigate the relationship between genetic type and alleles with the response to questionnaires diagnosing depression

Procedure: (Summarized)
Participants in New Zealand were allocated according to their alleles, and each groups were asked about their life events regarding employment, health and relationships.

Results:
The researchers from the study concluded that the 5-HTT gene or the serotonin transporter gene plays a significant role in depression but only if it is matched with the environment.

Serotonin Hypothesis:

States that a reduction in serotonin leads to a higher predisposition to depression.

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Depression Definition according to DSM:
Those who suffer from depression experience persistent feelings of sadness and hopelessness and lose interest in activities they once enjoyed. Aside from the emotional problems caused by depression, individuals can also present with a physical symptom such as chronic pain or digestive issues.

Kendler et al (2006) used 42,161 twins from the Swedish Twin Registry. They found that the concordance rate for female MZ twins was 44%, while for female DZ twins, only 16%.  In male MZ twins, the rate was 31% and for male DZ twins, the rate was 11%.   This seems to indicate that the disorder may be more genetic in origin for women than for men.

The researchers concluded that there is not a single gene that leads to depression, but rather that depression is the result of several genes that act together to produce a vulnerability to depression when other risk factors are present.

Caspi et al looked at the 5-HTT serotonin transporter gene, and how allele combinations and exposure to stressful events could result in participants having depression. From his study people who had inherited one or more short versions of the allele demonstrated more symptoms of depression and suicidal ideation in response to stressful life events. The effect was strongest for those with three or more stressful life events. Simply inheriting the gene was not enough to lead to depression, but the genes' interaction with stressful life events increased one's likelihood of developing depression.

The serotonin hypothesis of depression argues that a reduction in serotonin leads to increased predisposition to depression. Caspi's research on the genetic origins of Major Depressive Disorder demonstrates a potential link to serotonin, as the genes that were studied were responsible for serotonin transmission.

Neurogenesis theory of depression argues that depression is the result of the cessation of neuron birth in the hippocampus as well as in other neural networks related to serotonin, dopamine, and norepinephrine. Cortisol appears to be the reason for this lack of neurogenesis. Patients with major depressive disorder display a symptom called HPA-axis hyperactivity that results in the over-secretion of cortisol. This over-secretion of cortisol leads to reduced serotonin and other neurotransmitters in the brain, including dopamine, which has been linked to depression.

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A loss of interest in