Several studies have shown that the risk of suicide is higher in patients who suffer from Bipolar II than those who suffer from Bipolar I, and especially higher than patients who suffer from major depressive disorder.

In results of a summary of several lifetime study experiments, it was found that 24% of Bipolar II patients experienced suicidal ideation or suicide attempts compared to 17% in Bipolar I patients and 12% in major depressive patients. Bipolar disorders, in general, are the third leading cause of death in 15- to 24-year-olds. Bipolar II patients were also found to employ more lethal means and have more complete suicides overall.

Bipolar II patients have several risk factors that increase their risk of suicide. The illness is very recurrent and results in severe disabilities, interpersonal relationship problems, barriers to academic, financial, and vocational goals, and a loss of social standing in their community, all of which increase the likelihood of suicide. Mixed symptoms and rapid-cycling, both very common in Bipolar II, are also associated with an increased risk of suicide. The tendency for Bipolar II to be misdiagnosed and treated ineffectively, or not at all in some cases, leads to an increased risk.

As a result of the high suicide risk for this group, reducing the risk and preventing attempts remains a main part of the treatment; a combination of self-monitoring, close supervision by a therapist, and faithful adherence to their medication regimen will help to reduce the risk and prevent the likelihood of a completed suicide.

Comorbid conditions are extremely common in individuals with BP-II. In fact, individuals are twice as likely to present a comorbid disorder than not. These include anxiety, eating, personality (cluster B), and substance use disorders. For bipolar II disorder, the most conservative estimate of lifetime prevalence of alcohol or other drug abuse disorders is 20%. In patients with comorbid substance abuse disorder and BP-II, episodes have a longer duration and treatment compliance decreases. Preliminary studies suggest that comorbid substance abuse is also linked to increased risk of suicidality.

Bipolar disorder can cause suicidal ideation that leads to suicidal attempts. Individuals whose bipolar disorder begins with a depressive or mixed affective episode seem to have a poorer prognosis and an increased risk of suicide. One out of two people with bipolar disorder attempt suicide at least once during their lifetime and many attempts are successfully completed. The annual average suicide rate is 0.4 percent, which is 10–20 times that of the general population. The standardized mortality ratio from suicide in bipolar disorder is between 18 and 25. The lifetime risk of suicide has been estimated to be as high as 20 percent in those with bipolar disorder.

According to a substantial amount of epidemiology studies conducted, women are twice as likely to develop certain mood disorders, such as major depression. Although there is an equal number of men and women diagnosed with bipolar II disorder, women have a slightly higher frequency of the disorder.

In 2011, mood disorders were the most common reason for hospitalization among children aged 1–17 years in the United States, with approximately 112,000 stays. Mood disorders were top principal diagnosis for Medicaid super-utilizers in the United States in 2012. Further, a study of 18 States found that mood disorders accounted for the highest number of hospital readmissions among Medicaid patients and the uninsured, with 41,600 Medicaid patients and 12,200 uninsured patients being readmitted within 30 days of their index stay—a readmission rate of 19.8 per 100 admissions and 12.7 per 100 admissions, respectively. In 2012, mood and other behavioral health disorders were the most common diagnoses for Medicaid-covered and uninsured hospital stays in the United States (6.1% of Medicaid stays and 5.2% of uninsured stays).

A study conducted in 1988 to 1994 amongst young American adults involved a selection of demographic and health characteristics. A population-based sample of 8,602 men and women ages 17–39 years participated. Lifetime prevalence were estimated based on six mood measures:

1. major depressive episode (MDE) 8.6%,

2. major depressive disorder with severity (MDE-s) 7.7%,

3. dysthymia 6.2%,

4. MDE-s with dysthymia 3.4%,

5. any bipolar disorder 1.6%, and

6. any mood disorder 11.5%.

Endogenous depression occurs as the results of an internal stressor—commonly cognitive or biological—and not an external factor. Potential risk factors include these cognitive or biological factors. Patients with endogenous depression often are more likely to have a positive family history of disorders and fewer psychosocial and environmental factors that cause their symptoms. A family history of depression and perceived poor intimate relationships are internal risk factors associated with this type of depression. It is important to know these risk factors in order to take steps to recognize and help prevent this illness.

This type of depression often occurs due to biological reasons. Since symptoms are due to an internal phenomena, prevalence rates tend to be higher in older adults and more prevalent among women. Although endogenous depression has been associated with increased age, there has been few attempts to evaluate this fully. More research is needed to indicate factual prevalence rates on this type of depression in society.

Those suffering from post-schizophrenic depression are also commonly at risk for suicidal tendencies. There is a trend correlated between suicide and post-schizophrenic depression according to Mulholland and Cooper's research in "The Symptoms of Depression in Schizophrenia and its Management." Furthermore, depression and schizophrenia have both been studied individually to try to determine if there is a correlation, and research has indicated that there is a very strong tendency for people with depression or schizophrenia to attempt suicide. Statistically, out of all patients suffering from schizophrenia, "10%...commit suicide. Depressed patients with schizophrenia are at a particularly high risk for suicide the first few months after diagnosis and after hospital discharge." Risk factors increasing the chance of suicide are, from highest to lowest, previous depressive orders, previous suicide attempts, drug abuse, and several other factors. Surprisingly, the suicide risk actually decreased with the presence of hallucinations. "The ICD-10 Classification of MEntal and Behavioural Disorders" officially recognizes suicide as being a prominent aspect of post-schizophrenic depression. Because of this drastic increase in suicide, it can be difficult to study post-schizophrenic depression as many of its victims tragically take their own lives.

Socioeconomic status has also been looked at as a potential cause for personality disorders. There is a strong association with low parental/neighborhood socioeconomic status and personality disorder symptoms. In a recent study comparing parental socioeconomic status and a child's personality, it was seen that children who were from higher socioeconomic backgrounds were more altuistic, less risk seeking, and had overall higher IQs. These traits correlate with a low risk of developing personality disorders later on in life. In a study looking at female children who were detained for disciplinary actions found that psychological problems were most negatively associated with socioeconomic problems. Furthermore, social disorganization was found to be inversely correlated with personality disorder symptoms.

Depressed mood can be the result of a number of infectious diseases, nutritional deficiencies, neurological conditions and physiological problems, including hypoandrogenism (in men), Addison's disease, Cushing's syndrome, hypothyroidism, Lyme disease, multiple sclerosis, Parkinson's disease, chronic pain, stroke, diabetes, and cancer.

Child abuse and neglect consistently show up as risk factors to the development of personality disorders in adulthood. A study looked at retrospective reports of abuse of participants that had demonstrated psychopathology throughout their life and were later found to have past experience with abuse. In a study of 793 mothers and children, researchers asked mothers if they had screamed at their children, and told them that they did not love them or threatened to send them away. Children who had experienced such verbal abuse were three times as likely as other children (who did not experience such verbal abuse) to have borderline, narcissistic, obsessive-compulsive or paranoid personality disorders in adulthood. The sexually abused group demonstrated the most consistently elevated patterns of psychopathology. Officially verified physical abuse showed an extremely strong correlation with the development of antisocial and impulsive behavior. On the other hand, cases of abuse of the neglectful type that created childhood pathology were found to be subject to partial remission in adulthood.

Various factors have been found to be more associated with a diagnosis of AD than other axis I disorders, including:

- younger age

- more identified psychosocial and environmental problems

- increased suicidal behaviour, more likely to be rated as improved by the time of discharge from mental healthcare

- less frequent previous psychiatric history

- shorter length of treatment

Those exposed to repeated trauma are at greater risk, even if that trauma is in the distant past. Age can be a factor due to young children having fewer coping resources; children are also less likely to assess the consequences of a potential stressor.

A stressor is generally an event of a serious, unusual nature that an individual or group of individuals experience. The stressors that cause adjustment disorders may be grossly traumatic or relatively minor, like loss of a girlfriend/boyfriend, a poor report card, or moving to a new neighborhood. It is thought that the more chronic or recurrent the stressor, the more likely it is to produce a disorder. The objective nature of the stressor is of secondary importance. Stressors' most crucial link to their pathogenic potential is their perception by the patient as stressful. The presence of a causal stressor is essential before a diagnosis of adjustment disorder can be made.

There are certain stressors that are more common in different age groups:

Adulthood:

- Marital conflict

- Financial conflict

- Health issues with Oneself/Partner or Dependent children

- Personal tragedy (Death/personal loss)

- Loss of job or unstable employment conditions (e.g. Corporate takeover/redundancy)

Adolescence and childhood:

- Family conflict/parental separation

- School problems/changing schools

- Sexuality issues

- Death/illness/trauma in the family

In a study conducted from 1990 to 1994 on 89 psychiatric outpatient adolescents, 25% had attempted suicide in which 37.5% had misused alcohol, 87.5% displayed aggressive behaviour, 12.5% had learning difficulties, and 87.5% had anxiety symptoms.

Studies have shown that those who fall into minorities due to either their gender identity or sexual orientation (such as those that identify as LGBT), are more prone to depression.

The causes of bipolar disorder likely vary between individuals and the exact mechanism underlying the disorder remains unclear. Genetic influences are believed to account for 60–80 percent of the risk of developing the disorder indicating a strong hereditary component. The overall heritability of the bipolar spectrum has been estimated at 0.71. Twin studies have been limited by relatively small sample sizes but have indicated a substantial genetic contribution, as well as environmental influence. For bipolar disorder type I, the rate at which identical twins (same genes) will both have bipolar disorder type I (concordance) is estimated at around 40 percent, compared to about 5 percent in fraternal twins. A combination of bipolar I, II, and cyclothymia similarly produced rates of 42 percent and 11 percent (identical and fraternal twins, respectively), with a relatively lower ratio for bipolar II that likely reflects heterogeneity. There is overlap with major (unipolar) depression and if this is also counted in the co-twin the concordance with bipolar disorder rises to 67 percent in identical twins and 19 percent in fraternal twins. The relatively low concordance between fraternal twins brought up together suggests that shared family environmental effects are limited, although the ability to detect them has been limited by small sample sizes.

There is no clear cause to how certain patients with schizophrenia develop post-schizophrenic depression while others may surpass this stage. However, there are a few theories as to possible causes. Those suffering from post-schizophrenic depression often suffer from social isolation due to their illness, which may increase depression levels. There is strong evidence of stigma-related isolation against those suffering from mental illnesses in a variety of societies, especially those with schizophrenia as they are often viewed as dangerous and unpredictable. Because of this isolation and studies linking social isolation and depression, it is possible that patients under these stigmas eventually develop post-schizophrenic depression. Depression in patients with schizophrenia may also be caused by substance abuse, which is fairly common among those suffering from schizophrenia, as depressants such as alcohol and cannabis can relax the patient. Furthermore, with what little information is currently known about post-schizophrenic depression, the onset may be caused by not giving patients with schizophrenia antipsychotic medications. After being taken off of antipsychotic medication, schizophrenic patients' antidepressant medication had to be increased, while those under antipsychotic medication reported suffering fewer depressive symptoms, further giving reason to believe that a lack of antipsychotic medication in earlier stages of schizophrenia may lead to post-schizophrenic depression. However, some psychology professionals still push for the reduction of neuroleptic drugs, as there is a popular belief that post-schizophrenic depression is caused by neuroleptic treatment. Therapists are also believed to engage the depression in people with schizophrenia, having given too much psychotherapy after the patient had overcome their schizophrenic symptoms. Schizophrenia itself should not be overlooked as a key player in causing post-schizophrenic depression, though. A study done over a two-year time period shadowing patients with schizophrenia and monitoring their depression was unable to locate possible triggers such as the ones previously listed, so it is possible the nature of schizophrenia itself is the primary cause of post-schizophrenic depression.

Winter depression is a common slump in the mood of some inhabitants of most of the Nordic countries. It was first described by the 6th century Goth scholar Jordanes in his "Getica" wherein he described the inhabitants of Scandza (Scandinavia). Iceland, however, seems to be an exception. A study of more than 2000 people there found the prevalence of seasonal affective disorder and seasonal changes in anxiety and depression to be unexpectedly "low" in both sexes. The study's authors suggested that propensity for SAD may differ due to some genetic factor within the Icelandic population. A study of Canadians of wholly Icelandic descent also showed low levels of SAD. It has more recently been suggested that this may be attributed to the large amount of fish traditionally eaten by Icelandic people, in 2007 about 90 kilograms per person per year as opposed to about 24 kg in the US and Canada, rather than to genetic predisposition; a similar anomaly is noted in Japan, where annual fish consumption in recent years averages about 60 kg per capita. Fish are high in vitamin D. Fish also contain docosahexaenoic acid (DHA), which help with a variety of neurological dysfunctions.

There are no known biological causes that apply consistently to all cases of dysthymia, which suggests diverse origin of the disorder. However, there are some indications that there is a genetic predisposition to dysthymia: "The rate of depression in the families of people with dysthymia is as high as fifty percent for the early-onset form of the disorder". Other factors linked with dysthymia include stress, social isolation, and lack of social support.

In a study using identical and fraternal twins, results indicated that there is a stronger likelihood of identical twins both having depression than fraternal twins. This provides support for the idea that dysthymia is in part caused by heredity.

The exact cause of cyclothymia is unknown. It is known that major depression, bipolar disorder, and cyclothymia often occur together within families. There may be a genetic component to cyclothymia: In one study, it was found that an individual is 2–3 times more likely to have the disorder if an identical twin is affected.

It is also believed that a person suffering with PTSD may experience similar shifts in mood based on how many flashbacks they've been dealing with.

The cause of major depressive disorder is unknown. The biopsychosocial model proposes that biological, psychological, and social factors all play a role in causing depression. The diathesis–stress model specifies that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events. The preexisting vulnerability can be either genetic, implying an interaction between nature and nurture, or schematic, resulting from views of the world learned in childhood.

Childhood abuse, either physical, sexual or psychological are all risk factors for depression, among other psychiatric issues that co-occur such as anxiety and drug abuse. Childhood trauma also correlates with severity of depression, lack of response to treatment and length of illness. However, some are more susceptible to developing mental illness such as depression after trauma, and various genes have been suggested to control susceptibility.

Meta-analyses show that high scores on the personality domain neuroticism is a strong predictor for the development of mood disorders. A number of authors have also suggested that mood disorders are an evolutionary adaptation. A low or depressed mood can increase an individual's ability to cope with situations in which the effort to pursue a major goal could result in danger, loss, or wasted effort. In such situations, low motivation may give an advantage by inhibiting certain actions. This theory helps to explain why negative life incidents precede depression in around 80 percent of cases, and why they so often strike people during their peak reproductive years. These characteristics would be difficult to understand if depression were a dysfunction.

A depressed mood is a predictable response to certain types of life occurrences, such as loss of status, divorce, or death of a child or spouse. These are events that signal a loss of reproductive ability or potential, or that did so in humans' ancestral environment. A depressed mood can be seen as an adaptive response, in the sense that it causes an individual to turn away from the earlier (and reproductively unsuccessful) modes of behavior.

A depressed mood is common during illnesses, such as influenza. It has been argued that this is an evolved mechanism that assists the individual in recovering by limiting his/her physical activity. The occurrence of low-level depression during the winter months, or seasonal affective disorder, may have been adaptive in the past, by limiting physical activity at times when food was scarce. It is argued that humans have retained the instinct to experience low mood during the winter months, even if the availability of food is no longer determined by the weather.

Much of what we know about the genetic influence of clinical depression is based upon research that has been done with identical twins. Identical twins both have exactly the same genetic code. It has been found that when one identical twin becomes depressed the other will also develop clinical depression approximately 76% of the time. When identical twins are raised apart from each other, they will both become depressed about 67% of the time. Because both twins become depressed at such a high rate, the implication is that there is a strong genetic influence. If it happened that when one twin becomes clinically depressed the other always develops depression, then clinical depression would likely be entirely genetic.

Bipolar disorder is also considered a mood disorder. In the case of bipolar disorder several causes have been considered as possible, please see the wikipedia page Bipolar disorder for more details on the most common attributed causes. Recently, apart of the recent knowledge, it is hypothesized and there is evidence that bipolar disorder might be caused by mitochondrial dysfunction or mitochondrial disease.

Pseudoneurotic schizophrenia is a postulated mental disorder categorized by the presence of two or more symptoms of mental illness such as anxiety, hysteria, and phobic or obsessive-compulsive neuroses. It is often acknowledged as a personality disorder. Patients generally display salient anxiety symptoms that disguise an underlying psychotic disorder.

In the 1940s, psychiatrists Paul Hoch and Philip Polatin created the term pseudoneurotic schizophrenia. This mental illness, however, is no longer acknowledged as a clinical entity. In 1972 it went on to be called borderline personality disorder, a term coined by Otto Friedmann Kernberg, which referred to an expansive range of issues.

Pseudoneurotic schizophrenia is in the Russian adapted version of the ICD-10 (code F21.3).

Estimates of the numbers of people suffering from major depressive episodes and Major Depressive Disorder (MDD) vary significantly. In their lifetime, 10% to 25% of women, and 5% to 12% of men will suffer a major depressive episode. Fewer people, between 5% and 9% of women and between 2% and 3% of men, will have MDD, or full-blown depression. The greatest differences in numbers of men and women diagnosed are found in the United States and Europe. The peak period of development is between the ages of 25 and 44 years. Onset of major depressive episodes or MDD often occurs to people in their mid-20s, and less often to those over 65. Prepubescent girls and boys are affected equally. The symptoms of depression are the same in both children and adolescents though there is evidence that their expression within an individual may change as he or she ages.

In a National Institute of Mental Health study, researchers found that more than 40 percent of people with post-traumatic stress disorder suffered from depression 4 months after the traumatic event they experienced.

Cultural factors can influence the symptoms displayed by a person experiencing a major depressive episode. The values of a specific culture may also influence which symptoms are more concerning to the person or and their friends and family. It is essential that a trained professional knows not to dismiss specific symptoms as merely being the "norm" of a culture.

Women who have recently given birth may be at increased risk for having a major depressive episode. This is referred to as postpartum depression and is a different health condition than the baby blues, a low mood that resolves within 10 days after delivery.

According to the DSM IV-TR, the development of the emotional or behavioral symptoms of this diagnosis have to occur within three months of the onset of the identifiable stressor(s). Some emotional signs of adjustment disorder are:

However, the stress-related disturbance does not only exist as an exacerbation of a pre-existing axis I or axis II disorder and cannot be diagnostic as axis 1 disorder.

Suicidal behavior is prominent among people with AD of all ages, and up to one-fifth of adolescent suicide victims may have an adjustment disorder. Bronish and Hecht (1989) found that 70% of a series of patients with AD attempted suicide immediately before their index admission and they remitted faster than a comparison group with major depression. Asnis et al. (1993) found that AD patients report persistent ideation or suicide attempts less frequently than those diagnosed with major depression. According to a study on 82 AD patients at a clinic, Bolu et al. (2012) found that 22 (26.8%) of these patients were admitted due to suicide attempt, consistent with previous findings. In addition, it was found that 15 of these 22 patients chose suicide methods that involved high chances of being saved. Henriksson et al. (2005) states statistically that the stressors are of one-half related to parental issues and one-third in peer issues.

In many species, activity is diminished during the winter months in response to the reduction in available food, the reduction of sunlight (especially for diurnal animals) and the difficulties of surviving in cold weather. Hibernation is an extreme example, but even species that do not hibernate often exhibit changes in behavior during the winter. Presumably, food was scarce during most of human prehistory, and a tendency toward low mood during the winter months would have been adaptive by reducing the need for calorie intake. The preponderance of women with SAD suggests that the response may also somehow regulate reproduction.

Various proximate causes have been proposed. One possibility is that SAD is related to a lack of serotonin, and serotonin polymorphisms could play a role in SAD, although this has been disputed. Mice incapable of turning serotonin into N-acetylserotonin (by serotonin N-acetyltransferase) appear to express "depression-like" behavior, and antidepressants such as fluoxetine increase the amount of the enzyme serotonin N-acetyltransferase, resulting in an antidepressant-like effect. Another theory is that the cause may be related to melatonin which is produced in dim light and darkness by the pineal gland, since there are direct connections, via the retinohypothalamic tract and the suprachiasmatic nucleus, between the retina and the pineal gland. Melatonin secretion is controlled by the endogenous circadian clock, but can also be suppressed by bright light.

One study looked at whether some people could be predisposed to SAD based on personality traits. Correlations between certain personality traits, higher levels of neuroticism, agreeableness, openness, and an avoidance-oriented coping style, appeared to be common in those with SAD.

The 5-HTTLPR, or serotonin transporter promoter gene's short allele has been associated with increased risk of depression. However, since the 1990s results have been inconsistent, with three recent reviews finding an effect and two finding none. Other genes that have been linked to a GxE interaction include CRHR1, FKBP5 and BDNF, the first two of which are related to the stress reaction of the HPA axis, and the latter of which is involved in neurogenesis.

When pseudoneurotic schizophrenia was still being utilized as a diagnostic term, doctors were expected to be able to magically cure patients. Patients usually had very little understanding of themselves and the complexity of their illness. They were willing to employ any process in order to maintain mental stability. Their perception of mental stability, however, was also impaired, which made it much more difficult to make proper, helpful medication prescriptions.

Patients would often misuse medication in order to receive attention from their families. They would describe the dosage and effects of the medicine in some strange demeanor to demonstrate that their illness was physical rather than psychological. In like manner, taking medication also kept doctors concerned about the possibility of the patient developing substance dependence and/or drug addiction. Patients used this to get attention and sympathy from others.