Biological factors consist of anything physical that can cause adverse effects on a person’s mental health. This includes genetics, prenatal damage, infections, exposure to toxins, brain defects or injuries, and substance abuse. Many professionals believe that the sole cause of mental disorders is based upon the biology of the brain and the nervous system. Mind mentions genetic factors, long-term physical health conditions, and head injuries or epilepsy (affecting behaviour and mood) as factors that may possibly trigger an episode of mental illness.

Genetics

Family-linkage and twin studies have indicated that genetic factors often play an important role in the development of mental disorders. The reliable identification of specific genetic susceptibility to particular disorders, through linkage or association studies, has proven difficult. This has been reported to be likely due to the complexity of interactions between genes, environmental events, and early development or to the need for new research strategies. The heritability of behavioral traits associated with mental disorder may be greater in permissive than in restrictive environments, and susceptibility genes probably work through both “within-the-skin” (physiological) pathways and “outside-the-skin” (behavioral and social) pathways.  Investigations increasingly focus on links between genes and endophenotypes—more specific traits (including neurophysiological, biochemical, endocrinological, neuroanatomical, cognitive, or neuropsychological)—rather than disease categories. With regard to a prominent mental disorder, schizophrenia, for a long time consensus among scientists was that certain alleles (forms of genes) were responsible for schizophrenia, but some research has indicated only multiple, rare mutations thought to alter neurodevelopmental pathways that can ultimately contribute to schizophrenia; virtually every rare structural mutation was different in each individual.

Research has shown that many conditions are polygenic meaning there are multiple defective genes rather than only one that are responsible for a disorder. Schizophrenia and Alzheimer’s are both examples of hereditary mental disorders.

The increasing understanding of brain plasticity (neuroplasticity) raises questions of whether some brain differences may be caused by mental illnesses, rather than pre-existing and causing them.

Prenatal damage

Any damage that occurs to a fetus while still in its mother’s womb is considered prenatal damage. If the pregnant mother uses drugs or alcohol or is exposed to illnesses or infections then mental disorders can develop in the fetus. According to research, certain conditions, such as autism result from a disruption of early fetal brain progression.

Environmental events surrounding pregnancy and birth have been linked to an increased development of mental illness in the offspring. This includes maternal exposure to serious psychological stress or trauma, conditions of famine, obstetric birth complications, infections, and gestational exposure to alcohol or cocaine. Such factors have been hypothesized to affect specific areas of neurodevelopment within the general developmental context and to restrict neuroplasticity.

Infection, disease and toxins

A number of psychiatric disorders have often been tentatively linked with microbial pathogens, particularly viruses; however while there have been some suggestions of links from animal studies, and some inconsistent evidence for infectious and immune mechanisms (including prenatally) in some human disorders, infectious disease models in psychiatry are reported to have not yet shown significant promise except in isolated cases.

There have been some inconsistent findings of links between infection by the parasite Toxoplasma gondii and human mental disorders such as schizophrenia, with the direction of causality unclear. A number of diseases of the white matter can cause symptoms of mental disorder.

Poorer general health has been found among individuals with severe mental illnesses, thought to be due to direct and indirect factors including diet, bacterial infections, substance use, exercise levels, effects of medications, socioeconomic disadvantages, lowered help-seeking or treatment adherence, or poorer healthcare provision. Some chronic general medical conditions have been linked to some aspects of mental disorder, such as AIDS-related psychosis.

The current research on Lyme disease caused by a deer tick, and related toxins, is expanding the link between bacterial infections and mental illness.

Research shows that infections and exposure to toxins such as HIV and streptococcus cause dementia and OCD respectively. The infections or toxins trigger a change in the brain chemistry, which can develop into a mental disorder.

Injury and brain defect

Any damage to the brain can cause a mental disorder. The brain is the control system for the nervous system and the rest of the body. Without it the body cannot function properly.

Higher rates of mood, psychotic, and substance abuse disorders have been found following traumatic brain injury (TBI). Findings on the relationship between TBI severity and prevalence of subsequent psychiatric disorders have been inconsistent, and occurrence has been linked to prior mental health problems as well as direct neurophysiological effects, in a complex interaction with personality and attitude and social influences.

Neurotransmitter systems

Abnormal levels of dopamine activity have been correlated with a number of disorders (e.g., reduced in ADHD and OCD, and increased in schizophrenia). Dysfunction in serotonin and other monoamine neurotransmitters (e.g., norepinephrine and dopamine), and their associated neural networks, are also moderately correlated with certain mental disorders, including major depression, obsessive compulsive disorder, phobias, posttraumatic stress disorder, and generalized anxiety disorder. Studies of depleted levels of monoamine neurotransmitters show an association with depression and other psychiatric disorders, but “… it should be questioned whether 5-HT [serotonin] represents just one of the final, and not the main, factors in the neurological chain of events underlying psychopathological symptoms….” 

Substance abuse

Substance abuse, especially long-term abuse, can cause or exacerbate many mental disorders. Alcoholism is linked to depression while abuse of amphetamines and LSD can leave a person feeling paranoid and anxious.

Correlations of mental disorders with drug use include cannabis, alcohol and caffeine.  Caffeine use is correlated with anxiety  and suicide. Illicit drugs have the ability to stimulate particular parts of the brain which can affect development in adolescence. Cannabis has been found to worsen depression and lessen an individual’s motivation. Alcohol has the potential to damage “white matter” in the brain which affects thinking and memory. Alcohol has been found to be a serious problem in many countries due to many people participating in excessive drinking or binge drinking.