Investigated through family studies, mainly of monozygotic (identical) and dizygotic (fraternal) twins, often in the context of adoption. Monozygotic twins should be more likely than dizygotic twins to have the same disorder because they share 100% of their genetic material, whereas dizygotic twins share only 50%. For many disorders, this is exactly what research shows. But given that monozygotic twins share 100% of their genetic material, it may be expected of them to have the same disorders 100% of the time, but in fact they have the same disorders only about 50% of the time
These studies allow calculation of a heritability coefficient.
Biological Causal Factors
Neurotransmitter [imbalances of neurotransmitters like (1) Norepinephrine (2) Dopamine (3) Serotonin and (4) GABA (Gamma aminobutryic acid)] and hormonal imbalances in the brain
Constitutional liabilities [physical handicaps and temperament]
Brain dysfunction and neural plasticity
Physical deprivation or disruption [deprivation of basic physiological needs
Effects of urban/rural dwelling, gender and minority status on state of mind
Negatively Expressed Emotion playing a part in schizophrenic relapse and anorexia nervosa.