Depression as well as other major psychiatric disorders can be caused by external events. However it does not remit when the external cause dissipates. Also the reaction can be disproportionate to the cause. Some depressions start out of the blue. It is difficult to make a clear distinction between depressions with a clear cause and those without psychosocial precipitating events.
Depression is a heterogeneous disorder, as the other major psychiatric disorders, with a highly variable course, an inconsistent response to treatment, and no established mechanism.
Twin studies suggest a heritability for depressive disorder of about 37%, which is much lower than the heritability for bipolar disorder and schizophrenia.
A recent large register based cohort study of more than 2 million persons sheds light on risk factors for major psychiatric disorders: schizophrenia, bipolar disorder, unipolar disorder and schizoaffective disorder.
This study was performed in Denmark were they fortunately have large data registers on mental health. The Danish civil registration system was linked to the Danish Psychiatric Central Register. The researchers obtained information on inpatient psychiatric treatment of all cohort members and their family. They also linked the Cause of Death Register for the cause of death of parents and information on birth weight and gestational age was obtained from the Danish Medical Birth Register.
They included more than 2 million persons born in Denmark between January 1, 1955 and July 1, 1987. Overall follow-up began on January 1, 1973 and ended June 30, 2005.
In this study the incidence of affective and schizophrenic disorders was measured as well as risk factors that are supposed to operate at different stages of life. These risk factors were:
1. paternal age
2. urbanicity of place of birth
3. being born small for gestational age
4. parental loss.
- Women had a much higher incidence of unipolar depression than men
- In schizoaffective disorder gender was equally distributed between gender
- Incidence of schizophrenia was more than twice as high in men as in woman, a peak occurred in men aged 20 to 25 years
- Women had a higher incidence of bipolar disorder
- Schizophrenia peaked at an earlier age than bipolar disorder
- Loss of a parent was a risk factor for all disorders especially after unnatural death of a parent.
- High paternal age and urbanization at birth were risk factors for schizophrenia.
- No risk small for gestational age and born at term.
Overlap in risk factors examined in this study was found and the differences between the phenotypes were quantitative rather than qualitative which suggests a genetic and environmental overlap between the disorders. However large gender differences and differences in age specific incidences in the 4 disorders were present favoring the Kraepelinian dichotomization of schizophrenia versus the mood disorders.
J Clin Psychiatry. 2007 Nov;68(11):1673-81.
A comparison of selected risk factors for unipolar depressive disorder, bipolar
affective disorder, schizoaffective disorder, and schizophrenia from a danish
Munk Laursen T, Munk-Olsen T, Nordentoft M, Bo Mortensen P.