In this next section of this document, we will be surveying factors that can contribute to the development of depression. There are many factors that have been identified, and the relationship between one factor and another is often complex. To help explain this complexity, the mental health professions have adopted several models or theories that describe the way that various factors, which contribute to health and illness interrelate. We'll be discussing these models first, before going on to talk about the individual factors that "plug into" these models.
The biopsychosocial model, first developed by cardiologist Dr. George Engel, is today widely accepted by the mental health professions. This model suggests that biological, psychological and social factors are all interlinked and important with regard to promoting health or causing disease. In other words, the mind and the body are not independent and separate things (as was previously thought), but rather are connected and interdependent things (if they are indeed separate things at all). What affects the body will often affect the mind; and vice versa, what affects the mind will also often end up affecting the body. Wellness or illness is not simply a matter of someone's physical state, but is also influenced by that person's psychological and social status as well.
The biopsychosocial model encourages clinicians to explain phenomena such as depression by examining all relevant biological, psychological, and social factors that might be contributing to the development or maintenance of the disorder. With regard to biological factors, it is known that depressed individuals are often significantly disturbed with regard to endocrine (hormone), immune, and neurotransmitter system functioning. In addition, depression can make a person more vulnerable to developing a range of physical disorders. Similarly, a person who has a physical disorder is often more likely to develop depression. Research also suggest that genes can influence transmission of depression from generation to generation.
Psychological factors influencing depression include characteristic negative patterns of thinking, deficits in coping skills, judgment problems, and impaired emotional intelligence (the ability to perceive, understand, and express emotions) that depressed people tend to exhibit. To some degree, these psychological factors can be influenced by biology (e.g, people's innate temperament, or their biologically-based personality characteristics, can influence people to be more or less likely to act in ways characteristic of depression), and by social factors such as what coping behaviors are modeled for people (e.g., by their parents and teachers) as they are growing up.
People can also become depressed as a result of social factors such as: experiencing traumatic situations, early separation, lack of social support, or harassment (bullying). Research has shown that stressful social events are capable of serving as triggers for turning genes on and off, causing changes in brain functioning. Via this path, a social stressor can trigger a physical cause of depression. Environmental and social causes of depression can also be far more subtle than actual trauma. It is not necessary for people to have been abused as children to grow up feeling negatively about themselves or their prospects because of how they have learned to think about their self-worth or their ability to successfully respond to the tasks and stressors present in daily living.
The biopsychosocial model suggests, and the scientific evidence has tended to confirm, that the interdependent factors we have discussed above (biological, psychological and social factors) all end up influencing each other and feeding into each other in an interdependent way. Depression can be caused by any number of factors that would on their surface appear to be independent from one another. Also, as one factor tends to influence the other factors, it is possible to have a physical reaction to a social or psychological stressor, and vice versa. This interdependent nature; the way that the various causes of depression affect one another; make it urgent that all factors be taken into account when attempting to form a complete explanation of depression.
We've only here scratched the surface of the biopsychosocial model. In a later section, we will explain in more detail the biological, psychological and social factors that are known to contribute to depression.