Major depressive disorder can be caused by a variety of factors, both physical and/or social. A particular event such as a death, divorce or layoff can activate the symptoms which can then be extremely difficult to reverse without help. A person's sex can even play a role. Women are more likely to be affected by depression than men. Abuse -- whether physical, emotional or both -- can immediately lead to major depressive disorder or it may manifest itself in later life [source: WebMD].
Research also indicates that major depressive disorder can have a hereditary component. It tends to appear in each generation in some families. The fact that there may be no family history of depression doesn't, however, guarantee a person will live unaffected by the condition [source: WebMD].
While a blood test won't indicate whether or not clinical depression exists, a test can reveal other medical conditions that can cause the same symptoms associated with it. A malfunctioning thyroid, for example, can result in sluggishness. A stroke can produce changes that manifest themselves in the form of MDD. Alcoholism or other drug abuse can lead to feelings of despair and hopelessness. Even some medications can have side effects that cause depression [source: WebMD].