Depression is the most commonly occurring psychological disorder in the United States. In the US, it's estimated that twenty percent of the population is depressed. Taken as an actual number, that twenty percent affliction rate amounts to roughly sixty million people. It's also worth noting that most depressed people in the United States don't get professionally diagnosed or treated. Given how widespread depression seems to be, the question of what actually causes depression is an important one.

There are essentially two reasons for depression: biological factors and environmental interactive factors. A combination of biological and environmental factors is obviously another possibility. When discussing biological causes of depression, essentially what's being referenced is a chemical imbalance in the brain. There are different chemicals at work in the brain, operating in complex functions. There's a correlation between low levels of particular chemicals within the brain and emotional and psychological dysfunctions, depression included.

One of the chemicals believed to have a primary function in the regulation of mood is serotonin. Low serotonin levels have been associated not only with depression but with obsessive-compulsive disorder, anxiety, sleep difficulties, and anger and aggression abnormalities. The discovery of serotonin in the role of mood was key in the development of the selective serotonin reuptake inhibitor class of drugs, also known as SSRI's. Prozac was the first of the SSRI's on the market, and is still the best known of the SSRI's that are available. Prozac in fact is one of the most widely prescribed pharmaceuticals in history, a testament to the prevalence of depression and emotional malaise.

The belief that depression is entirely an environmental interactive issue is the other side of the coin in the thinking on what causes depression. That is to say, depression isn't caused by any biological imbalance at all but instead occurs because of how a person responds to their environment. As one example, an individual may consider the break up of a relationship as a significant loss and fall into a depression as a result. An environmental interactive perspective would place the cause for this case of depression entirely on individual perspective and not on any sort of chemical imbalanance: a relationship break up in this particular case. Using this sort of explanation, all forms of depression can be explained by a person's perspective as opposed to any biological factors.

Is depression then caused by biological factors or by how one considers the environment they inhabit? The answer is likely a combination of both. The effectiveness of drugs like Prozac demonstrate that there is a biological element to depression, but there's certainly also issues of perspective at work as well.