The World Health Organization currently ranks depression as the fourth largest contributor to the global burden of disease and estimates it will rank second by 2020. And, yet, despite its widespread importance and the enormous amount of research that has gone into the construct, there remains significant debate regarding the precise nature of the construct. For example, the question as to whether depression is a normal reaction to chronic stress, a psychological disorder, or a brain disease has not been satisfactorily resolved.

To get a flavor for why depression might mean different things, imagine two different television commercials. The first begins with an attractive woman isolating herself at a party. Everyone else appears to be having a good time, yet she stands in the background, apparently gripped in the throes of a seemingly inexplicable sadness. The cultural milieu is of upper middle class suburbia. A soft voice inquires and informs, "Have you experienced periods of depressed mood? Have you lost interest in things you used to enjoy? Do you feel tired, guilty, ineffective, or hopeless? Depression is an illness. Ask your doctor about new antidepressant treatments available." The implicit message of this advertising is clear. When people are suffering from depression, something has gone wrong with the physiology of the brain.

Now imagine a different commercial. This one begins with an impoverished woman getting slapped by her husband. Her three children are having difficulties in school. Her husband controls her, and she has little in the way of social support. She recently immigrated to the United States and cannot get a job because she does not speak English well. She frequently faces prejudice and racism. The voice overlay asks, "Have you been feeling down or depressed, guilty or hopeless? Have you lost interest in things you usually enjoy? Depression is an illness. Ask your doctor about new antidepressant treatments available." Somehow the "depression as disease" message in this commercial is less convincing.

As these two vignettes illustrate, different portrayals can lead to radically different notions regarding the nature of depression. The unified theory of psychology provides the needed framework to get clarity on these issues. As discussed previously, Behavioral Investment Theory, posits that animal behavior can be thought of as the process of expending energy to control the flow of resources. Control of larger territories, access to better food, higher social status, etc. are obviously advantageous. However, the behavioral investment needed to acquire and maintain these resources can be expensive. It costs energy both in basic calories and in increasing risk of injury and loss. Resources are frequently not available or cannot be acquired, which means behavioral investments are fruitless. Additionally, competition over valuable resources can be fierce, often resulting in injury.

What does this analysis have to do with depression? It suggests that if an animal is consistently getting a poor return (i.e., high costs, little benefit) from its actions, it should start decreasing its investment. That is, if an animal is spending eight behavioral units and only getting back four units of value, then that is a bad ratio. If it tries everything in its behavioral repertoire yet the ratio remains the same, the best in a bad situation solution is to decrease the amount of the behavioral investment in an effort to reduce net loss. It is better to expend two and get back one unit over the same period of time than the eight to four ratio previously obtained.

This understanding gives rise to the Behavioral Shutdown Model (BSM) of depression, which suggests that depression arises out of an evolved tendency to decrease behavioral expenditure in response to chronic danger, stress, or consistent failure to achieve one's goals. Put slightly different, according to the BSM, we should think about depression as a state of behavioral shutdown. (It is worthwile to note here that the while the BSM is a psychological model, it shares many parallels with economic models of societal depressions).

The BSM offers a potential explanation for many features of depression. For example, it strongly predicts that depression should be more likely to occur in situations that are chronically dangerous, humiliating, or repeatedly result in failure to achieve one's goals. These are circumstances in which the cost-to-benefit ratio is the worst and therefore the most effective strategy is to reduce costs. Consistent with this prediction, situations in which the individual feels chronically trapped or humiliated are most likely to produce symptoms of depression. To give just one example, almost 50 percent of battered women are depressed.

In addition to offering an explanation as to why certain situations are more likely to result in depression, the BSM also explains many of the symptoms of depression. The model explains why emotional pain is such a prominent feature of depression, as the pain is a signal that things are not going well. Additionally, behavioral shutdown is the antithesis of active behavioral investment, and thus the BSM explains why loss of interest is such a fundamental characteristic of depressive conditions. It also directly accounts for why low energy is such a prominent complaint. The model also explains why negative cognitions are so prominent in depression. Cognitive theorists have clearly documented how depressed individuals are hypersensitive to any indications of loss, failure, or rejection. In direct accordance with the BSM, recent cognitive models have conceptualized depressed individuals as investors with few resources who take risk-aversive strategies to avoid loss.

The BSM also provides explanations for findings that are difficult to explain from a disease model perspective. Because so many different things can result in difficulties in solving important problems, the model accounts for why so many different causal pathways result in depression. Behavioral shutdown should be a matter of degree, thus the BSM also accounts for why symptoms of depression exist on a continuum that range from chronic, severe depressions to minor depressions to adjustment disorders to low mood. Since the model suggests depression should be associated with difficulties in functioning, the model explains why depressive symptoms evidence such a high comorbidity with other mental disorders, especially anxiety.

The BSM also offers a way to understand the various conceptual confusions that exist about the nature of depression. The BSM suggests that depression, including Major Depressive Episodes, should be considered a state of behavioral shutdown. That is what depression is. With this conception, then the question of whether depression is a normal mood, a psychological disorder, or a biological disease is found in the cause of the behavioral shutdown.            

When an individual is depressed as an obvious consequence of serious loss or chronic frustration, we can see depression as a normal reaction. For example, if my wife and children were killed in a car accident, I may well experience a state of profound "shutdown" as my entire psychological system would need to become recalibrated given that, as a consequence of the trauma, my primary pathways of investment would be gone. Interestingly, the founders of the DSM seem to have recognized this when they offer bereavement as an exception to diagnosing depression if the loss occurred in the past two months. Yet, this exception for grief raises significant questions. Consider, for example, the woman in the second imaginary commercial? Her shutting down also could be conceptualized as a very understandable reaction to the inability to find pathways of productive investment. However, as it currently stands, only bereavement exempts one from a diagnosis of Major Depression, whereas racism, poverty, isolation and abuse do not.

In contrast, when we see depression arising as a result of maladaptive, vicious cycles of behavioral investment, we can consider the condition a psychological disorder. This is essentially the behavioral conception of depression and it is consistent with a common behavioral treatment for depression called behavioral activation. The cognitive perspective similarly emphasizes how negative interpretations can lead to vicious depressive cycles. From a more psychodynamic perspective, consider how self-criticisms so prominent in depressed individuals might sometimes function to justify submission and the inhibition of aggressive impulses, and how this would lead to greater shutdown. All of these models are conceptions of depression as a psychological disorder, whereby the shutdown is resulting in vicious cycles.

The BSM also allows for a bio-psychiatric conceptualization and clarifies the distinction between a mental disease and a mental disorder. Severe depressive episodes that occur in the absence of behavioral ineffectiveness or loss can be considered depressive diseases because such occurrences reflect a breakdown in the functioning of the basic bio-psychological architecture.

In sum, thinking of depression as a state of behavioral shutdown offers much potential to clarify the nature of depression and when, how, and why depression can be considered a normal reaction, a psychological disorder, or a biological disease. It all depends on the cause of the shutdown.

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