Evolutionary approaches to depression are attempts by evolutionary psychologists and evolutionary psychiatrists to use the theory of evolution to further understand mood disorders. Depression is generally thought of as dysfunction or a mental disorder, but its prevalence does not increase with age the way dementia and other organic dysfunction commonly does. Some researchers have surmised that the disorder may have evolutionary roots, in the same way that others suggest evolutionary contributions to schizophrenia, sickle cell anemia, psychopathy and other disorders. The proposed explanations for the evolution of depression remain controversial.
Background
Depression is a mental disorder characterized by pervasive low mood, diminished motivation, and loss of pleasure from normally enjoyable activities. It is a risk factor for disability and suicide, and contributes significantly to disease burden worldwide. Depression is hence thought to be a pathological state of one's brain.
In most cases, rates of organ dysfunction increase with age, with low rates in adolescents and young adults, and the highest rates in the elderly. These patterns are consistent with evolutionary theories of aging, which posit that selection against dysfunctional traits decreases with age (because there is a decreasing probability of surviving to later ages). In contrast to these patterns, prevalence of clinical depression is high across all age categories. In one 1993 study of the US population, the 12 month prevalence of major depressive episodes was highest in the youngest age category (15- to 24-year-olds), making it an outlier when compared to the prevalence of other mental disorders such as intellectual disability, autism, and schizophrenia, where all of the latter have prevalence rates about one tenth that of depression, or less.Epidemiologic Catchment Area, National Comorbidity Study , depression is the second most common mental disorder behind anxiety.
The common occurrence and persistence of a trait like clinical depression with such negative effects early in life is difficult to explain. (Rates of infectious disease are high in young people, of course, but clinical depression is not thought to be caused by an infection.) Evolutionary psychology and evolutionary psychiatry, and their application in evolutionary medicine suggest how behaviour and mental states, including seemingly harmful states such as depression, may have been beneficial adaptations of human ancestors which improved the fitness of individuals or their relatives. For example, Shenk (2005) suggests that Abraham Lincoln's lifelong depression was a source of insight and strength. Some even suggest that "we aren't designed to have happiness as our natural default" and so a state of depression is the evolutionary norm.
The following hypotheses attempt to identify a benefit of depression that outweighs its obvious costs.
Such hypotheses are not necessarily incompatible with one another and may explain different aspects, causes, and symptoms of depression.
Psychic pain hypothesis One reason depression is thought to be a pathology is that it causes so much psychic pain and distress. However, physical pain is also very distressful, yet it has an evolved function: to inform the organism that it is being damaged, to motivate it to withdraw from the source of damage, and to learn to avoid such damage-causing circumstances in the future. Sadness is also distressing, yet is widely believed to be an evolved adaptation. In fact, perhaps the most influential evolutionary view is that most cases of depression are simply particularly intense cases of sadness in response to adversity, such as the loss of a loved one.
According to the psychic pain hypothesis, depression is analogous to physical pain in that it informs them that current circumstances, such as the loss of a friend, are imposing a threat to biological fitness. It motivates them to cease activities that led to the costly situation, if possible, and it causes him or her to learn to avoid similar circumstances in the future. Proponents of this view tend to focus on low mood, and regard clinical depression as a dysfunctional extreme of low mood—and not as a unique set of characteristics that are physiologically distanced from regular depressed mood.
Alongside the absence of pleasure, other noticeable changes include psychomotor retardation, disrupted patterns of sleeping and feeding, a loss of sex drive and motivation—which are all also characteristics of the body's reaction to actual physical pain. In depressed people there is an increased activity in the regions of the cortex involved with the perception of pain, such as the anterior cingulate cortex and the left prefrontal cortex. This activity allows the cortex to manifest an abstract negative thought as a true physical stressor to the rest of the brain.
Behavioral shutdown model The behavioral shutdown model states that if an organism faces more risk or expenditure than reward from activities, the best evolutionary strategy may be to withdraw from them. This model proposes that emotional pain, like physical pain, serves a useful adaptive purpose. Negative emotions like disappointment, sadness, grief, fear, anxiety, anger, and guilt are described as "evolved strategies that allow for the identification and avoidance of specific problems, especially in the social domain." Depression is characteristically associated with anhedonia and lack of energy, and those experiencing it are risk-aversive and perceive more negative and pessimistic outcomes because they are focused on preventing further loss. Although the model views depression as an adaptive response, it does not suggest that it is beneficial by the standards of current society; but it does suggest that many approaches to depression treat symptoms rather than causes, and underlying social problems need to be addressed.
A related phenomenon to the behavioral shutdown model is learned helplessness. In animal subjects, a loss of control or predictability in the subject's experiences results in a condition similar to clinical depression in humans. That is to say, if uncontrollable and unstoppable stressors are repeated for long enough, a rat subject will adopt a learned helplessness, which shares a number of behavioral and psychological features with human depression. The subject will not attempt to cope with problems, even when placed in a stressor-free novel environment. Should their rare attempts at coping prove successful in a new environment, a long lasting cognitive block prevents them from perceiving their action as useful and their coping strategy does not last long. From an evolutionary perspective, learned helplessness also allows a conservation of energy for an extended period of time should people find themselves in a predicament that is outside of their control, such as an illness or a dry season. However, for today's humans whose depression resembles learned helplessness, this phenomenon usually manifests as a loss of motivation and the distortion of one uncontrollable aspect of a person's life being viewed as representative of all aspects of their life – suggesting a mismatch between ultimate cause and modern manifestation.
Analytical rumination hypothesis This hypothesis suggests that depression is an adaptation that causes the affected individual to concentrate his or her attention and focus on a complex problem in order to analyze and solve it.
One way depression increases the individual's focus on a problem is by inducing rumination. Depression activates the left ventrolateral prefrontal cortex, which increases attention control and maintains problem-related information in an "active, accessible state" referred to as "working memory", or WM. As a result, depressed individuals have been shown to ruminate, reflecting on the reasons for their current problems. Feelings of regret associated with depression also cause individuals to reflect and analyze past events in order to determine why they happened and how they could have been prevented. The rumination hypothesis has come under criticism. Evolutionary fitness is increased by ruminating before rather than after bad outcomes. A situation that resulted in a child being in danger but unharmed should lead the parent to ruminate on how to avoid the dangerous situation in the future. Waiting until the child dies and then ruminating in a state of depression is too late.
Some cognitive psychologists argue that ruminative tendency itself increases the likelihood of the onset of depression.
Another way depression increases an individual's ability to concentrate on a problem is by reducing distraction from the problem. For example, anhedonia, which is often associated with depression, decreases an individual's desire to participate in activities that provide short-term rewards, and instead, allows the individual to concentrate on long-term goals. In addition, "psychomotoric changes", such as solitariness, decreased appetite, and insomnia also reduce distractions. For instance, insomnia enables conscious analysis of the problem to be maintained by preventing sleep from disrupting such processes. Likewise, solitariness, lack of physical activity, and lack of appetite all eliminate sources of distraction, such as social interactions, navigation through the environment, and "oral activity", which disrupt stimuli from being processed.
Possibilities of depression as a dysregulated adaptation Depression, especially in the modern context, may not necessarily be adaptive. The ability to feel pain and experience depression, are adaptive defense mechanisms, but when they are "too easily triggered, too intense, or long lasting", they can become "dysregulated". In such a case, defense mechanisms, too, can become diseases, such as "chronic pain or dehydration from diarrhea". Depression, which may be a similar kind of defense mechanism, may have become dysregulated as well.
Thus, unlike other evolutionary theories this one sees depression as a maladaptive extreme of something that is beneficial in smaller amounts. In particular, one theory focuses on the personality trait neuroticism. Low amounts of neuroticism may increase a person's fitness through various processes, but too much may reduce fitness by, for example, recurring depressions. Thus, evolution will select for an optimal amount and most people will have neuroticism near this amount. However, genetic variation continually occurs, and some people will have high neuroticism which increases the risk of depressions.
Rank theory Rank theory is the hypothesis that, if an individual is involved in a lengthy fight for dominance in a social group and is clearly losing, then depression causes the individual to back down and accept the submissive role. In doing so, the individual is protected from unnecessary harm. In this way, depression helps maintain a social hierarchy. This theory is a special case of a more general theory derived from the psychic pain hypothesis: that the cognitive response that produces modern-day depression evolved as a mechanism that allows people to assess whether they are in pursuit of an unreachable goal, and if they are, to motivate them to desist.
Social risk hypothesis This hypothesis is similar to the social rank hypothesis but focuses more on the importance of avoiding exclusion from social groups, rather than direct dominance contests. The fitness benefits of forming cooperative bonds with others have long been recognised—during the Pleistocene period, for instance, social ties were vital for food foraging and finding protection from predators.