According to the World Health Organization, around 450 million people (6-7 million Canadians) struggle with problems regarding their mental health, making it the leading cause of disability worldwide. Depression (major depressive disorder) is a common and serious mental illness that adversely affects aspects in daily life, such as physical health, relationships and cognitive performance . Individuals experiencing depression enter an affective state characterized by sad mood, anhedonia and changes in psychomotor, sleeping, and eating patterns. As an indicator of its adverse effects, depression has been studied to have the same predictive ability for mortality as does smoking. One such predictor of depression is stress in the form of challenges across a range of areas, which may inhibit an individual’s ability to withstand stress.
Despite the challenges from stress, the brain plays a crucial role in promoting survival and reproduction, which would mean the pressures of evolution would make the brain resistant to adverse states such as depression. This would generally mean mental ailments should be rare but yet depression is not. From an evolutionary perspective, the prevalence of depression poses a serious puzzle. The discipline of evolutionary psychology views modern human behaviour as by-products of natural selection that acted on the psyche of our ancestors. The relative frequency of apparently adverse behaviours caused by depression may have been reinforced over time by natural selection. Contrary to wide spread belief about depression, it may have evolved to be an adaptive mechanism. Various studies propose that depression has evolved to solve difficult and complex problems in social interactions and significant in pathogen host defense. Researchers have begun to view depression as an evolved mechanism that modulates behaviour. As such, there is a paradigm shift in looking at depression. as an adaptive mechanism used by humans today.
Recent studies of depression have suggested many evolutionary advantages depressed individuals may possess. While depressive behaviour may not provide long term benefits to the depressed individual, it provides some immediate benefits in the form of withdrawal from an adverse environment . Multiple papers have hypothesized an evolutionary model of depression which focuses on the apparent benefit from emitting depressive behaviour in response to stressor . Research suggests that depression has evolved to withdraw from the stressor, allowing the individual time to analyze stressful situations. It has also been suggested that depression elicits assistance from partners, friends and family. Individuals with depression often tend to think about their problems. Numerous studies have shown this thinking style to be highly analytical, as they are able to break down problems into smaller components. It has also been discovered that depressed individuals working on complex problems in an intelligence test tend to score higher on the test . Analysis requires high mental focus and uninterrupted thought, and depressive conditions allow for changes in the body to help people analyze their problems without getting distracted. The ventrolateral prefrontal cortex (VLPFC) must fire for people to avoid being distracted. Continuous firing can cause neurons to break down but it has been found individuals with depression have receptors involved in supplying the fuel they need to fire and preventing them from breaking down.
A multitude of studies have also found depressed mood states are better for solving social dilemmas. Depression can act as a functional mechanism when there is danger and can also be functional when one has to recover from loss or from physical ailments . The symptoms of depression only become a problem when they are out of control. Depression has been interpreted as a form of communication to manipulate others into providing resources . The ability to instill pity in others may be helpful. Humans simply need the capacity to be depressed in specific circumstances. Eliminating genes that are associated with depressive behaviour would be disadvantageous. Low mood resulting from depression can be advantageous in certain situations. Decreased motivation would be useful in mainly situation where actions would be futile or dangerous. Humans need to regulate when and how they exert energy, as such being in a depressed state can be advantageous by disengaging individuals from unproductive efforts.
Studies conducted have identified the evolutionary significance of depression in pathogen host defense. Researchers proposed that some of the alleles that increase the risk of depression, also enhance immune response to infections. Acute stress does not only lead to depression but also can empower the immune system. Environments in which our ancestors lived in, acute stress was often associated with the threat of physical harm or wounds. Immune response to acute stress would have been necessary for our ancestors to fight off infections. Depressive symptoms such as social withdrawal and a lack of energy was also advantageous to ancestors. Social withdrawal would minimize the likelihood of being exposed to infectious agents. As such, there is a link between inflammatory pathways formed from depression in the brain, which can lead to behavioural responses such as avoidance, to evolutionary advantages ancestors possessed in their interactions with pathogens.
Although, depression appears to only provide adverse effects upon an individual, it also has proved to provide some advantages evolutionary for our ancestors. Like other physiological functions that may cause discomfort (like inflammation), depressive withdrawal and avoidance also has a functional aspect behind the mechanism. Despite many adaptive functions, the evolutionary model of depression is still being reviewed. It may not truly be classified as evolutionary or adaptive in the long-term due to high levels of stress and an inability to pass it on through birth. Investigation of these advantages and its stressors, which cause the individual to withdraw, presents a potentially fruitful area of research. withdrawal from potential danger, is advantageous for the person who exhibits such behaviour and therefore may be seen as adaptive. Depression as an adaptive mechanism should still be subject to critical critique but the avoidance of depressive like symptoms may have negative impact on individuals that we previously suspected.