Depression
This section provides a basic overview of depression, including symptoms of depression, common pathways to depression, an evolution-informed conceptualization of depression, circumstances under which depression may be adaptive, and the evolution-informed treatment of depression.
Some suffering is part of a vital mechanism shaped by natural selection to help people survive in their environment.
Symptoms of depression
The DSM-5 identifies nine potential symptoms of a major depressive episode:
Low mood
Anhedonia
Weight loss or gain; appetite decrease or increase
Insomnia or hypersomnia
Psychomotor agitation or retardation
Loss of energy or fatigue
Feelings of guilt or worthlessness
Difficulty concentrating or making decisions
Thoughts of death, suicidal ideation/plan/attempt
Common pathways to depression
Adverse childhood experiences
The risk of depression is increased by adverse childhood experiences such as neglect, physical or sexual abuse, witnessing domestic violence, poverty, and other experiences.
Traumatic events
Traumatic events can sometimes result in depression, in addition to other mental disorders such as post-traumatic stress disorder.
Neurobiological abnormalities
Depression is often mediated by neurobiological abnormalities, including neurotransmitter hypoactivity, grey matter volume reduction, and hormone dysregulation.
Bereavement
Although bereavement and depression are not considered the same syndrome, some complex forms of bereavement can result in comorbid depression.
Genetic and epigenetic traits
Some genetic traits are risk factors for depression. Some epigenetic traits—heritable traits that are not encoded in DNA—may also be risk factors for depression.
Personality traits
Some personality traits—such as high levels of neuroticism and lows levels of extraversion—are risk factors for developing depression.
Chronic Stress
Chronic stress is a risk factor for depression. Chronic stress has a detrimental effect on multiple biological systems like the immune system and the neuroendocrine system.
Others
Other pathways to depression include seasonal depression, substance-induced depression, disease-induced depression, among others.
Depression: One disorder or many?
There is a remarkable variability in the symptoms of people with depression. Some people with depression suffer from anhedonia, while others do not. Some people with depression suffer from feelings of worthlessness, while others do not. This kind of intradiagnostic heterogeneity raises an important question: Is depression one disorder or many?
There is also remarkable variability in the common pathways to depression. Some people become depressed after traumatic events, while others do not. Some people become depressed during winter months, while others do not. This kind of etiological heterogeneity again raises the question of whether depression is one disorder or many.
No definitive answer to this question currently exists. It is therefore important that conceptualizations of depression accommodate the variability observed in people with depression while at the same time providing some understanding of what people with depression frequently have in common.
Depression: A psychological or biological phenomenon?
A number of alternative conceptualizations of depression currently exist. Some are based on theories and findings from psychological science. Others are based on theories and findings from biological science. Many researchers have called for more integrative conceptualizations of depression that incorporate finding from both psychological and biological science.
This is reflected in the National Institute of Mental Health’s recent Research Domain Criteria Initiative (RDoC), which is an effort to promote an interdisciplinary science of psychopathology that incorporates theories and findings from both psychological and biological science.
Below is an evolution-informed conceptualization of depression. It incorporates theories and findings from both psychological and biological science. It also accommodates the variability observed in people with depression while providing some understanding of what people with depression frequently have in common.
Applying evolutionary theory to depression
Evolved capacities
Species-typical mental and physiological processes are evolved capacities that control adaptive behaviors. From the perspective of evolutionary psychopathology, specific mental disorders can be understood as characteristic patterns of disturbance to the mental and physiological processes that control adaptive behaviors.
Depression
Depression is a disturbance to the mental and physiological processes that control adaptive behaviors characterized by some combination of the following: perceptual fixation, cognitive pessimism, emotional dysphoria, motivational depletion, behavioral inhibition, and/or physiological dysregulation.
Common features of depression
Perceptual fixation
People with depression tend to perceptually fixate on negative stimuli more than people without depression. For example, people with depression are more likely to visually fixate on faces with negative expressions than those with positive expressions.
Motivational depletion
People with depression tend to have reduced motivation compared to people without depression. For example, people with depression are more likely to struggle with the activities of daily living such as working, socializing, and self-care.
Cognitive pessimism
People with depression tend to have negative thoughts and beliefs more than people without depression. In particular, people with depression are more likely to have negative thoughts and beliefs about themselves and their future.
Behavioral inhibition
People with depression tend to be more inhibited than people without depression. For example, people with depression are more likely to be reluctant to try new things, including things they recognize may be beneficial for them.
Emotional dysphoria
People with depression tend to have negative moods and emotions more than people without depression. In addition to low moods, people with depression are more likely to experience negative emotions like sadness, anger, guilt, and shame.
Physiological dysregulation
People with depression tend to be physiologically dysregulated more than people without depression. For example, people with depression are more likely to have dysregulated appetite, sleep, energy, movement, or sexual interest.
The adaptive significance of depression
The principle of adaptive significance suggests that many clinical symptoms and syndromes commonly identified as psychopathology can be either adaptive or maladaptive, depending on the circumstances. Fear and anxiety, for example, can be adaptive when individuals engage in high-risk behaviors. Undoubtedly, many cases of depression are maladaptive. But is depression ever adaptive? If so, under what circumstances?
Researchers in evolutionary psychopathology have advanced a number of hypotheses about circumstances under which depression may be temporarily adaptive. Below, I summarize two such hypotheses. First, depression may be temporarily adaptive if it helps an individual to better manage finite resources. Second, depression may be temporarily adaptive if it helps a person to remain social integrated in a group after social conflicts.
Depression can help people to allocate their limited resources
The resource allocation hypothesis about depression is based partly on optimal foraging theory from ethology. Animal foraging can be analyzed in terms of both productivity and expenditure. Productivity is the yield of consumable plants or animals. Expenditure is the amount of resource depletion measured in time, energy, or material resources. Optimal foraging theory suggests that animals that maximize the ratio of productivity to expenditure tend to have better biological fitness, i.e. the likelihood of survival and reproduction.
The resource allocation hypothesis about depression suggests that, for ancestral humans, the valence of emotions (positive/negative) and moods (high/low) helped optimize foraging behaviors. Successful foraging sites and behaviors produced positive emotions and high moods, whereas unsuccessful foraging sites and behaviors produced negative emotions and low moods. In this way, the valence of emotions and moods helped ancestral humans to manage their limited resources.
The resource allocation hypothesis about depression suggests that negative emotions and low moods can help people to make better resource allocation decisions by directing them away from unfavorable settings and behaviors and toward more favorable ones. Simply put, negative emotions and low moods help people to make better decisions about how to spend their limited time, energy, and material resources.
Depression can help people to remain socially integrated after social conflicts
The social rank hypothesis about depression is also based partly on research from non-human animal behavior. Many species of social animals form social hierarchies, i.e. patterns of social rank. Social rank is the relative dominance or subordination of individuals living within the same social group. Social rank is mediated largely through dominance disputes, i.e. behaviors related to conflict between animals, including ritualized threat displays, physical aggression, and submission.
Social animals that lose a dominance dispute often display a collection of behaviors known as the involuntary defeat strategy (IDS). In non-human animals, the IDS includes submissive behaviors, reduced motivation, and behavioral inhibition. In humans, the IDS also includes low mood, lack of energy, and diminished self-worth. In short, the IDS induces symptoms of depression.
The social rank hypothesis about depression suggests that the loss of a dominance dispute often induces symptoms of depression that help people to adjust to the loss of social rank or to the failure to achieve higher social rank. The capacity to adjust to the loss of a dominance dispute helps people to remain integrated in a social group even when their social rank is diminished or when their efforts to gain greater social rank fail.
Evolution-informed treatment of depression
Evolutionary psychopathology encourages clinicians to consider the adaptive significance of depressive symptoms for the patients and clients they treat. Making an informed judgment about the adaptive significance of depressive symptoms requires the clinician to (1) consider whether an individual’s depressive symptoms might have been produced by selection processes in ancestral or developmental environments; and (2) consider the ways that an individual’s depressive symptoms might produce beneficial or detrimental effects in their current environment.
Evolutionary psychopathology can also help clinicians to select treatment goals and interventions for people with depression. By helping clinicians to focus their attention on the adaptive deficits of the person in treatment, evolutionary psychopathology can help make the treatment of depression more individualized. The evolution-informed conceptualization of depression described above suggests six potential treatment goals for people with depression, namely promoting perceptual flexibility, cognitive optimism, emotional euthymia, motivational induction, behavioral activation, and physiological regulation.
Treatment goals for people with depression
Goal 1: Perceptual flexibility
For people experiencing perceptual fixation on negative stimuli, an appropriate therapeutic goal is to promote perceptual flexibility. Perceptual flexibility can be promoted through attention bias modification treatment, which trains people to direct their attention to positive and rewarding stimuli.
Goal 4: Motivational induction
For people experiencing motivational depletion, an appropriate therapeutic goal is motivational induction, i.e. the stimulation of mental processes that initiate goal-directed behavior. Motivational induction can be promoted through motivational interviewing, acceptance and commitment therapy, as well as other currently available treatments.
Goal 2: Cognitive optimism
For people who have a pessimistic cognitive outlook, an appropriate therapeutic goal is to promote optimism. Optimism can be promoted through cognitive restructuring, an approach to cognitive therapy that identifies and modifies maladaptive thoughts and beliefs.
Goal 5: Behavioral activation
For people experiencing behavioral inhibition, an appropriate therapeutic goal is behavioral activation. Behavioral activation can be promoted through behavior activation therapy, which helps people in treatment to initiate health-promoting behaviors that naturally result in positive reinforcement.
Goal 3: Emotional euthymia
For people experiencing emotional dysphoria, an appropriate therapeutic goal is to promote emotional euthymia. Emotional euthymia can be promoted through emotion-focused therapy, dialectical behavior therapy, positive psychotherapy, as well as other currently available treatments.
Goal 6: Physiological regulation
For people experiencing physiological dysregulation, an appropriate treatment goal is to promote physiological regulation. Physiological regulation can be promoted through mindfulness, meditation, exercise, as well as other currently available treatments.
Environmental mismatch
Ancestral environments
Ancestral humans lived in environments where they had more exercise, more sleep, fewer calories, less inflammatory diets, less social competition, and more social integration.
Modern environments
People living in modern environments fare less well on all of those metrics, resulting in a variety of negative physical and mental health outcomes, including depression.
Reducing environmental mismatch can help reduce symptoms of depression
One approach to treating depression supported by research in evolutionary psychopathology is to try to reduce the discrepancies between modern environments and ancestral environments. A growing body of research suggests the significant physical and mental health benefits of living in ways that, in certain respects, more closely resemble our human ancestors.