life event, such as those that have been implicated in depression (e.g., death, divorce), "cheap" signals of need, such as crying, might not be believed when social partners have conflicts of interest. the symptoms of major depression, such as loss of interest in virtually all activities and suicidality, are inherently costly, but, as costly signaling theory requires, the costs differ for individuals in different states. for individuals who are not genuinely in need, the fitness cost of major depression is very high because it threatens the flow of fitness benefits. for individuals who are in genuine need, however, the fitness cost of major depression is low, because the individual is not generating many fitness benefits. thus, only an individual in genuine need can afford to have major depression. major depression therefore serves as an honest, or credible, signal of need. for example, individuals suffering a severe loss such as the death of a spouse are often in need of help and assistance from others. such individuals who have few conflicts with their social partners are predicted to experience grief—a means, in part, to signal need to others. such individuals who have many conflicts with their social partners, in contrast, are predicted to experience depression—a means, in part, to credibly signal need to others who might be skeptical that the need is genuine. bargaining theory depression is not only costly to the affected person, it also imposes a significant burden on family, friends, and society at large—yet another reason it is thought to be pathological. yet if people with depression have real but unmet needs, they might have to provide an incentive to others to address those needs. the bargaining theory of depression is similar to the honest signaling, niche change, and social navigation theories of depression described below. it draws on theories of labor strikes developed by economists to basically add one additional element to honest signaling theory: the fitness of social partners is generally correlated. when a wife has depression and reduces her investment in offspring, for example, the husband's fitness is also put at risk. thus, not only do the symptoms of major depression serve as costly and therefore honest signals of need, they also compel reluctant social partners to respond to that need in order to prevent their own fitness from being reduced. this explanation for depression has been challenged. depression decreases the joint product of the family or group as the husband or helper only partially compensates for the loss of productivity by the depressed person. instead of being depressed the person could break their own leg and gain help from the social group, but this obviously is a counterproductive strategy. and the lack of a sex drive certainly does not improve marital relations or fitness. social navigation or niche change theory the social navigation or niche change hypothesis proposes that depression is a social navigation adaptation of last resort, designed especially to help individuals overcome costly, complex contractual constraints on their social niche. the hypothesis combines the analytical rumination and bargaining hypotheses and suggests that depression, operationally defined as a combination of prolonged anhedonia and psychomotor retardation or agitation, provides a focused sober perspective on socially imposed constraints hindering a person's pursuit of major fitness enhancing projects. simultaneously, publicly displayed symptoms, which reduce the depressive's ability to conduct basic life activities, serve as a social signal of need; the signal's costliness for the depressive certifies its honesty. finally, for social partners who find it uneconomical to respond helpfully to an honest signal of need, the same depressive symptoms also have the potential to extort relevant concessions and compromises. depression's extortionary power comes from the fact that it slows the flow of just those goods and services such partners have come to expect from the depressive under status quo socioeconomic arrangements. thus depression may be a social adaptation especially useful in motivating a variety of social partners, all at once, to help the depressive initiate major fitness-enhancing changes in their socioeconomic life. there are diverse circumstances under which this may become necessary in human social life, ranging from loss of rank or a key social ally which makes the current social niche uneconomic to having a set of creative new ideas about how to make a livelihood which begs for a new niche. the social navigation hypothesis emphasizes that an individual can become tightly ensnared in an overly restrictive matrix of social exchange contracts, and that this situation sometimes necessitates a radical contractual upheaval that is beyond conventional methods of negotiation. regarding the treatment of depression, this hypothesis calls into question any assumptions by the clinician that the typical cause of depression is related to maladaptive perverted thinking processes or other purely endogenous sources. the social navigation hypothesis calls instead for analysis of the depressive's talents and dreams, identification of relevant social constraints (especially those with a relatively diffuse non-point source within the social network of the depressive), and practical social problem-solving therapy designed to relax those constraints enough to allow the depressive to move forward with their life under an improved set of social contracts. this theory has been the subject of criticism. depression as an incentive device this approach argues that being in a depressed state is not adaptive (indeed quite the opposite), but the threat of depression for bad outcomes and the promise of pleasure for good outcomes are adaptive because they motivate the individual toward undertaking effort that increase fitness. the reason for not relying on pleasure alone as an incentive device is because happiness is costly in terms of fitness as the individual becomes less cautious. this is most readily seen when an individual is manic and undertakes very risky behavior. the physiological manifestation of the incentives are most noticeable when an individual is bipolar with bouts of extreme elation and extreme depression as anxiety which is about the (possibly immediate) future is highly correlated with being bipolar. as noted earlier, bipolar disorder and clinical depression, as opposed to event depression, are viewed as dysregulation just as persistently high (or low) blood pressure are viewed as dysregulation even though at times high or low blood pressure is fitness enhancing. prevention of infection it has been hypothesized that depression is an evolutionary adaptation because it helps prevent infection in both the affected individual and his/her kin. first, the associated symptoms of depression, such as inactivity and lethargy, encourage the affected individual to rest. energy conserved through such methods is highly crucial, as immune activation against infections is relatively costly; there must be, for instance, a 10% increase in metabolic activity for even a 1°c change in body temperature. therefore, depression allows one to conserve and allocate energy to the immune system more efficiently. depression further prevents infection by discouraging social interactions and activities that may result in exchange of infections. for example, the loss of interest discourages one from engaging in sexual activity, which, in turn, prevents the exchange of sexually transmitted diseases. similarly, depressed mothers may interact less with their children, reducing the probability of the mother infecting her kin. lastly, the lack of appetite associated with depression may also reduce exposure to food-borne parasites. however, it should also be noted that chronic illness itself may be involved in causing depression. in animal models, the prolonged overreaction of the immune system, in response to the strain of chronic disease, results in an increased production of cytokines (a diverse group of hormonal regulators and signaling molecules). cytokines interact with neurotransmitter systems—mainly norepinephrine, dopamine, and serotonin, and induce depressive characteristics. the onset of depression may help an individual recover from their illness by allowing them a more reserved, safe and energetically efficient lifestyle. the overproduction of these cytokines, beyond optimal levels due to the repeated demands of dealing with a chronic disease, may result in clinical depression and its accompanying behavioral manifestations that promote extreme energy reservation. the third ventricle hypothesis third ventricle the third ventricle hypothesis of depression proposes that the behavioural cluster associated with depression (hunched posture, avoidance of eye contact, reduced appetites for food and sex plus social withdrawal and sleep disturbance) serves to reduce an individual's attack-provoking stimuli within the context of a chronically hostile social environment. it further proposes that this response is mediated by the acute release of an unknown inflammatory agent (probably cytokine) into the third ventricular space. in support of this suggestion, imaging studies reveal that the third ventricle is enlarged in depressives. reception clinical psychology and psychiatry have historically been relatively isolated from the field of evolutionary psychology. some psychiatrists raise the concern that evolutionary psychologists seek to explain hidden adaptive advantages without engaging the rigorous empirical testing required to back up such claims. while there is strong research to suggest a genetic link to bipolar disorder and schizophrenia, there is significant debate within clinical psychology about the relative influence and the mediating role of cultural or environmental factors. for example, epidemiological research suggests that different cultural groups may have divergent rates of diagnosis, symptomatology, and expression of mental illnesses. there has also been increasing acknowledgment of culture-bound disorders, which may be viewed as an argument for an environmental versus genetic psychological adaptation. while certain mental disorders may have psychological traits that can be explained as 'adaptive' on an evolutionary scale, these disorders cause individuals significant emotional and psychological distress and negatively influence the stability of interpersonal relationships and day-to-day adaptive functioning.