EVOLUTIONARY THEORIES OF DEPRESSION

Social competition theory of depression (Price et al., 1994; Nesse and Williams, 1995) is based on the idea that depression is an evolved response to loss of status, or to an unsuccessful attempt to gain status. In response to such a loss, it might be adaptive to abandon the strategy you were previously using in your attempts at status enhancement. Similarly, perhaps you should change behaviours if your previous behaviours were tied to reproductive potential you have now lost.

According to the social competition theory, depression provides a marker that can be recognised by the brain which indicates when switching strategies to seek another niche is in order. If you are living in a small group, as we believe our ancestors did, switching strategies might well result in considerably greater success. Depressed mood is nature's way of telling you to accept that your current behaviour will not improve your reproductive lot and motivating you to try behaving differently. In the circumstances, you should evaluate your behaviour thoroughly, dwelling on the negative. You might also try to stay out of social situations altogether if you think you lack the resources to do well in them, and it has been found that "depressed individuals report being uncomfortable in interactions with others, often perceiving these interactions as unhelpful, or even as unpleasant or negative."(Gotlib, 1992,).

In modern societies, one's chance of excelling - of being the best at anything, or indeed anywhere near the best - are remote. If we have inherited a mechanism which is triggered when we believe ourselves to be outcompeted, then that mechanism will fire frequently as we are inundated with information about accomplished people.

If the mechanism is set off by the realisation that one is not even close to being the best at anything in the global village of the information age then getting depressed is not likely to be an effective reaction. Typically, there is no other strategy to adopt - no other niche one could fill - which would do significantly better than the present one in that global competition.

Also, the mechanism will frequently be set off even though its owner is actually doing very well in the local environment.

The social competition hypothesis for depression sees it as a formerly adaptive trait which causes problems in our current environment.

1) Write a list of the features of depression – then explain why each one might be viewed as adaptive in the EEA.

2) Explain the idea that depression is a ‘strategy switch’.

3) Why might depression be viewed as non-adaptive in modern times ?

WHAT ABOUT BI-POLAR DEPRESSION? (Why are depressives sometimes manic as well?)

RANK THEORY OF DEPRESSION(Stevens & Price)

In terms of conflict

Mania = winning subroutine

(status preservation)

Depression = yielding subroutine

(damage limitation)

Defection Theory of depression (Watson and Andrews, 1998; Hagen 1998) maintains that in the ancestral environment postpartum (i.e. postnatal) depression was an adaptive response which led women to limit their investment in the new child when, because of social, biological or environmental factors, a major investment in the infant would be likely to reduce the total number of offspring produced by that woman during her lifetime who would reach reproductive age and reproduce successfully.

Social conditions in the ancestral environment that would have been good cues for triggering a sharply reduced maternal investment would be insufficient investment from the father and / or other appropriate kin.

Biological cues would include problems with the pregnancy or birth, or other visible indications that the infant was not likely to be viable and healthy.

Environmental cues would include harsh winters, famine conditions and other indications that material resources would be inadequate.

In modern societies with elaborate support systems provided by the state and other organisations, it may be much less likely that these cues are reliable indicators that a mother who "defects" and sharply reduces her investment in her baby will increase her own reproductive fitness. But there is a growing body of evidence suggesting that these situations are indeed significantly correlated with postpartum depression. (Hagen). I.e. postpartum depression is another example of a condition produced by an adaptive mechanism that is functioning just as it was designed to function, though in an environment that is quite different from the one in which it evolved.

1) In what way does this theory also view depression as a ‘strategy switch’ ?

2) Why was post-natal depression adaptive in the EEA ?

3) Why might it not be adaptive in the modern world ?

Anxiety disorders

These provide another possible example of disorders that result from a mismatch between the contemporary environment and the environment in which our minds evolved.

Marks and Nesse (1994) point out that in the ancestral environment -

· fear of public places and fear of being far from home might well have been adaptive responses "that guard against the many dangers encountered outside the home range of any territorial species."

• fear of heights accompanied by "freezing instead of wild flight" would have had obvious adaptive value to our hunter-gatherer forebears.

They argued that emotions are response patterns that have been shaped by natural selection to offer a survival advantage in certain situations. Anxiety is one such emotion that helps an organism defend against a wide variety of threats. There are different sub-types of anxiety responses depending on the perception of the situation encountered e.g:

· High places induce freezing behaviours.

• Blood or injury produce fainting which reduces blood loss.
• Public places and being lost arouse mild fears that guard against the dangers encountered outside of the home range.
• Traumatic experiences produce excessive fear and avoidance behaviours.
• Social threats invoke submissive behaviours.
• Anxiety is beneficial only if it is regulated, natural selection has shaped our nervous system to make us attend to certain cues (prepotency or salience) and we are predisposed to learn certain reactions to specific stimuli (preparedness) i.e. snakes induce fear while bad food produces nausea.
• Stimuli that caused most danger to our ancestors can produce phobic behaviours in modern humans (e.g. snakes, spiders, heights, storms, darkness, blood, strangers, social threats, being isolated etc).
• We rarely develop fears to harmless stimuli (flowers, leaves, wood etc).
• It is also difficult to develop fear of evolutionary recent dangers (e.g. cars, knives, electric sockets, cigarettes, saturated fats). Our cognitive system that has built in domain-specific biases

These traits, like most traits, could be expected to show considerable phenotypic variation even in a population of individuals who are genotypically identical with respect to the relevant genes (the expression of genes varies according to the environment, including the individual's 'genetic' environment). People who are toward the sensitive end of these distributions -(those who become anxious more readily when far from home or when they find themselves in high places) might well have functioned quite normally in ancestral environments. In a modern urban environment, however, people who become extremely anxious when they are away from home or when they are in public places will find it all but impossible to lead a normal life.

People who become extremely anxious in high places will find it difficult or impossible to travel in airplanes, ride in glass enclosed elevators or work on the higher floors of modern buildings. Thus, because the modern environment is so different from the ancestral environment, people who are toward the sensitive end of the distribution of phenotypic variation may be incapable of coping with many ordinary situations despite the fact that all of their mental mechanisms are functioning in just the way that natural selection designed them to function.

1) What does ‘phenotypic variation’ mean ?

2) Summarise the evidence for the idea that our fears evolved in the EEA.

3) Why is it difficult to argue that phobias are learnt behaviours ?

4) Why are phobias non-adaptive in the modern world ?

 A02

Buss - no fear of cars, sockets, Seligman - genetic 'preparedness', Bennet et al. - fear of 'strange' not 'harmful', Daly & Wilson - threat of 'infanticide'

EXHB 2 - Evolutionary explanations of human mental disorders including depression

(e.g. unipolar and bipolar depression) and anxiety disorders.

Mental disorders appear to have a genetic components

- so must be a product of evolution ?

Classification	Neurotic disorder	Psychotic (affective) disorder
Disorder	ANXIETY DISORDERS	DEPRESSION
Detail	e.g. phobias, stress, obsessive-compulsive disorders	especially Manic ('bi-polar') depression -much more linked to genetics (Price)
Theory 1	GENETIC 'PREPAREDNESS' – nature prepares us to avoid danger. (Seligman) PHOBIAS ARE ADAPTIVE (Nesse) We fear ; Public places, far from home Heights Blood / injury Spiders. All useful things to fear in the EEA OUR PHOBIC REACTIONS ARE ADAPTIVE AS WELL Stand ‘frozen to the spot’ – we don’t draw attention to ourselves. Faint at sight of blood – fainting prevents further blood loss	SOCIAL COMPETITION THEORY -There was conflict for resources / mates -Therefore there were winners and losers -Losers need to CHANGE THEIR STRATEGY -Depression does this in three ways; WITHDRAWN – leave situation to fight another day LACK OF ENERGY – avoids danger of bad strategy INTROSPECTION – helps develop new strategy
AO2	Buss – phobias must be inborn, if we learnt them, we’d be afraid of cars not spiders.	Found in all societies as a response to 'losses' e.g. financial insecurity, death, redundancy (Stevens & Price) In modern day, we are constantly bombarded with successes of others by the media – can never be the ‘winner’ – therefore more likely to be depressed ? Depression no longer adaptive ? (genome lag)
Theory 2	GENOME LAG - Used to be adaptive - still around due to fact that evolution takes long time to make changes	DEFECTION THEORY OF DEPRESSION Postnatal depression was adaptive (put bluntly – it was a bad time to have a baby, so mother doesn’t care for it) Triggered by; Social cue – poor help from father Biological cue – unhealthy baby Environmental cue – harsh winter / famine
AO2		This is another STRATEGY SWITCH