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Evolutionary Psychology Lecture 11: Health & Psychopathology.
Learning Outcomes. • At the end of this session you should be able to: • 1. Evaluate the adaptive benefits of pregnancy sickness. • 2. Discuss evolutionary explanations for certain pathological behaviours.
Levels of Explanation. • In the past medicine and psychiatry have attempted to explain physical and behavioural disorders, and to seek appropriate treatments for those disorders based on straightforward considerations of anatomy and physiology. • These are proximate explanations. • In contrast, the relatively new field of Darwinian medicine/psychiatry focuses upon: • The way the body is designed that make it vulnerable to certain disorders and ailments. • The operation of adaptations that may once have served a useful purpose but the operation of which may now result in maladaptive or pathological consequences in modern environments. • These are ultimate explanations.
1. Health & Illness • According to Nesse & Williams (1998) we can place evolutionary explanations for disease in several categories. • Of key importance is that some discomforting conditions (pain, fever, coughing, sneezing, diarrhoea, inflammation, nausea, anxiety etc), are not diseases or design faults. • Instead they are evolved defences aimed at prolonging the reproductive fitness (survival) of the individual. • E.g, a high temperature is assumed to reflect a dangerous side effect of invasion by pathogens, which should be reduced. • However, fever is a carefully regulated rise in the set point of the body's thermostat and a rise in temperature will facilitate pathogen destruction; reducing the temperature by drugs may actually prolong the illness.
Example: Pregnancy Sickness • Pregnancy sickness (or nausea and vomiting during pregnancy - NVP) is a collection of symptoms (food aversions, nausea, and vomiting) which occur during the first trimester (first 3 months) of pregnancy. • Many smells and tastes that women normally find palatable become intolerable to them; and these smells or tastes can trigger nausea and vomiting. • This condition has been viewed by the standard medical account as an uncomfortable by-product of the hormonal alterations of pregnancy. • From an evolutionary perspective, one can however focus on the possible defence mechanisms and thus the adaptive benefits that NVP might serve (Profet, 1992).
Benefits of NVP? • It is at first difficult to see how feeling ill during pregnancy can benefit the mother or her embryo. • However, Klebanoff et al., (1985) found that women who experienced severe nausea and vomiting in the first trimester had much lower risks of having a miscarriage. From Flaxman & Sherman, 2000)
Toxin Avoidance. • Plants, vegetables and fruits manufacture toxins to poison their predators. • Such toxins exist in plants normally considered harmless and which form an integral part of a healthy diet e.g. apples, bananas, celery, cabbage, brussel sprouts, parsnips, cherries, oranges. • Many plant toxins smell or taste bitter or pungent (e.g. onions), and animals have developed chemoreceptors in the nose and tongue to detect these toxins. • The levels of these toxins are very low, and will only affect adults in large doses, however small doses are very dangerous to embryos. • E.g Potato’s contain high levels of toxins which can cause neural tube defects in the embryos of mammals (Renwick et al., 1984).
Pregnancy Sickness as An Adaptation. • Pregnancy sickness coincides with organogenesis - the embryonic period of maximum vulnerability to teratogens (environmental poisons), 20-56 days after conception. • Pregnancy sickness begins 2-4 weeks after conception, peaks at 6-8 weeks, and falls off after 8 weeks, (the end of organogenesis). • It is not present in the first 2 weeks as the embryo has not yet formed a placenta. • Sickness is unnecessary after organogenesis as the foetus is much less susceptible to teratogens. • The nutritional costs of sickness are low because ovulation and conception does not occur unless the female has accumulated a certain threshold of fat reserves (Frisch, 1987).
Why Create an Aversion to Vegetables? • It seems odd that pregnancy sickness should cause an aversion to vegetables which are rich in folates and other vitamins essential to normal embryonic development. • However, our ancestors would not have been vitamin deficient at conception because much of their calorific intake came from vegetables and fruits. • Folates are stored in the liver for 4 months and so would have covered the temporary deficiencies experienced during pregnancy sickness. • Modern women are low in folates and are thus particularly susceptible to folate deficiency (Eaton & Konner, 1985).
Olfactory Cues. • Pregnancy sickness heightens smell and taste sensitivity. • This induces food aversions and vomiting with little provocation. • Vegetable toxins produce pungent aromas and so provide good clues about their possible dangers. • Non-plant foods also produce olfactory cues, e.g pungency in meat and dairy products is a sign of parasitisation by bacteria. • The best-tolerated foods should be those which are bland and do not spoil easily (bread, cereals etc). Studies analysing food aversions show that women have the strongest aversions to tea, coffee, vegetables and meat and have little or no aversions to bread and cereals (Flaxman & Sherman, 2000).
Evolutionary Novel Toxins. • However, evolutionary novel toxins lack such perceptual triggers and fail to elicit the symptoms of pregnancy sickness. • Many modern drugs are derivatives of plant toxins and are very potent; however they are typically delivered in pills and injections which do not give off volatile odours or tastes. • Studies of food aversion show that strong alcohol aversion is much less pronounced than are strong food aversions, as the main component of alcohol (ethanol) is a non-bitter chemical. • Alcoholic drinks containing bitter-tasting plant constituents (e.g. whiskey, wine) are more aversive in pregnancy than ones with no such extracts (e.g. vodka).
Flaxman & Sherman (2000). • They reviewed the theory that pregnancy sickness acts as an adaptation, and confirmed that: • Symptoms peak when embryonic organogenesis is most susceptible to chemical disruption. • Women who experience morning sickness are significantly less likely to miscarry. • Women who vomit suffer fewer miscarriages than those who experience nausea alone. • Many pregnant women have aversions to alcoholic and caffeinated drinks, and strong-tasting vegetables, though the greatest aversions are to meat, fish, poultry and eggs. This is because animal products are especially dangerous as they contain pathogens and parasites.
Food Aversions and Cravings. Meat N-A drinks Veg Alcohol Spicy Dairy Sweets Grains Fruits From Flaxman & Sherman, 2000 p 127.
Psychopathology. • Crawford & Salmon (2002) hold two views of psychopathology: • 1. It is a neurodevelopmental disorder (physical cause). • 2. It could represent the activation of strategies that may once have been adaptive but which now are maladaptive. • Nesse & Willams (1995) noted that certain pathologies may be perpetuated by genetic mechanisms, because: • Their effects were beneficial in ancestral environments (e.g. preference for high calorie foods - which now leads to obesity and illness). • Genes that influenced disease expression may also have had benefits in certain environments (e.g. sickle-cell anaemia and malaria protection). • Such explanations might also be true for psychopathological conditions.
Example: Psychopathy • Psychcopaths are rare but cause many problems. • Most are male, manipulative, deceitful, impulsive, charismatic, charming, aggressive, promiscuous, and show little guilt for their antisocial behaviours. • They are impatient, unable to control their impulses and are less affected by the consequences of their actions (Howard et al., 1997). • They do not react in a normal physiological manner to cues of distress or to aversive stimuli (Patrick, 1994).
Psychopathy as an Adaptation. • Lalumière et al., (2001) point out that psychopathy has traditionally been viewed as a brain pathology stemming from a disturbance in the developmental process.They provide an alternative explanation. • The behavioural, emotional and cognitive impairments displayed by psychopaths may represent a set of adaptive features that would have once thrived during human evolutionary history. • Harpending & Sobus (1987) used game research to show that cheaters could thrive under certain conditions: • When they are difficult to detect. • When they are highly mobile. • If they are highly persuasive. • If they are very attractive (physically or possess charisma).
Psychopathy and Developmental Instability? • Lalumière et al., (2001) noted that if psychopathy is the result of an early maladaptive development, than psychopaths should display markers of developmental perturbations. • They assessed prevalence of psychopathy and number of reported obstetric problems in 800 violent male offenders. • Those scoring highest on the psychopathy checklist were less likely to have experienced developmental perturbations.
Psychopathy and Developmental Instability? • In a second study they measured fluctuating asymmetry in psychopathic and non-psychopathic offenders and a control group of non-offenders. • FA was indeed lower in the non-offenders and was highest in the non-psychopathic offenders, the psychopathic offenders fell in-between these groups. • The authors argued that their research showed some support for the idea that psychopathy is not caused by developmental disorder.
Anorexia. • Standard explanations of eating disorders revolve around proximate explanations, e.g. cultural pressures, neurochemical disturbances, genetic predisposition's. • These factors are undoubtedly important yet do not address why females suffer disproportionately from eating disorders? • Evolutionary explanations attempt to take a more ultimate view to take account of the possible adaptive nature of such behaviours and there are several models:
a) Reproductive Suppression Hypothesis (RSH). • Biologists have noted that female mammals are able to suppress their reproductive capability when environmental conditions are poor. • In modern industrial societies adolescent females eat high calorie foods, enter menarche much earlier, and thus spend a longer time before marriage and children than previous generations. • During this time young girls may receive attention from many males so sexual activity is difficult to avoid, and is costly if it results in fertilisation. • Voland & Voland (1989) suggested that anorexia nervosa is an emergency strategy that suppresses reproduction achieved through reducing critical fat mass below that at which ovulation is possible.
b) Sexual Competition Model. • Abed (1998) pointed out the following: • Female body shape and relative thinness is (and was) an honest marker of age and reproductive potential. • A specific body size/shape is attractive to males and serves as a competitive signal to other females. • Humans have the cognitive capacity to recognise their own mate value and act accordingly to change their appearance. • Abed (1998) hypothesised that eating disorders are an example of 'runaway' intra-sexual competition. • Sexual selection would favour females who not only possessed a nubile shape but who also possessed the psychological adaptations that enabled them to recognise that this attractive body shape was under threat (i.e. by obesity or age).
Sexual Competition Model continued • It would thus be adaptive for females to monitor their own body size/shape and constantly compare this with surrounding females. • If (as in modern Western societies) women are increasingly surrounded by images (or real-life) young, thin, nubile females, then this would produce a drive for relative thinness in order to improve ones mate value. This may manifest itself in different ways: • In younger females anorexia nervosa is an attempt to engage in intra-sexual competition where the nubile female shape is set at an abnormally thin level. • Amongst older females, bulimia nervosa should predominate, as it is an attempt to reactivate their once nubile shape in an attempt to maintain mate value.
Non-Pathological Outcomes. • Natural selection would have used powerful emotions such as guilt, depression and anxiety to ensure that a redeeming strategy is conducted. • These emotions would generate a feeling of dissatisfaction with one's own body size and shape and motivate the individual to address this. • Many women are concerned with their weight, are on (or have been on) diets, exercise regularly, are aware of calorie-contents of foods etc.
c) Perceived Social Rank. • Treasure & Owen (1996) noted that sheep, goats and pigs may all display eating disorders when healthy and where food is plentiful. • In these cases the disorder appears to have been triggered by defeat in competition for social rank. • Such defeat is accompanied by submissive behaviours in order to avoid being attacked, where an animal cannot escape dominating individuals this situation leads to stress-related disease and depression. • Human societies also characterised by social ranking in competition for resources. • Depression may be an evolved manifestation of submissive behaviours when there is no opportunity to escape a dominating situation (Gilbert, 1992).
Test of the Hypothesis. • Troop et al., (2003) reviewed a series of studies showing that indicators of low social rank (feelings of inadequacy, low self-esteem, lack of control, lack of assertiveness etc) are common in patients with eating disorders. • They then explored perceived social comparison and submissive behaviour in women and men with eating disorders. • Patients displayed significantly lower social comparison scores and significantly higher submissiveness scores than matched controls. • The authors argue that patients with eating disorders do compare themselves less favourably with others and also report themselves to be more submissive than others in everyday life.