Pszichológia | Tanulmányok, esszék » Genetic inheritance and intelligence, The nature nurture debate

Genetic Inheritance and Intelligence - The Nature-Nurture Debate Overview The issue of what causes individual differences in intelligence goes beyond psychology, and involves moral, political, ethical, educational, social, physiological and statistical issues to name just a few. The issue of how differences in intelligence come about between individuals and groups is a topic of much fascination and controversy - it can arouse strong reactions and elicit personal beliefs and biases. This document outlines some of the main psychological concepts and evidence that relates to explaining individual differences in intelligence. As a student, it is your responsibility to develop a familiarity with the basic arguments, strengths, and weaknesses for and against the causal influences and correlates of intelligence Nature vs Nurture - What do you think? In looking for the causes of individual differences in intelligence, a major issue is the relative contribution of genetics and environment.

100% Genetics (Nature) 90% 80% 70% 60% 50- 60% 70% 80% 90% Gen. Gen. Gen. Gen. 50 Env. Env. Env. Env. 100% Environ. (Nurture) Rate the extent to which you believe nature and nature influence (cause) human intelligence. Warning! As you learn more about the theory and research on genetic and environmental influences on human intelligence, you may find that you change some of your beliefs and assumptions. Pendulum of opinion on Nature vs. Nurture through history Over time with regard to the amount of influence that nature vs. nature has on human intelligence the zeitgeist (the intellectual and culture ‘flavour’ of a time and place) has swung back and forth. For example, in the late 1800s in the UK, as Darwinism took off, the role of genetically determined intelligence was considered very important. This was in contrast, for example, to the 1960s in the USA, when views were more in favour of a "tabula rasa" (blank state) view of human intelligence - in

other words, all people are capable of much more, if given conducive environmental conditions in which to reach their potential. Currently the thinking in the Western psychological world is somewhere in between both genetics and environment are seen as playing important roles. To be more precise, the modern view about nature vs nurture in intelligence is "interactionist". This view is well expressed by Ridley (1999); "Mother Nature has plainly not entrusted the determination of our intellectual capacities to the blind fate of a gene or genes; she gave us parents, learning, language, culture and education to program ourselves with." Late 19th century - early 20th century (Nature) From the mid to late 1800s through to the early 1900s opinions rested in the nature camp. This was consistent with the scientific discoveries of the role of inheritance and natural selection by Mendel and Darwin. The major contributor to the psychological argument was Francis Galton in his

book "Hereditary Genius: Its Laws and Consequences" (1869). Galton had observed that gifted individuals tended to come from families which had other gifted individuals. He went on to analyze biographical dictionaries and encyclopaedias, and became convinced that talent in science, the professions, and the arts, ran in families. Galton took this observation one step further, to argue that it would be "quite practicable to produce a high gifted race of men by judicious marriages during several consecutive generations". This suggestion became known as eugenics, "the study of the agencies under social control that may improve or repair the racial qualities of future generations, either physically or mentally." Galton wanted to speed up the process of natural selection, stating that: "What Nature does blindly, slowly, and ruthlessly, man may do providently, quickly, and kindly". Galton was convinced that "intelligence must be bred, not

trained". Such arguments have had massive social consequences and have been used to support apartheid policies, sterilization programs, and other acts of withholding basic human rights from minority groups. Post WWI: 1920s-1930s After World War I, careful re-analysis of the mass of intelligence test data took place. This began to challenge the commonly held view that intelligence was directly, genetically linked to racial differences: • e.g blacks from Illinois had higher IQ scores than whites from 9 southern states a finding difficult to reconcile with the simple idea that whites are intellectually superior to blacks. Evidence now seemed to support a closer link between social class and intelligence, rather than race and intelligence. As a result, a number of psychologists in the 1920s and 1930s shifted their position towards the environmental camp. The shift against nature views was given momentum by the backlash against the social consequences of government policies e.g

sterilization laws had been passed in 24 US States, resulting in 20,000 people being sterilized against their will. 320,000 people suffered the same fate in Germany. QUESTION: What is meant by sterilization and why would this have been carried out? What kinds of groups do you think were sterilized and on what basis? 1940s-1990s The backlash faded, and the pendulum swung back towards the middle. From the early 1940s, it seemed there was a rejection of simplistic nature or nurture views, with more common recognition of their complex interaction. Nevertheless, social prejudices and inequalities were still evident and growing. Thus, in the 1960s, the focus of the problem was shifted away from the individual as the cause of the problem, and centred on social determinants. The pendulum swung towards the nurture/environmental end and away from the nature/genetic end. Efforts were made to arrest poor educational achievement through special schooling, and to alleviate poor living conditions

through welfare. It became politically correct to minimize talk and discussion of the role of nature in contributing to any individual differences, let alone intelligence. The evidence of differences in intelligence between socioeconomic groups and racial groups, however, did not go away Recent Trends "The Bell Curve" Controversy From time to time, there have been inflammatory articles which present and interpret evidence of IQ differences between groups. The most recent, and most major of these publications was Herrnstein and Murrays (1994) "The Bell Curve". This book provided momentum to swing the pendulum in the direction of nature, at least in the publics eye, but even more so, it generated massive debate and controversy in psychology, sociology, education, and politics, not to mention the media and household. The 800+ page book, written for laypersons, hit the best-seller lists in the U.S The works main thesis is that an individuals intelligence - no less than

40% and no more than 80% of which is inherited genetically from his or her parents - has more effect than socioeconomic background on future life experiences. In addition to the premise that measured intelligence (IQ) is largely genetically inherited, a second important premise was that IQ is correlated positively with a variety of measures of socioeconomic success in society, such as a prestigious job, high annual income, and high educational attainment; and is inversely correlated with criminality and other measures of social failure. It was suggested that Socio-Economic Status (SES) successes (and failures) are largely genetically caused. Some sample controversial quotes from "The Bell Curve" • "IQ has more effect on future life experiences than SES" • "school intervention efforts to boost attainment in poorer families are largely a waste of time and money" • "increasing population of lower caste intelligences is lessening the nations

genetic capital" QUESTION: How do you feel about some of the claims being made in The Bell Curve"? Do you agree or disagree with any of them in particular? What might critics say about them? Reactions to The Bell Curve: • "The Bell Curve" re-ignited the nature-nurture debate. • The public debate was (and is) divided. o The politically left saw the authors as "un-American"; "pseudo-scientific racists"; and the book as "alien and repellent". o The politically right saw the authors as: "brave and respectable scholars,"; whose book was "lucid" and "powerfully written" The part of The Bell Curve that captured public attention was on the differences in IQ between African and Caucasian Americans. Further to this were the suggestions made by Herrnstein and Murray about the implications of a predominantly geneticallyinherited intelligence for government policy. Since IQ was largely seen as genetically

determined, the authors expressed resistance to educational and environmental interventions. They argued that money spent in this way is wasted The authors also argued that America is becoming a society of cognitive castes, with the lower caste including a large proportion of African-Americans. Hence their statement that the genetic capital of society is being eroded because the less intelligence, lower class is reproducing at a greater rate than high IQ classes. QUESTION: They make the claim that educational intervention is a waste of time. Do you agree? Does sending your child to a school like Haileybury make a difference to their intellectual ability? Evidence in Favour of "Nature" In the heyday of eugenic IQ testing in the 1920s there was no evidence for the heritability of IQ. It was just an assumption of the practitioners Today that is no longer the case. The heritability of IQ (whatever IQ is!) is now a hypothesis that has been tested - on twins and adoptees. The

results really are quite startling No study of the causes of intelligence has failed to find a certain and often substantial heritability. What varies from study to study is the amount that can be attributed to heritability. Concordance rates of IQ scores Evidence from family studies provides the main supporting evidence from which arguments about the relative roles of genetics and environment are constructed. Minnesota Twin Study - A large number of the study of twins reared apart was undertaken by Thomas Bouchard of the University of Minnesota starting in 1979. He collected pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies at the same time concentrated on comparing the IQs of adopted people with those of their adopted parents and their biological parents or their siblings. Put all these studies together, which include the IQ tests of tens of thousands of individuals, and the table looks like this: • Same

person tested twice 87% • Identical twins reared together 86% • Identical twins reared apart 76% • Fraternal twins reared together 55% • Biological siblings reared together 47% (studies show that reared apart about 24%) • Parents and children living together 40% • Parents and children living apart 31% • Adopted children living together 0% • Unrelated people living apart 0% QUESTION: Are there any other factors besides genes that could be responsible for these correlations? Meta-analysis of the heritability of intelligence A meta-analysis of 9 family studies was conducted by Daniels, Devlin and Roeder (1997): it included 212 correlations and produced very similar results to those quoted by Matt Ridley. These authors conclude that heritability can account for 48% of the variation in peoples IQ. The highest estimates have come from reviews of research by Herrnstein & Murray, 1994 (74%) and Eysenck (80%). A safer bet is probably to sit on the fence 50%!

Some Problems with the Genetic Evidence Heritability indices, however, are not pure measures of genetic inheritance - they are distorted because they include any effects of the prenatal environmental (e.g whether the mother smokes, what she eats, etc.) and the postnatal material environment eg how much a child is challenged or read to at a young age. Thus these heritability indices are likely to overestimate the role of genetics Twins reared apart are not assigned at random to foster or adoptive parents - since homes are selected on purpose to be similar to the childs natural family. This would partially account for the IQ correlations attributed to inheritance Twin studies may not be generalizable to the population at large as twins are more susceptible to prenatal trauma leading to retardation. The inclusion of retarded cases may increase the twin correlation in intelligence test scores. Evidence in Favour of "nurture" "Give me a dozen healthy infants & my own

specific world to bring them up in, & Ill guarantee to take any one at random & train him to become any type of specialist I might select - doctor, lawyer, artist, merchant, chef & yes, even beggar & thief, regardless of his talents, penchants, tendencies, abilities, vocations, and race of his ancestors." - John B. Watson, 1924 This was a famous quote in the heyday of behaviourism, when the child was considered to be a tabula rasa (blank slate) onto which anything could be sculpted through environmental experience. This would be a 100% environmental view, but virtually no psychologists would accept such an extreme position today. The Flynn effect: Are we getting smarter? In the 1980s, a New Zealand-based political scientist, James Flynn, noticed that IQ was increasing in all countries all the time, at an average rate of about 3 IQ points per decade i.e the average IQ across the world has risen over 1 standard deviation (ie 15 points) since WWII - predominantly one

assumes due to environmental effects. As a result, new norms continue to be used to rescale IQ tests to 100. QUESTION: Why might this be? How can you explain an increase in IQ over the last 100 years? Could this be due to diet? Possibly but IQ scores are still rising just as rapidly in wellnourished western countries. Could it be schooling? But interruptions to schooling only have temporary effects on IQ. Importantly, it is those tests that test abstract reasoning ability that show the steepest improvements. One researcher, Ulric Neisser suggests that the Flynn effect is due to the way we are being saturated with sophisticated visual images: ads, posters, videogame and TV graphics etc - rather than written messages. He suggests that children experience a much richer visual environment than in the past and that this assists them with visual puzzles of the kind that dominate IQ tests. All of this shows the powerful effect of the environment on IQ since none of this can be attributed

to genetic changes. Intelligence varies with at least 21 environmental factors Here are some of the other factors that have been found to vary to a significant extent in relation to IQ. All of these support the view that the environment is an important factor in IQ • • • • • • • • • • • • • • • • Infant malnutrition (-ve) Birth order Number of siblings (-ve) Number of years in school Social status of parental home Fathers profession Fathers economic status Degree of parental rigidity (-ve) Parental ambition Mothers education Average TV viewing (-ve) Average book-reading Self-confidence according to attitude scale measurement Age (negative relationship, applies only in adulthood) Degree of authority in parental home (-ve) Criminality (-ve) • • • Alcoholism (-ve) Mental disease (-ve) Emotional adaptation So, it would appear that there are many environmental, psychological and biological factors each contributing a small a small fraction to the

variance in IQ scores. QUESTION: Look at the environmental factors listed above. Explain why each might be important. Why might some people question that some of these are truly environmental? Could any be attributed to genes as well? Intelligence & race Herrnstein and Murray (1994) in The Bell Curve state that: 1. Asians and Asian-Americans have a significantly higher average IQ than white Americans, and that 2. White Americans have a significantly higher mean IQ than black Americans Further, they claim that this difference is not a function of cultural testing bias in the questions. Herrnstein and Murray (1994) acknowledge that the causes of these differences could be environmental; however the differences in IQ appear to be too large to be accounted for by environmental influences alone. They provide much qualification, cautioning, and warnings about how their evidence should be interpreted and used. In particular, they remind the reader that -IQ is not strongly linked to many

so-called desirable human qualities; and -The fallacy of drawing conclusions about an individual on the basis of group findings. It would be incorrect to characterize "The Bell Curve" as out-and-out a racist, eugenicist, etc. book Even detractors acknowledge the importance of its contribution to psychological and social debate. But the book does, in general, support a view that intelligence is largely heritable. The problem is differences in IQ scores between races do not necessarily imply genetic causes. Even if the tests are considered fair they are confounded by differences in socioeconomic status and education levels This is supported by the following evidence: 1. Children with black fathers, brought up in a white family show no evidence of lower IQs. 2. Black children adopted by white families show only slightly lower IQs than white adopted children and this could be accounted for by lower teacher expectations / mild racism (Howe, 1997). Some theorists suggest that

socioeconomic disadvantages are the main causes of ethnic differences in IQ. Even if the variation within a group reflects genetic differences, the average differences between the groups (black and white) could be wholly due to environmental factors. Imagine, two pots, with randomly allocated seeds from the same batch. The two plots have equivalent genetic potential. One plot received fertilizer (an environmental condition), the other pot receives no fertilizer. The average height (ie intelligence) differences between each pot will be due to environmental differences (fertilizer), however the height differences amongst individual plants within a pot are due to genetic differences (assuming similar conditions throughout the pot). QUESTION: Think about this analogy. How does this work for racial differences in intelligence? There is debate about whether heritability estimates even matter, since they cant be applied to an individual or be used to help people: "It does not matter

whether the field of human behaviour genetics finally decides that the heritability of IQ in the United States is 25%, 40%, 50%, or 70%. Any such estimate will be utterly useless to anyone seeking better ways to improve the intelligence of the nation through health care and education." Wahlsten (1997, p 84) In the US a large nationwide programme, the Head Start programme, aims to enrich the schooling of disadvantaged children. On the whole, the results have been mixed Head Start and programmes like it have been criticized for not living up to expectations in changing IQ. The main defence is that the primary aim is not to improve IQ, but to accelerate academic development - IQ change is a bonus, academic development is more important. Head Start type programmes have also been criticized for not causing lasting improvements. This is not surprising if children return to poor, unsupportive, deprived environments. And, in fact, such a finding supports the idea that IQ is changeable

by the environment - in both directions. New and better ways of educating, improving, and maximizing individual potentials in intelligence are likely to be developed. As this happens, more and more of the environmentally-influenced variation in IQ is likely to come under control and estimates of the environment proportion could increase. The role of "interaction" An under-researched area, while the nature vs. nurture debate has raged, is the interaction between genetics and environment. In the overfocus on nature vs nurture issues, in reality, "gene expression is environment dependent" and it impossible to obtain pure estimates of genetic vs. environmental contribution - one could not exist without the other. The environment a child experiences is partly a consequence of the childs genes as well as external factors. To some extent a person seeks out and creates his or her environment. If she is of a mechanical bent she practices mechanical skills; if a bookworm, she

seeks out books. Thus genes may create an appetite rather than an aptitude. Remember that the high heritability of short-sightedness is accounted for not just by the heritability of a gene for short sightedness but by the heritability of reading habits! What have we learnt about intelligence? So, what can we say about nature vs. nurture as causal determinants of intelligence? A conservative, seemly safe position is that: "In the field of intelligence, there are three facts about the transmission of intelligence that virtually everyone seems to accept 1. Both heredity and environment contribute to intelligence 2. Heredity and environment interact in various ways 3. Extremely poor as well as highly enriched environments can interfere with the realization of a persons intelligence, regardless of the persons heredity 4. Genes are poorly understood Genetic influence on intelligence is certainly not a single gene, but a complex combination of genes. 5. But likewise, it is difficult

to pin-down single, identifiable elements of the environment which directly influence IQ scores. Several environmental factors influence intelligence. QUESTION: Reflect upon what you have learned here. What is the most interesting thing that you would want to share with someone like your parents?