Unit #8 – Twin Studies and IQ 8.1 Twin studies which occurred during the decades following World War II and the exposure of Nazi atrocities contributed to a rebalancing of the nature-nurture debate in favour of nurture. By the 1960s, the Standard Social Science Model (SSSM) had achieved near hegemonic status with respect to ideas about how human psychological and social traits develop. Notwithstanding the growing stature of the SSSM, there were a few areas of social and psychological research underway in the post-World War II years that suggested we would be making a mistake if we completely discounted the importance of genetics in shaping human social and psychological traits.

The results of so-called “twin studies” presented one of the more significant challenges to the nurture side of the nature-nurture controversy during the first few decades after World War II. One of the most prominent efforts in twin research is a set of studies referred to collectively as the Minnesota Twin Family Study. This study examined approximately 8,000 pairs of twins, beginning with the study of twins born in Minnesota from 1935-1961.

Twin studies purportedly allow researchers to test ideas about the degree to which human personality and intellect are shaped by genetic inheritance and/or the environment. These studies enabled researchers to get around the methodological challenges confronting scholars such as John B. Watson, who could only hypothesize what might happen were he to raise a group of children in a highly controlled environment. The research ethics of the mid-20th century prevented researchers in most advanced countries from exercising the degree of control over human subjects that would be required to test Watson’s hypothesis. (Although the ethics of the early 20th century did not prevent Watson from conducting his Little Albert experiment.)

Twin researchers proposed that identical (or monozygotic twins) could be assumed to share virtually all their genes. They also claimed that identical twins tend to be reared in a very similar environment. They typically grow up in the same household at the same time and would have the same parents and siblings. By way of comparison, fraternal/sororal (or dizygotic twins) share only 50% of their genetic inheritance, the same amount as any full brother or sister. However, like identical twins, they tend to grow up in a very similar environment to each other; especially if they are the same sex.

Researchers speculated that if the personality and intelligence traits of identical twins were more similar than those for fraternal twins, it would suggest that genes must play a role in fostering such traitst. After all, twins raised in the same household had a very similar socialization experience. In other words, the researchers assumed they had found a way to control for environmental differences. If, for example, they discovered that a pair of same

sex identical twins were more alike with respect to certain psychological and personality traits, such as intelligence, than were pairs of fraternal twins, it would suggest that nature (or genetic inheritance) plays a role in the acquisition of those traits.

Also intriguing to researchers was the ability to examine identical twins who had been separated at birth and adopted into different households (a practice that many adoption agencies do not support today). The researchers proposed that a set of identical twins separated by adoption would still share the same genetic code but would grow up in different household environments. If they discovered that identical twins reared apart were more alike in terms of personality and intelligence than fraternal twins or other siblings separated by adoption, or that they were different from outcomes for both types of twins who were not adopted, they could potentially identify the influence of genes relative to environmental influences.

While psychologists can employ a variety of personality assessment tools to compare individuals, IQ testing became one of the more popular ways to compare twins. IQ testing supposedly offered a well-known empirical instrument that could not be influenced by factors such as the skill of the examiner – which might be the case when administering a personality inventory. Students will recall that IQ testing was also viewed as a handy and reliable measure of intelligence by eugenicists. In several jurisdictions during the 20th century a low score on an IQ test could result in eugenic sterilization.

There is significant controversy today about the real value of IQ tests and whether they tell us what they purport to say with respect to measuring intellect. Critics of the tests contend that they are culturally biased and fail to account for the influences of different forms of nurturing and socialization on people’s performance on the tests. The reading by Malcolm Gladwell, “None of the Above: What IQ doesn’t tell us about race,” assigned for this unit, presents some of the academic criticism of the value of IQ testing. In particular, it debunks the social Darwinian idea that because some racial and ethnic groups perform less well than others that they are genetically and intellectually inferior to those groups who perform better on the tests. We will discuss Gladwell’s article again later in this unit.

Notwithstanding the various criticisms of IQ tests, they seem to allow for an apples to apples comparison between siblings with different levels of genetic inheritance. If, for example, identical twins reared apart have IQ scores which are on average more similar than the scores for fraternal twins raised apart (who are ½ genetically similar) something is probably at work to cause the degree of similarity and it is probably genetics.

8.2 The heritability statistic

Researchers employ what is referred to as the “heritability statistic” to illustrate the degree to which genetics plays a role in fostering a trait such as intelligence. The heritability statistic is a mathematical measure. It is in the same family of empirical measurement as an average or standard deviation which help us make sense and use of data. The early work on the heritability statistic was done by Francis Galton, who we have already discussed in relation to his study of “hereditary eminence” and his coining of the term eugenics.

The heritability statistic can be used to compare the differences in IQs between people with different degrees of genetic relationship. We might compare the IQs of identical twins raised together, who have a 100% genetic relationship to those of fraternal twins raised together who have a 50% genetic relationship. We could compare a variety of genetically identifiable groups such as full siblings, half siblings, mothers and their children, etc.

Table 8.1 provided below presents a number of statistics that identify genetic groups and the degrees of similarity in their IQ scores. These figures were presented by Rutherford (2011:119) and are based on a meta analysis of correlation coefficients for heritability conducted by Bouchard and McGue in 1981.

Correlation coefficients might be thought of as a measure of the degree of similarity that exists between members of a group for a particular trait. If, for example, members of a group were exactly alike with respect to a given trait the correlation coefficient would be close to 100% (which is expressed as 1.0). Another factor contributing to our use of the heritability statistic is genetic relatedness. The table indicates that identical twins have a genetic relationship of 1.0 or 100%. They are close to genetically identical. This relationship is represented in the table by the letter “k” for kinship. The right hand column in the table presents the correlation coefficients for IQ test scores of people in the various kinship categories such as identical twins. The correlation coefficient is represented by the letter “r.” If on average identical twins raised together scored exactly same as their co-twins on IQ tests, this number would also be 1.0 or 100%. However, we see from the table that the number is only 0.86. This means that for the population of identical twins raised together their IQs are 86% similar – they are highly similar but they are not the same. Furthermore, they are more similar than are the IQs of groups which are less genetically similar.

Table 8.1 IQ Correlations

Genetic group k – kinship r – correlation coefficient

Identical twins raised together 1.0 0.86

Identical twins raised apart 1.0 0.72

Fraternal twins raised together 0.5 0.6

Fraternal twins raised apart 0.5 0.52

Siblings raised together 0.5 0.47

Siblings raised apart 0.5 0.24

Biological parent and child living together 0.5 0.42

Biological parent and child living apart (adoption)

0.5 0.22

Half siblings raised together 0.25 0.31

Unrelated siblings 0.0 0.34

Calculating the heritability statistic

The next step in calculating the heritability statistic is to plug the numbers from two of the groups identified in the table into the mathematical equation provided below. For example, we can compare the degree of similarity in IQ correlation coefficients (represented by the letter “r”) between identical twins raised together and fraternal twins raised together.

Heritability statistic comparing identical twins raised together and fraternal twins raised together.

H = the heritability statistic (number)

r = correlation coefficient

H = (r identical twins raised together – r fraternal twins raised together) x 2

H = (0.86 – 0.6) x 2

H = 0.26 x 2

H = 0.52

Based on the above calculations we have determined that the heritability statistic, “H,” that compares IQs for identical twins raised together with IQs for fraternal twins raised together is 0.52. This means that 0.52 of the differences in IQ between identical twins raised together and fraternal twins raised together is probably attributable to genetics as opposed to environmental differences.

However,, when we apply the heritability statistic calculation to the data for other groups in the table we do not obtain exactly 52%. When we calculate the H (heritability statistic) for siblings raised together and siblings raised apart.

Heritability statistic calculation for IQ comparing siblings raised together and siblings raised apart.

H = (r siblings raised together – r siblings raised apart) x 2

H = (0.47 – 0.24) x 2

H = 0.23 x 2

H = 0.46

We find the figure for siblings raised together and apart is only 0.46 – or 46%. One might reasonably maintain that 52 and 46 are similar enough to strongly suggest that nature and nurture are close to equally involved in determining IQ. But we still need to be careful about assuming this is a universally applicable claim. consider the example provided in the next paragraph.

Suppose, for example, we have a pair of identical twins (twin A and twin B) who are separated at birth and adopted by different families. Let’s further assume that the parents of twin A do not do a good job of keeping their stairways clear of objects, e.g. toys, laundry baskets, etc. Twin A trips on the stairs resulting in traumatic brain injury that produces a significant cognitive deficit. Twin B on the other hand has a relatively normal childhood. Further assume that we give each twin an IQ test and find that twin B’s IQ is 105 ( a little above normal) and twin A’s IQ is 50, a very low score. Can we assume that the difference between the IQs of twins A and B is approximately 50% due to genetic factors, as is the

average case identified by the heritability statistic? We clearly cannot attribute the cluttered staircase to the genetics of twin A. In this case environment clearly plays an important role in the differences in IQ between the two twins. The relative role of genetics would be much lower than 50.0

One of the odd, or counter intuitive features of the heritability statistic is demonstrated by what might be referred to as the “five fingers example.” Suppose one wanted to calculate the heritability of the trait of having five fingers (one thumb and four fingers) on each hand by comparing five fingeredness among identical twins raised together and fraternal twins raised together. With very rare exceptions we can reasonably assume that virtually all identical twins raised together and identical twins raised apart will be born with five fingers on each hand. Exceptions to this are close to statistically irrelevant. It seems logical to assume that the tendency to have five fingers on each of our hands is inherited virtually all the time – close to 100% inherited. Let’s see what happens when we plug the “r” figure of 1.0 (representing a 100% correlation coefficient) for five fingeredness into the calculation.

Heritability statistic calculation for five fingeredness for identical twins raised together and fraternal twins raised together.

H = (r for five fingeredness in identical twins – r for five fingeredness in fraternal twins) x2

H = (1.0 – 1.0) x 2

H = 0 x 2

H = 0

In this example we see that the heritability statistic is not exactly a precise measure of the degree to which a trait is genetically inherited as opposed to environmentally generated. If that is what it actually meant the statistic would tell us we have a 0.0 chance of passing on the five fingers trait to our offspring. Clearly, when commentators in the media and modern day social Darwinists attempt to emphasize the power of genetic inheritance over environmental influences by making use of heritability statistics – we need to evaluate their claims with a grain of salt. However, one should not dismiss the use of the statistic out of hand. Regardless of what we might think of IQ tests, the results from twin studies demonstrate that genetics do indeed play a prominent role in the development of whatever skills or abilities might be measured by the tests.

8.3 Is the glass half full or half empty?

Glass half full. Wikipedia.

https://upload.wikimedia.org/wikipedia/commons/a/a4/Glass_Half_Full_bw_1.JPG
Notwithstanding the reservations just described with regard to the heritability statistic, we are nonetheless confronted by data that suggests genetic inheritance plays an important role in shaping intelligence or whatever it is that is measured by IQ tests. The fact that the IQs of identical twins are more similar than the IQs of fraternal twins is strong evidence in support of the contention that genes have a powerful effect.

Unfortunately, modern day social Darwinists have employed the data from twin studies to provide scientific support for contentions about the superiority and inferiority of various ethnic groups based on differences in IQ scores between groups. The argument goes like this, if IQ is strongly influenced by genetic factors and some ethnic groups perform less well on IQ tests than other ethnic groups, it is potentially due to some groups having superior genes for intelligence than others.

Judith Rich Harris (2009) proposes an interesting counter argument to those who assume that twin studies have provided the narture side of the debate with the ammunition required to win the argument. Harris suggests that rather than focusing on the similarities between twins identified by correlation coefficients for traits such as IQ (e.g. .86 for IQ for identical twins raised together), we should place greater emphasis on the fact that there is any significant difference at all between people who are genetically identical. Harris states that we should make much more of the amazing fact that there are such large difference between the IQs of identical twins which are due to environmental factors. Harris suggests that people who support the notion that primary socialization is important in shaping personality traits and traits such as intelligence should take more of a glass half-full approach to the results of twin studies. Rather than allow the nature side of the debate to claim a victory they should take comfort from the fact that the data does not indicate that nurture is unimportant.

But the flip side of the coin is seldom mentioned. The flip side of the coin is that identical twins reared in the same home are not nearly as alike as you would expect them to be. Given how similar the reared-apart twins are, you would probably think that the reared together ones must be as alike as two copies of your annual Christmas letter… Here are two people raised in the same home with the same parents, and yet they do not have the same personality [or IQ].

(Harris 2009: 31, 32)

Harris makes the point that even the subtle differences that exist in the nurturing environments shared by identical twins are enough to cause them to grow up to be different across a range of personality measures and IQ. This suggests that despite the role that genes might contribute to traits such as intelligence, we would be making a huge mistake if we failed to recognize the powerful effects of nurturing environments.

8.4 Moore’s Criticism

Happy Twin Girls flickr photo by donnierayjones shared under a Creative Commons (BY) license

In his book, The Dependent Gene, David Moore, also discounts the value that proponents of the nature side of the debate place on twin studies. As a starting point, Moore contends that there is probably no such thing as truly genetically identical twins. Moore notes that there are conditions during gestation in the womb which can have subtle effects on the genotypes twins (Moore 2003: 49-51). There appears to be evidence for this claim in modern genetic testing that shows it is unlikely to find a pair of twins or clones that do not have a few differences in the arrangement of nucleotides in the three billion or so base pairs that make up our genetic codes. Other observers have noted that cloning experiments on animals have sometimes failed to produce viable offspring capable of living to the ages of the parents from who they were cloned. Even when a clone is constructed from the supposedly identical DNA of its parent there are things that seem to go wrong.

Perhaps more relevant to our criticism of the way twin studies are interpreted are Moore’s observations about the actual environmental conditions experienced by twins reared apart. For example, back in the day when much of the early twin study data was collected, many identical twins put up for adoption were adopted by other family members. If let’s say the parents of twins were killed in a car accident, one twin might be adopted by the family of an uncle and the other to the family or an aunt or grandparent. This, according to Moore, indicates that the homes in which the twins were raised might be more similar than people assume when looking at twin study data. If a pair of twins is adopted by two different aunts, who were themselves raised by the same parents as one of the twins’ deceased parents, there are bound to be more similarities in the environments in which the adopted twins were raised than if they were sent to more randomly chosen households (Moore 2003: 49-51).

Furthermore, even when the orphan twins went to the households of different people who they were not related to, those households could still be similar in important respects. For example, until a few decades ago it was rare for children to be placed for adoption with

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families of a different race. And, not surprisingly, adoption agencies endeavoured to place children in stable homes that could provide a safe and healthy environment. In other words, twins separated at birth would tend to be placed in homes that reflected the standards of middle class respectability common during the mid-decades of the 20th century. It is not as though one twin would be placed with a family living in abject poverty and another went to live with an affluent couple from the right side of the tracks. One twin did not go to Venus while the other was sent to Mars – they grew up in very similar households in terms of social class, race and culture.

8.5 The Flynn effect Malcolm Gladwell’s 2007 article “None of the Above,” describes a phenomenon known as the Flynn effect. In a nutshell, the Flynn effect is the worldwide increase in IQ scores of approximately 3.0 points per decade (or 0.3 points per year) over the course of the 20th century. Gladwell’s article is an assigned reading for this course and is posted along with this unit.

Gladwell’s article provides a critique of the value and meaning of IQ test scores. He challenges the notion that IQ tests are capable of measuring native intelligence – that is the sort of intelligence that is independent of social learning, e.g. schooling. More importantly, from the perspective of the nature-nurture debate, Gladwell challenges the idea that some ethnic or racial groups are innately superior or inferior to other groups based on the supposed evidence provided by IQ scores.

Gladwell’s article helps us see how modern-day social Darwinists can take advantage of group differences in IQ scores to support specious claims about certain racial groups being more or less intelligence than others. However, Gladwell is not entirely clear about how the arguments he presents show that claims about a racial basis for IQ are incorrect.

Question 300 words Do the results of twin studies employing IQ test scores provide meaningful evidence to the effect that genetic inheritance plays a role in determining one or more human psychological, behavioural or intellectual traits? Do the arguments and evidence presented by Malcolm Gladwell, James Flynn and others about the validity of IQ tests as a measure of intelligence mean that the twin studies using IQ scores tells us nothing about the relative roles of genes and the social or nurturing environment in shaping human development?

Response to student 50 words or more T S

The results of the twin studies prove that genetic inheritance plays a crucial role in determining human traits. With regards to the twin studies, intelligence/IQ is the only factor measured. However, I have read other articles that suggest psychological and behavioural traits are also very similar in twins. Table 8.1 shows that identical twins raised together (86%) and identical twins raised apart (72%). This goes to show that genes do play a significant role in IQ, but one’s environment also plays a role. Personally, I think environment plays a bigger role in one’s development. As for IQ, these results are making me think that genes impact one’s intelligence more than I initially thought.

The course notes state that Gladwell “challenges the notion that IQ tests are capable of measuring native intelligence – that is the sort of intelligence that is independent of social learning, e.g. schooling”. Further, Gladwell claims that “An I.Q., in other words, measures not so much how smart we are as how modern we are”. IQ tests do not measure our intelligence, but rather our modernity. That said, I do not think that IQ tests determine who we are or impact our development.

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