Twin research

The twin research is a research method that in human genetics and psychology is used. Their applicability is largely dependent on a large amount of reliable data provided by twin registers (e.g. the Danish twin register or Swedish twin register ).

description

In the classic twin research, identical twins (monozygous twins, MZ) and dizygotic twins (dizygote twins, DZ) are examined. If identical twins resemble each other more closely than dizygotic twins with regard to a certain trait, this can be interpreted as an indication that the trait examined is genetically influenced to a particular degree.

Mathematical analyzes can be used to approximately determine the genetic share ( heritability ) and the influence of shared environmental factors (e.g. in the family). In connection with methods of molecular genetics , single genes can now also be examined in twin studies. By linkage analysis and association analysis, the heritability of single or multiple genes can be attributed to this. Among other things, this can benefit research into diseases (see Boomsma et al. ).

Heredity is often misunderstood as the proportion of hereditary disposition in the expression of a trait in a certain person. But it determines the genetic share of the differences . People differ because they carry different gene variants and because they live in different environments. A heredity of z. B. 88% for the body mass index would mean that 88% of the differences in the body mass index in the population are due to genetic differences, but not that a single person is only responsible for 12% of their weight .

On the basis of the assumption (see also section Criticism of the twin method ) that twins are largely subject to the same environmental influences, twin pairs [both identical (EZ) and dizygoti (ZZ)] were identified in criminological research (especially in the 1930s and 1940s) investigated whether criminal behavior is inherent (ie genetically determined ). For this purpose, the test subject pairs were checked for an existing concordance (or disconcordance). The existence of a “criminal record” for both the one and the other part was regarded as a concordant expression. In fact, this research did show a higher level of concordance in development cooperation - which indicated a confirmation of the initial hypothesis. However, the results obtained can rightly be questioned due to the limitation of the criminal record to brightfield , the small number of twins examined in each case, and the retrospective character of the examinations (preselection). In any case, twin research has so far not provided any empirical evidence for the genetic determination of criminal behavior.

In the field of psychological twin research, which temporarily fell into disrepute due to falsification allegations against Cyril Burt , the work of Kurt Gottschaldt should be mentioned.

Twin research on intelligence

The twin method aims to clear up the ambiguities of the plant and the environment. It is used to get to the bottom of existing differences and therefore does not serve to change people. When it comes to the question of whether and to what extent intelligence is genetically determined, the IQ values within families must be considered. To this end, researchers examined the influences of shared genes and shared surroundings separately from one another. The twin research method has proven to be the ideal strategy for this. On the one hand, identical twins who grow up separately and together are compared with one another in the twin method. On the other hand, this strategy is based on comparing the similarity of identical and dizygotic twins.

The heredity of intelligence is estimated at 50-80%.

Logic of the Twin Studies

The twin method provides important findings on the genetic and environmental influences on intelligence. Twin studies are largely based on natural observation rather than experimentation. Twin studies have shown that people who share the same genes have comparable mental abilities.

When considering different twin studies, it must be noted that a large number of them have significant shortcomings and are subject to strong biases (see section Limits ).

Identical twins (raised together)

Growing up together in this context means growing up in the same / identical environment. The similarity is most pronounced in identical twins who grew up together ( correlation of 0.87). With these twin pairs, not only the system but also the environmental conditions are almost the same. However, it must be noted at this point that children who grow up together do not necessarily have a completely identical environment. Since children, for example, attend different leisure groups and are sometimes brought up differently, the correlation of the environment of two people from the same family can never be exactly 1, but is always smaller. However, the test scores for identical twins are as similar as if the same person had performed the test twice.

In addition, studies of the brains of identical twins have shown that they are built and function in similar ways. They show comparable activity patterns when dealing with mental processes. In addition, the brains in the various brain areas match almost perfectly for linguistic and spatial intelligence. Identical twins are also particularly similar in terms of specific talents.

Identical twins (raised separately from each other)

In this context, growing up separately from one another should mean growing up in different environments / in uncorrelated environments. Since growing up separately can encompass all transitions between perfect positive and perfect negative correlation , not all twin studies meet this requirement.

Since identical twins have identical genes , differences between them can be attributed to environmental influences (if one can assume that the twins will be separated from birth). Identical twins raised in different environments have a correlation of 0.75. These twin pairs often resemble each other in certain characteristics, which are therefore not environmental, but genetic.

A study carried out by Newman et al. In 1937 aimed to get to the bottom of the plant-environment problem with the help of identical twins who were raised separately. While the mean IQ difference of the identical twin pairs who grew up together was 5.9 points, Newman found a value of 8.2 points for identical twins growing up separately. With this difference, identical twins who grew up separately are still below the average difference between dizygotic twins who grew up together. At first glance, these findings allow us to draw the conclusion that the influence of the genetic make-up greatly exceeds that of the environment. On closer inspection of the twin pairs, however, it becomes apparent that the children grew up in very similar environments (e.g. both children live in the city, attend similar schools). In the scope of his study, Newman had examined only four pairs of twins who grew up with significant environmental differences. While z. For example, if one twin grew up on a farm, the other lived in the city. Since the four couples with the greatest environmental differences also had the greatest IQ differences of 12, 17, 19 and 24 IQ points, it was found that the IQ differences increase with increasing differences in the educational environment. If identical twins grow up in uncorrelated environments, a clear difference in IQ values can be seen, which can thus be attributed to environmental influences.

Comparison of the twin pairs

Identical pairs of twins who grew up together have the most similar intelligence scores. Important to note: 1.0 means a perfect correlation, while 0 means no correlation at all.

Identical twins vs. dizygotic twins

Dizygoti twins usually have 50% of their genetic makeup in common. For dizygoti twins that grew up together, a correlation of 0.60 is visible. This lower association with identical twins suggests a genetic influence.

If one assumes that identical twins are consistently more similar to dizygotic twins, this assumption is wrong. Assuming that a trait is essentially environmental, dizygoti twins who grew up together are just as similar to identical twins who grew up together. The dizygoti twins are even more similar in this respect than identical twins who grew up separately. However, if dizygoti twins grown together are less similar than identical twins grown together or even identical twins grown separately, this feature can be characterized as predominantly hereditary. Nevertheless, all relevant studies confirm that identical twins are more similar than dizygoti twins of the same or different sex.

Same-sex dizygoti twins vs. opposite sex twins

A large number of studies confirm the assumption that same-sex dizygoti twins are more similar than opposite-sex twins in areas of behavior that are defined according to gender. It has been found that boys and girls are brought up differently. In addition, further studies confirm that boys have a high heredity coefficient for mathematical abilities and dominance behavior. The same applies to girls' social behavior and language skills.

Identical twins vs. dizygoti twins (both grew up together)

Identical twins who grew up together have, on average, more similar intelligence quotients than dizygoti twins who grew up together. This fact is initially attributed to the genetic commonality. In addition, it must now be borne in mind that identical twins have been shown to have a more similar environment than dizygoti twins: They are more often dressed in the same way, spend more time together, are treated more similarly, and often have the same teachers. The more similar IQ of identical twins can therefore also be attributed to environmental influences.

Identical twins are treated more equally than dizygoti twins and thus have a more similar environment than dizygoti twins.

siblings

The findings of the twin study also apply to normal siblings. If there are greater differences in siblings who grow up separately than in siblings who grow up together, then environmental influences become clear here.

Results overview

Increase in heredity

The similarities between identical twins do not decrease in the course of their lives, but can increase up to the 8th decade of life. Researchers showed the increase in heredity through a study that lasted over 13 years. The IQ of 209 pairs of twins was recorded repeatedly over several years (first point in time at the age of 5 years, last point in time at the age of 18 years). At the end of the study, the researchers compared the correlations between monozygotic and dizygotic twins for each age group in order to be able to estimate the heredity. An impressive increase was seen in both non-verbal and verbal IQ. While the hereditary estimate of the non-verbal IQ rose from 64% to 74%, that of the verbal IQ rose from 46% to 84%.

Jensen's findings

A major psychologist of intelligence is Arthur Jensen , who found the following in his research:

The greater the genetic similarity, the higher the correlation. This relationship is independent of whether the twin pairs grew up separately or together. Jensen always took the view that intelligence is genetic. He substantiates this point of view on the basis of certain individual results.

While the IQ of foster parents and their adopted children shows a correlation of 0.2, parents and their biological children have a more similar IQ (0.5).
While the IQ of dizygotic twins of the same sex who grew up together has a correlation of 0.56, identical twins grown together have a more similar IQ (0.87).
While the IQ of dizygotic twins of the same sex who grew up together has a correlation of 0.56, identical twins who were raised separately have a more similar IQ (0.75).
In addition, Jensen's studies reveal gender-specific differences. While the IQ of fraternal twins of the same sex has a correlation of 0.56, fraternal twins of different sex have less similarity (0.49).

Even if the previous view gives the impression that intelligence is exclusively genetic, this is not true. Jensen's studies also reveal certain indications of environmental influences. While growing up together increases the similarity values, these are reduced by growing up separately.

When comparing unrelated people who grew up together with unrelated people who grew up separately, it can be seen that people who grew up together have a more similar IQ (0.24) compared to those who did not grow up together are (0,0).
The slight similarity of the intelligence quotient (0.20) of adopted children and foster parents indicates environmental influences.
The IQ scores of siblings grown up together (0.55) are more similar to the IQ scores of siblings who grew up separately (0.45).
The IQ scores of identical twins raised together (0.87) are more similar than the IQs of identical twins raised separately (0.75).

Jensen comes to the following conclusion: the greater the difference between identical twins, the greater the environmental impact.

If one considers the difference between a perfect correlation of 1.00 and the actual correlation of 0.75 in identical twins that grew up separately, one can see an estimate for the proportion of variance in the IQ, which is attributable to the differences in the environment .

These findings lead to the following simple calculation: 1.00 - 0.75 = 0.25

According to Jensen, 75% of the IQ variance is due to genetic variation and 25% to environmental variation.

Jensen also confirms his results with a second calculation. He now offsets the perfect correlation of 1.00 with the mean correlation between adopted children who grew up together of 0.24. Since these do not have a common genetic component, correlations between these children can be associated with environmental influences. The proportion of genetic variation can be calculated from the following calculation: 1.00 - 0.24 = 0.76.

It was also found that environmental influences play a role in the variance of IQ scores for people who score in the top range.

Limits

With regard to twin studies, growing up separately should mean that the twins are growing up in uncorrelated environments. Since this is often not the case, the results are often distorted within twin studies. There is also the problem that growing up together does not always mean that the twins grew up in the same environment.

When examining separated twins, who are very similar in many characteristics, the majority is explained by heredity. However, it is neglected that the twins may have grown up in very similar environments. In addition, it should be noted that the IQ value is ultimately a one-dimensional index that comes about by reducing various forms of expression. Since two people with the same IQ value can still have completely different intelligence profiles and personality structures, the question arises whether this concept is at all suitable for investigating investment and environmental issues. It is also noticeable that the work of the psychologist Arthur Jensen relates exclusively to the nativist position . Other results are largely hidden by him.

Twins raised separately

Identical twins who grew up separately from one another are also examined occasionally. Since identical twins are genetically almost identical, one can draw conclusions about which traits are innate and which are learned by comparing identical twins raised separately . If common characteristics are found despite different surroundings, this is an indication (not proof) of genetic determination. Compared to classic twin research, the focus here is not on the differences, but on the similarities. An example: the twins examined were more similar to their birth parents than their adoptive parents in terms of their weight characteristics ( overweight , normal weight ).

Identical twins growing up separately are extremely rare, for example when twins are given up for adoption after birth.

Criticism of the twin method

Twin research is based on the assumption that identical and dizygotic twins experience common environmental factors to the same extent and that differences in the similarity between monozygotic and dizygotic twins are therefore solely due to the different proportion of common genes (100% vs. 50%). However, this assumption is controversial, since the similarity of the identical twins can lead to identical environmental influences (gene-environment correlation). As a result, the inheritance can be overestimated in individual cases. However, the behavior of the parents towards identical twins is not identical either, but differentiates according to the behavior and needs of the individual twins. In addition, especially in the field of medical twin research, the relative influence of common environmental factors is small or non-existent; the variability is essentially based on genetic differences and individual environmental factors.

Another basic assumption is that identical twins are genetically identical to one hundred percent. This premise is also only partially true, since in the course of life somatic mutations and epigenetic modifications in the DNA accumulate and lead to differences. However, in the same way, the cells of an individual are not identical to themselves, since mutations occur locally and lead to genetic differences between the cells.

literature

Dorret Boomsma , Andreas Busjahn and Leena Peltonen. Classical twin studies and beyond . Nature Reviews Genetics 2002; 3: 872-882
Robert Plomin, John C. DeFries, Gerald E McClearn, and Michael Rutter: Genes, Environment, and Behavior - An Introduction to Behavioral Genetics . (1999) Huber Verlag, ISBN 3456831854

Ilona Federenko: Influence of genetic factors on the endocrine, cardiovascular and psychological stress response. (2003) Cuvillier Verlag, ISBN 3-8987-3757-8

Henning, J. & Netter, P. (2005) (Ed.): Biopsychological foundations of personality. Heidelberg: Spectrum ISBN 3-8274-0488-6

Jay Joseph: The Missing Gene. (2006), Algora Publishing

Web links

Historical films on twin research from the collection of the Federal Institute for Scientific Film (ÖWF) in the online archive of the Austrian Media Library

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Hermann Rosemann: Intelligenztheorien . Research results on the plant-environment problem in a critical overview. Ed .: Wolfgang Müller. Rowohlt, Reinbek near Hamburg 1979, ISBN 3-499-17254-2 .
↑ ^a ^b ^c ^d ^e ^f David G. Myers: Psychology . 3. Edition. Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-40781-9 .
^ Hermann Rosemann: Intelligenztheorien . Research results on the plant-environment problem in a critical overview. Ed .: Wolfgang Müller. Rowohlt, Reinbek near Hamburg 1979, ISBN 3-499-17254-2 .
↑ John C. Loehlin, Robert C. Nichols: Heredity, Environment, and Personality . A Study of 850 Sets of Twins. Texas, the United States of America 1976, ISBN 0-292-73003-9 , pp. 50 .
↑ Richard J. Gerrig et al .: Psychology and Life . 2nd Australasia edition. Pearson Australia, Frenchs Forest 2012, ISBN 978-1-4425-3981-5 , pp. 339 .

[:0-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j Hermann Rosemann: Intelligenztheorien . Research results on the plant-environment problem in a critical overview. Ed .: Wolfgang Müller. Rowohlt, Reinbek near Hamburg 1979, ISBN 3-499-17254-2 .

[:1-2] ↑ ^a ^b ^c ^d ^e ^f David G. Myers: Psychology . 3. Edition. Springer, Berlin / Heidelberg 2014, ISBN 978-3-642-40781-9 .

[3] Hermann Rosemann: Intelligenztheorien . Research results on the plant-environment problem in a critical overview. Ed .: Wolfgang Müller. Rowohlt, Reinbek near Hamburg 1979, ISBN 3-499-17254-2 .

[4] John C. Loehlin, Robert C. Nichols: Heredity, Environment, and Personality . A Study of 850 Sets of Twins. Texas, the United States of America 1976, ISBN 0-292-73003-9 , pp. 50 .

[5] Richard J. Gerrig et al .: Psychology and Life . 2nd Australasia edition. Pearson Australia, Frenchs Forest 2012, ISBN 978-1-4425-3981-5 , pp. 339 .