Twin Studies Research Paper

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In the social sciences, the twin study has become an important source of information about the contribution of “nature” to human traits and dispositions. Twin studies are particularly used in clinical and psychological research aimed at attempting to find the genetic component of certain disorders, such as schizophrenia, as well as of intelligence and various personality traits. The British scientist Francis Galton (1822-1911) was the first to suggest that the study of twins might unlock the mysteries of nature. In the 1870s Galton, who sought a methodology that could conclusively show the primacy of hereditary factors in human nature, suggested that the life histories of twins could be used to weigh the relative powers of both nature and nurture. However, Galton cannot be credited with the invention of what is now known as the classical twin study because he merely collected life histories from twin pairs in England through the use of self-report surveys.

The first two classical twin studies, published in 1924, were conducted in Germany and the United States. In the context of understanding the pathologies of races, families, and twins, the German dermatologist Hermann W. Siemens (1891-1969) suggested comparing the correlations (r) of identical or monozygotic (MZ) twins on a given trait to correlations of fraternal or dizygotic (DZ) twins on the same trait. The American psychologist Curtis Merriman (1875-1975), who at this time still needed to convince his readership that two distinct types of twins actually existed, reported correlations of identical and fraternal twins on a number of attributes, with the former showing higher correlations than the latter. Several statistical calculations for estimating the heritability coefficient have been reported, the most basic being 2 (r^. – rDz). This formula provides an estimate of the total variance of a given trait that may be attributed to genetic variance.

Twin studies are used to estimate the heritability of a trait, although the concept of heritability is often misunderstood. It was first described in 1936 by Jay Lush (1896-1982), a professor of animal breeding. Lush distinguished between two types of heritability: broad (H2) and narrow (h ). Narrow heritability is of most interest to the social sciences. The heritability coefficient obtained through the classical twin study is an estimate of h 2. Narrow heritability can be understood as the proportion of the variance in a given population on a given trait that can be attributed to genetic variance.

This definition encompasses the following important points. (1) Heritability refers to variance within a population; it does not describe the relative importance of genetic factors at an individual level. (2) Heritability is an estimate of genetic variance in a specific population, at a specific point in time, under specific environmental conditions; these estimates are variable over time, location, and population. (3) Estimates of heritability describe the genetic variance within a given population; they are not valid for comparisons between different populations. (4) Environmental conditions may improve the strength of the heritability estimates. (5) Heritability does not necessarily measure the genetic contribution to a trait. Suppose, for example, a researcher is interested in using the classical twin-study method to find a heritability estimate for the condition of having two eyes. The researcher would find that, in most cases, all of their MZ and DZ participants have two eyes, thereby expressing no variance, and resulting in a heritability estimate of zero.

Calculating h 2 from classical twin studies rests on an important premise: the equal environment assumption (EEA). Since MZ twins share 100 percent of their genetic material, and DZ twins share on average only 50 percent, one might assume that any observed differences between the two types of twins are due to genetic variance. However, in order to calculate h 2 from classical twin studies, researchers must first assume that MZ twins and DZ twins share the same environment, thus allowing researchers to isolate the magnitude of genetic influences on a trait without environmental confounds. Critics of twin studies have long pointed out that MZ twins generally experience environmental conditions and treatment that are more similar than that experienced by DZ twins, particularly if the DZ twins are of different genders.

In response to such criticism, twin-study proponents reformulated the EEA into the equal trait-relevant environment assumption (trait-relevant EEA), which assumes that MZ and DZ twins have equal exposure to only those environmental influences of known relevance to the trait under study. But critics of the trait-relevant EEA argue that MZ twins spend more time together, are more likely to engage in similar activities, are similarly treated by others, and are more likely to have similar friends than DZ twins. Therefore, since MZ twins experience a more similar environment than DZ twins overall, they are also more likely to have greater exposure to environmental influences that affect a given trait than DZ twins.

In addition to the classical twin study, other methodologies are available for comparing the differences between twins. The co-twin control method is the only twin-study design that attempts to manipulate environmental influences. In this type of study, the genetic component is held constant, and the researcher manipulates the environment. Only MZ twins can be used in such a study. One twin becomes the control participant, while the other is given an environmental intervention. Scores on the trait of interest are measured before and after environmental manipulation. The correlation between the twin pairs before the intervention is measured against the correlation in their scores afterward. The resulting coefficient is a measure of the effects of the environment on the trait under study. This method, however, has largely been discontinued due to its history of misuse, most notably in the twin studies conducted by Nazi doctor Josef Mengele (1911-1979) at the Auschwitz concentration camp during World War II (1939-1945).

Adoption studies are often referred to as twins-reared-apart studies, since one or both of the twins is separated from the biological parents and placed with a relative or an adoptive or foster family. Typically, MZ twins who are reared apart (MZA) are compared to MZ twins reared together (MZT). In adoption studies, correlations of MZAs on a given trait are compared to MZTs. Occasionally, DZ twins reared apart are used as well. In most cases, researchers are interested in establishing trait similarity across a number of different measures. The underlying assumption of adoption studies is that twins have been placed randomly into homes, with minimal or no contact with each other. Although this assumption is almost never accurate, the results from twins-reared-apart studies have been very influential in the social sciences.

The most significant twin-study findings resulted from two large-scale systematic adoption studies: the Minnesota Study of Twins Reared Apart (beginning in 1983) and the Swedish Adoption/Twin Study of Aging (beginning in 1984). These studies reported medium to high correlations between MZAs and MZTs on a number of variables, including IQ, physical traits, and a long list of personality traits. In addition, the Swedish Adoption/Twin Study found moderate to high correlations for processes specifically related to aging, such as memory decline. The conclusion reached in these studies is that genetics are responsible for trait similarity, and that environmental factors have little influence. Critics of these results have pointed to researcher bias, vague or missing data, the denial of access to collected raw data, and the dubious separation of the twins involved in the studies.

Genetics research has at times generated heated ethical and political debates. For example, some authors have commented that studying the genetic components of traits will lead to a resurgence in the eugenics movement and, in particular, the misuse of heritability research on intelligence by some proponents of genetic engineering. Despite these concerns, twin studies and the field of behavioral genetics have had an undeniable impact in the sciences and culture. The progress of this field of research has helped to shift the public discourse on genetic research in favorable directions. With advances in microgenetic research and the continuing development of genomic sci-ences—particularly as they relate to health promotion and disease prevention—twin studies are likely to become a less-prominent technique for estimating genetic effects than they have been in the past. In any case, because of the methodological shortcomings of twin studies, the estimates that such studies provide for the heritability of attributes and processes should be analyzed carefully and critically.


  1. Bouchard, Thomas J., Jr., David T. Lykken, Matthew McGue, et al. 1990. Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart. Science 250 (4978): 223–228.
  2. Joseph, Jay. 2004. The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope. New York: Algora.
  3. Merriman, Curtis. 1924. The Intellectual Resemblance of Twins. Psychological Monographs 33 (4): 1–58.
  4. Plomin, Robert, and Gerald E. McClearn, eds. 1993. Nature, Nurture, and Psychology. Washington, DC: American Psychological Association.
  5. Siemens, Hermann Werner. 1924. Die Zwillingspathologie: Ihre Bedeutung, ihre Methodik, ihre bisherigen Ergebnisse. Berlin: Springer.

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