Twin studies

Significance: Studies of twins are widely considered to be the best way to determine the relative contributions of genetic and environmental factors to the development of human physical and psychological characteristics.

The Origin of Twin Studies

Sir Francis Galton, an early pioneer in the science of genetics and a founder of the theory of eugenics, conducted some of the earliest systematic studies of human twins in the 1870s. Galton recognized the difficulty of identifying the extent to which human traits are biologically inherited and the extent to which traits are produced by diet, upbringing, education, and other environmental influences. Borrowing a phrase from William Shakespeare, Galton called this the “nature vs. nurture” problem. Galton reasoned that he could attempt to find an answer to this problem by comparing similarities among people who obviously shared a great deal of biological inheritance, with similarities among people sharing less biological inheritance. Twins offered the clearest example of people who shared common biological backgrounds.

Galton contacted all of the twins he knew and asked them to supply him with the names of other twins. He obtained information on ninety-four sets of twins. Of these, thirty-five sets were very similar, people who would now be called identical twins. These thirty-five pairs reported that people often had difficulty telling them apart. Using questionnaires and interviews, Galton compared the thirty-five identical pairs with the other twins. He found that the identical twins were much more similar to one another in habits, interests, and personalities, as well as in appearance. They were even much more alike in physical health and susceptibility to illness. The one area in which all individuals seemed to differ markedly was in handwriting.

Modern Twin Studies

Since Galton’s time, researchers have discovered how biological inheritance occurs, and this has made possible an understanding of why twins are similar. It has also enabled researchers to make more sophisticated use of twins in studies that address various aspects of the nature vs. nurture problem. Parents pass their physical traits to their children by means of genes in chromosomes. Each chromosome carries two genes (called alleles) for every hereditary trait. One allele comes from the father and one comes from the mother. Any set of full brothers and sisters will share many of the same alleles, since all of their genes come from the same parents. However, brothers and sisters usually also differ substantially; each zygote (ovum, or egg, fertilized by a sperm cell) will combine alleles from the father and the mother in a unique manner, so different zygotes will develop into unique individuals. Even when two fertilized eggs are present at the same time, as in the case of dizygotic or fraternal twins, the two will have different combinations of genes from the mother and the father.

Identical twins are an exception to the rule of unique combinations of genes. Identical twins develop from a single zygote, a cell created by one union of egg and sperm. Therefore, monozygotic twins (from one zygote) will normally have the same genetic makeup. Differences between genetic twins, researchers argue, must therefore be produced by environmental factors following birth.

The ideal way to conduct twin studies is to compare monozygotic twins who have been reared apart from each other in vastly different types of families or environments. This is rarely possible, however, because the number of twins separated at birth and adopted is relatively small. For this reason, researchers in most twin studies use fraternal twins as a comparison group, since the major difference between monozygotic and dizygotic twins is that the former are genetically identical. Statistical similarities among monozygotic twins that are not found among dizygotic twins are therefore believed to be caused by genetic inheritance.

Researchers use several types of data on twins to estimate the extent to which human characteristics are the consequence of genetics. One of the main sources for twin studies is the Minnesota Twin Registry. In the 1990s, this registry consisted of about 10,500 twins in Minnesota. They were found in Minnesota birth records from the years 1936 through 1955, and they were located and recruited by mail between 1985 and 1990. A second major source of twin studies is the Virginia Twin Registry. This is a register of twins constructed from a systematic review of public birth records in the Commonwealth of Virginia. A few other states also maintain records of twins. Some other organizations, such as the American Association of Retired Persons (AARP), keep records of twins who volunteer to participate and make these records available to researchers.

Zygosity, or degree of genetic similarity between twins, is usually measured by survey questions about physical similarity and by how often other people mistake one twin for the other. In some cases, zygosity may be determined more rigorously through analysis of DNA samples. The emerging field of epigenetics has also found that the epigenomes of twins can display the differences caused by environmental factors after birth. For example, a study from the university of Utah's Genetic Science Learning Center found that fifty-year-old twins have significantly more variation in epigenetic tags than do three-year-old twins. This understanding can help reveal the influence of both genetics and the environment on various characteristics and conditions.

Problems with Twin Studies

Although twin studies are one of the best available means for studying genetic influences in human beings, there are a number of problems with this approach. Although twin studies assume that monozygotic twins are biologically identical, some critics have claimed that there are reasons to question this assumption. Even though these twins tend to show greater uniformity than other people, developmental differences may emerge even in the womb after the splitting of the zygote.

Twins who show a great physical similarity may also be subject to environmental similarities so that traits believed to be caused by genetics may, in fact, be a result of upbringing. Some parents, for example, dress twins in matching clothing. Even when twins grow up in separate homes without being in contact with each other, their appearances and mannerisms may evoke the same kinds of responses from others. Physical attractiveness, height, and other characteristics often affect how individuals are treated by others so that the biologically based resemblances of twins can lead to common experiences. The discovery of the influence of epigenetics on twins further suggests that the nature-nurture relationship is extremely complicated and may be oversimplified by relying on a traditional understanding of twin studies.

Finally, critics of twin studies point out that twins constitute a special group of people and that it may be difficult to apply findings from twin studies to the population at large. Some studies have indicated that intelligence quotient (IQ) scores of twins, on average, are about five points below IQ scores in the general population, and twins may differ from the general population in other respects. It is conceivable that genetics plays a more prominent role in twins than in most other people.

Impact and Applications

Twin studies have provided evidence that a substantial amount of human character and behavior may be genetically determined. In 1976, psychologists John C. Loehlin and Robert C. Nichols published their analyses of the backgrounds and performances of 850 sets of twins who took the 1962 National Merit Scholarship test. Results showed that identical twins showed greater similarities than fraternal twins in abilities, personalities, opinions, and ambitions. A careful examination of backgrounds indicated that these similarities could not be explained by the similar treatment received by identical twins during upbringing.

Later twin studies continued to provide evidence that genes shape many areas of human life. Monozygotic twins tend to resemble each other in probabilities of developing mental illnesses, such as schizophrenia and depression, suggesting that these psychological problems are partly genetic in origin. A 1996 study published in the Journal of Personality and Social Psychology used a sample from the Minnesota Twin Registry to establish that identical twins are similar in probabilities of divorce. A 1997 study in the American Journal of Psychiatry indicated that there is even a great resemblance between twins in intensity of religious faith. Twin studies have offered evidence that homosexual or heterosexual orientation may be partly a genetic matter, although researcher Scott L. Hershberger has found that the genetic inheritance of sexual orientation may be greater among women than among men. Similar studies have suggested that there may be some genetic influence in the development of transgender identities.

Twin studies have been particularly insightful regarding the genetic or environmental origin of various diseases and disorders. For example, evidence from research on identical and fraternal twins suggests that there is a strong genetic influence on conditions such as reading disability, autism, and Alzheimer's disease. In comparison, Crohn's disease, strokes, and rheumatoid arthritis are much more dependent on environmental factors. These studies allow researchers to potentially identify triggers and risk factors for diseases as well as their mechanisms of action, aiding the development of preventative measures and treatments.

In 2015 and 2016, retired American astronauts Scott and Mark Kelly performed a unique twin study to gauge the effects of spaceflight on the human body. The brothers, born in 1964, are identical twins. Scott Kelly spent nearly a year aboard the International Space Station from March 27, 2015 to March 2, 2016. Mark Kelly remained on Earth. Teams of researchers monitored the brothers during that time and examined Scott Kelly upon his return. Among the most notable findings, the researchers discovered that telomeres, protective “caps” on the end of DNA strands, actually lengthen while Scott was in space. Telomeres typically shorten as people age, leaving the body more susceptible to genetic changes that could cause cancers and other illnesses. The twin study also showed that Scott Kelly’s body adapted relatively well to space-flight, and readjusted to life on earth when he returned.

Key terms

• dizygotic: developed from two separate zygotes; fraternal twins are dizygotic because they develop from two separate fertilized ova (eggs)
• monozygotic: developed from a single zygote; identical twins are monozygotic because they develop from a single fertilized ovum that splits in two
• zygosity: the degree to which two individuals are genetically similar
• zygote : a cell formed from the union of a sperm and an ovum

Bibliography

Asbury, Kathryn, and Robert Plomin. G is for Genes: The Impact of Genetics on Education and Achievement. Hoboken: Wiley, 2014. Print.

Edwards, Monica, and Laurie Abadie. "NASA’s Twins Study Results Published in Science Journal." National Aeronautics and Space Administration, 6 Feb. 2020, www.nasa.gov/feature/nasa-s-twins-study-results-published-in-science. Accessed 1 Nov. 2022.

Fletcher, H. L., and G. I. Hickey. Genetics. New York: Garland Science, 2013. Print.

Hershberger, Scott L. “A Twin Registry Study of Male and Female Sexual Orientation.” Journal of Sex Research 34, no. 2 (1997). Print.

Joseph, Jay. The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope. New York: Algora, 2004. Print.

Kendler, Kenneth S., and Carol A. Prescott. Genes, Environment, and Psychopathology: Understanding the Causes of Psychiatric and Substance Use Disorders. New York: Guilford, 2006. Print.

"Insights from Identical Twins." Learn.Genetics. University of Utah Health Sciences, 2016. Web. 27 Jan. 2016.

Loehlin, John C., and Robert C. Nichols. Heredity, Environment, and Personality: A Study of 850 Sets of Twins. Austin: U of Texas P, 1976. Print.

Piontelli, Alessandra. Twins: From Fetus to Child. New York: Routledge, 2002. Print.

Spector, Tim D. Identically Different: Why You Can Change Your Genes. London: Phoenix, 2013. Print.

Spector, Tim D., Harold Snieder, and Alex J. MacGregor, eds. Advances in Twin and Sib-Pair Analysis. New York: Oxford UP, 2000. Print.

Steen, R. Grant. DNA and Destiny: Nurture and Nature in Human Behavior. New York: Plenum, 1996. Print.

"Twin Studies." Genetics and Social Science. NCHPEG, 2016. Web. 27 Jan. 2016.

TwinStudies.org. International Society for Twin Studies, 2016. Web. 27 Jan. 2016.

Wright, Lawrence. Twins: And What They Tell Us About Who We Are. New York: John Wiley and Sons, 1997. Print.