Twin Studies

Greta G Boers. 21st Century Anthropology: A Reference Handbook. Editor: H James Birx. Volume 1. Thousand Oaks, CA: Sage Reference, 2010.

Studying twins offers a unique opportunity to identify and differentiate physical and behavioral characteristics according to their origin. How much of who we are is genetically determined and how much is the result of our environment? By comparing the consistency of phenotypes, that is, observable physical and behavioral characteristics, in twins we can hypothesize about their heritability—whether or not a specific characteristic is genetically determined. Based on the assumption that all twins share their environment and that monozygotic twins also share genomes, phenotypes that vary between monozygotic twins must be the result of environmental factors, while those that are consistently the same must be genetic in origin.

Though they must be inextricably linked, there is a difference between research on twins and research that uses the twin studies method. The following chapter will follow both.

Sir Francis Galton: The Twin Study Method

Sir Francis Galton (1822-1911) was the first to articulate the value of twins to the nature-nurture debate. While his cousin, Charles Darwin, culminated his theory of natural selection in the 1850s, Galton was beginning to articulate his related theory of eugenics. By the beginning of the 20th century, eugenics was considered the new liberating science, promising to help control disease and improve the quality of life. The reasoning was that if a species adapted through the process of natural selection, then it should be possible to control disease and other undesirable characteristics by intentional selection. Breeding in animals and plants had made successful contributions to agriculture; using carefully documented bloodlines among the nobility to identify marriage partners was an accepted cultural practice in Europe, the New World, and many other cultures. Deliberately manipulating reproduction was a recognizable and acceptable idea.

Galton argued that because of their observable similarities, twins were a primary source of data. In his 1876 article, “The History of Twins, as a Criterion of the Relative Powers of Nature and Nurture,” Galton pointed to the unique potential that twins offered to genetic research: “Their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture” (p. 391). He coined the terms identical and fraternal to describe the degree of similarity between the twins.

Harris Hawthorne Wilder was the first to speculate that identical, fraternal, and conjoined twins originated at different points in the same process. Identical twins were the result of the egg splitting into two parts; fraternal twins were the result of fertilization of two separate eggs. Conjoined twins—unfortunately referred to as “double monsters” during that time—were part of the same process, but in those cases, the splitting of the egg was incomplete (Wilder, 1904, p. 391). It was not until 1909, when Miguel Fernandez and in a separate study J. Thomas Patterson and H. H. Newman, that scientists were able to observe polyembryology in mammals—specifically armadillos—and to verify this developmental sequence from the point of conception to multiple fetuses to birth (Newman, 1941, pp. 102-103).

Research on Twins: Fetal Development

We now know that monozygotic, or identical, twins develop by the division of a single zygote over the first several weeks of pregnancy. Dizygotic, or fraternal, twins form at conception by the fertilization of two eggs by two sperm, so their genetic similarity to one another would be like that of any two singleton siblings. The rate of monozygotic twins is consistent, while dizygotic twins varies across ethnicities (Piontelli, 2002, pp. 14, 16). For example, 1 in 11 births in Nigeria are twins, but in Japan, the ratio is 1 for every 250 births (Hall, 2003, p. 735). Twin births are more likely in later pregnancies. Different cultural factors affect women’s decisions to start their families later in life, but the trend is consistent across many countries and continents over the last century. In vitro fertilization and other fertility treatments have increased the incidence of twins as well as other multiples in populations where such technologies are used (Piontelli, 2002, p. 14). From 1980 to 2004, the twin birthrate rose by 70% (Martin et al., 2009, p. 2). In the United States in 2006, twins occurred with a frequency of 32 in every 1,000 live births. Almost all of the twins resulting from fertility treatments are dizygotic.

Medical research continues to illuminate the different phases of fetal development in twins and other multiples in order to anticipate attendant risks. In 1995 in the United States, the mortality rate of twins was 32 deaths for every 1,000 births; in the same year, the rate for singletons was 6 deaths for every 1,000 births (Parker, Schoendorf, & Kiely, 2001, p. 14). Approximately 8 out of 10 twin pregnancies are premature, and approximately half of all twins have a low birth weight—a major factor in infant mortality (Martin et al., 2009, pp. 10, 21). Some of these conditions further distinguish situational versus genetic factors, such as vulnerability to specific diseases, sleep-wake patterns in utero, and the complexities of twins and identity such as bonding and loss. Already, in the uterine environment, genetic predispositions and environmental chance begin to interweave.

Four major physiological considerations affect twin parturition: the time of division and the way that the amnion, chorion, and placenta are situated. There are four approximate phases over the first weeks of pregnancy that are significant: Days 1 through 4; 4 through 8; 8 through 14; and after the 14th day. The degree of genetic similarity, the particular risk factors, and the further development of the amnion, chorion, and placenta are determined by the day on which the zygote divides. The amnion is the inner sac filled with fluid, which surrounds the fetus, protecting it and allowing movement. The chorion is a stronger sac that surrounds the amnion. The placenta is embedded in the uterus and is connected to the fetus via the umbilical cord and serves its circulatory system through the exchange of oxygen and carbon dioxide. It also provides nutrition and antibodies to the fetus from the mother. Dizygotic twins always have their own placentas, though sometimes monozygotic twins will share one.

Vanishing-Twin Syndrome

If division takes place during Days 1 through 4, then the two resulting fetuses will each develop its own amnion, chorion, and placenta. In Days 4 through 8, there is a higher likelihood that each twin will have a separate amnion and placenta but share one outer sac, the chorion. In these first two phases, the “vanishing twin” syndrome is possible in which one of the fetuses is absorbed by the mother, the uterus, or its twin. Researchers H. J. Landy and L. G. Keith (1998) estimated that approximately 30% of twin pregnancies are subject to the loss of one of the embryos, something scientists—and parents—weren’t aware of until the 1980s when sonography made it possible to see them (pp. 181-182).

Twin-to-Twin Transfusion Syndrome

From Days 4 through 8, the probabilities lean toward two fetuses in one amnion, one chorion, and with one placenta. In Phases 2 through 3, twin-to-twin transfusion syndrome, or fetofetal transfusion, becomes a risk for monozygotic twins as the blood flow between the placenta and one twin affects the flow of those substances toward the other. As a result of this uneven flow of blood, oxygen, and nutrients, one twin may become anemic while the other becomes polycythemic—with too high a red blood cell count. There’s also a danger that the one twin will be surrounded by too much amniotic fluid and the other not enough. The concern with polyhydramnios—too much amniotic fluid—is that the fetus has not absorbed enough. With oligohydramnios—too little—the fetus’s kidneys may be adversely affected. Growth and development of the larger twin may continue to constrict in the available space of the smaller one, causing further developmental risks. Unless diagnosed within the first 15 weeks of pregnancy, mortality from fetofetal transfusion is approximately 80% to 100%. Also, twins who survive are at risk for immune problems. The more closely the twins are situated in one amnion or even one chorion, the more likely they are to be affected by each other’s sleep-wake cycle and the subsequent separation after birth. The chance of tangling up in one another’s umbilical cords and the developmental effects of constricted movement are also important factors.

Conjoined Twins

After Day 14, an incomplete division is probable—conjoined twins also share the uterine environment completely. According to Lewis Spitz (2005), 60% of conjoined twins die during pregnancy or at birth; live births of conjoined twins approximate 1 for every 250,000 (p. 814). Most fetal deaths occur during the 1st trimester.

Such twins can be joined at a number of different locations. The most frequent fusions are thoracopagus, where the twins are joined at the chest, possibly sharing heart, liver, or intestines. Sharing the heart and cardiovascular system poses the greatest risk, which is why the mortality rate of thoracopagi is also highest. Other major organs can be shared. For example, the famous P. T. Barnum Siamese twins, Chang and Eng, were thoracopagus. After their deaths in 1874, it was discovered that they shared their liver. Though Eng was healthy, Chang’s alcoholism contributed to his death. Additional complications occur when the thoracopagus are also omphalopagus—that is, joined at the umbilical region. Additional classifications in degree of frequency are pygopagus—joined at the surface close to the sacrum and coccyx either side by side or through the posterior. In those cases, the gastrointestinal and/or urinary tracts may be shared. Less frequent are ischiopagus, joined at the pelvis, and craniopagus, joined at the head.

Each pair of conjoined twins is fused in unique ways, sometimes symmetrically but not always. When the attachment has resulted in unequal fetal development, only one of the twins may survive birth or even the first days and weeks of life. In such cases, emergency separation surgery is required to rescue at least one but hopefully both of the twins. Such instances are not the only difficult decisions: Because the attachments are so unique, medical care, surgery, and survival must be determined individually. Fetal imaging makes early diagnosis possible. The sooner the degree and kind of fusions are determined, the more quickly decisions about abortion, separation, and medical care can be made.

Separation surgery excites the medical community, offering some of the most complicated surgical interventions, because there is relatively little similarity from one case to another. The different kinds of surgical interventions are common. The number of procedures strain the infants’ bodies and the interdependence of biological systems that require the planning and performing of the surgery to be so exquisitely careful. In each case, a separate cadre of specialists are required. Each twin will need his or more frequently her (female conjoined twins have a frequency of 3 to 1 over male conjoined twins) own anesthesiologist and monitoring system. Former Surgeon General C. Everett Koop, famous for creating the first intensive-care pediatric-surgery unit, reported that it took 100 hours to plan, 4 hours of preoperative preparation, two surgical teams, 23 operating-room staff, and 10.5 hours of surgery to separate twins Clara and Alta at Children’s Hospital of Philadelphia in 1974 (Children’s Hospital of Philadelphia, 1974). The separation of Yurelia and Fiorella Rocha-Arias at Stanford University Hospital in 2007 took approximately 200 experts and cost between $1 and $2 million.

Separation surgery also excites a range of controversial ethical considerations. Alice Domurat Dreger (2004) argued that separation surgery is not always necessary but is chosen by the parents and doctors because they subscribe to socially defined expectations of normal appearance and identity (pp. 56-60). In contrast, in 2000, the parents of Maltese conjoined twins Jodie and Mary were ordered to submit their daughters to a sacrifice surgery. It was evident to physicians at St. Mary’s Hospital in Manchester, England, that Mary was severely deformed and that she would prevent her sister’s survival, as well as her own. As Roman Catholics, the parents believed that the death of the twins would be God’s will. The High Court argued that Jodie’s right to life was a higher consideration.

Theories in Twin Studies: An Overview

To understand the place of twin studies over time and across national and cultural boundaries, it helps to understand the philosophies driving the fashions in research focus and study design as well as the sociopolitical contexts that determined funding, support, and ethical standards of the research itself. What kind of research were twin studies used for? What are the experiences of twin studies’ subjects in such research?

The theories that predominantly influenced twin studies during the 20th and the beginning of the 21st centuries are eugenics, phenogenetics, behaviorism, and genomics. A rough survey of their popularity breaks up into three time periods: from 1876, when Galton published his twin study method, to the end of World War I; from the end of World War I to the end of World War II; and from 1948, when B. F. Skinner published his theory of behaviorism, to 1985 when DNA “fingerprinting” made it possible to actually see twins’ identical genomes.


Sir Francis Galton (1876) coined the term eugenics. It was his belief that scientists could breed a better kind of human, eliminating illness and disease by controlling reproduction. This required determining the heritability of specific characteristics. It would be over 100 years before this could be done by looking at the actual genome. Galton’s solution to this mystery was to study the variability or consistency of characteristics in monozygotic and dizygotic twins and make generalizations based on statistical computations.

The eugenicist ambition troubles us today because of the corollary belief that there were inferior human “species” or races, a belief possible at a time when the mutability—the ability to change over generations—of all phenotypes was improperly understood. Human races were not really considered to be different species, but it was believed that they were differentiated by immutable characteristics, some of which were physically visible, such as skin color and hair texture, while others such as moral propensities were not. The purpose of research was to determine how such characteristics were connected. For those who leaned heavily on nature as the determining influence in human destiny, the obvious step toward a new and “healthy” population was to control its evolution. Nationalism as a matter of collective identity would further magnify these theories in the social and political infrastructures of the early 20th century (Adams, Garland, & Weiss, 2005, p. 233). From today’s vantage, it can be surprising to see how pervasive and acceptable eugenics research, legislation, and mythology were in 20th-century life but then not so surprising that they came to be exploited.

The confusion about genetic versus environmental origins, which drove twin studies, had a long reach from science into politics even in the United States. The base for such research in the United States was the Eugenics Record Office (ERO) at Cold Spring Harbor, established in 1910. Charles B. Davenport created and then directed the ERO; his assistant, Harry H. Laughlin, a passionate leader in the eugenicist movement, initiated the Johnson-Reed Immigration Restriction Act, which was ratified in 1924. This federal legislation set quotas for “inferior” populations to control the number who entered the country, including people who were trying to escape the Holocaust during World War II (Adams et al., 2005, pp. 238-239). Sterilization of the “unfit” was common practice in the early 20th century in many countries, including Sweden, Norway, Canada, Denmark, and Germany. In the United States, the Supreme Court upheld the decision of Buck v. Bell, permitting the involuntary sterilization of Cary Buck in 1927. In the 1970s Iowa, North Carolina, and Oregon were still operating sterilization programs: “No revulsion against Nazi sterilization policy seems to have curtailed American sterilization programs” (Reilly, 1987, pp. 161, 167).


In Germany, as well as other countries, eugenics became institutionalized to a greater degree—resulting in the concept of Rassenhygiene or “racial hygiene,” that is, racial purity. Almost all of the political machinery during the Nazi era was tied up with this mission. The Kaiser Wilhelm Institute was established in Berlin in 1927, partly funded by the Rockefeller Foundation, which also supported the ERO in the United States. Among the many scientists, anthropologists, and researchers who worked there, Otmar Verschuer and Joseph Mengele were particular proponents of the twin study method for their research on heredity. Their object—and the object of most of the Nazi medical and biological scientists—was the opposite of Galton’s original idea. Instead of breeding out negative characteristics of a population through controlled reproduction, they wanted to identify and isolate specific, desirable “Aryan” phenotypes and breed them into the population. Increasing reproduction in the Aryan population was a major effort of the Nazi regime. Phenogenetic research looked to link specific characteristics in physical appearance, intelligence, and abilities such as musical or mathematical talent to genetic material so that it could be manipulated. Most of this research was done in the concentration camps.

History’s description of Mengele is almost as disturbing for his reputed charm and popularity among the inmates as for his diabolical experiments. Mengele’s research used one twin as a control with the other as subject. His experiments included injecting bacteria, including typhus, into one twin while using the other as a control; injecting chemicals into subjects’ eyes to see if the color of the iris would change; surgically attaching (i.e., conjoining) identical twins to see if and how they would die (Lagnado & Dekel, 1992). Twin survivor Eva Mozez Kor describes entering Auschwitz-Birkenau at 10. She was injected with bacteria that made her so ill that Mengele refused her any medicine because he wanted her to die. Knowing that her twin Miriam would be put to death so that Mengele could compare their corpses, Eva focused on a determination to live (Hercules, Pugh, & Simpson, 2005). The experiments on Miriam Mozez affected the development of her kidneys and eventually caused her death—among the further difficulties of twins surviving Auschwitz-Birkenau was finding out what injections and substances they had been given. Much of the documentation was shoddy or lost. Because Mengele escaped before the doctor’s trial at Nuremberg, twin survivors of Mengele’s experiments finally gave public testimony at a substitute trial in 1985 at Yad Vashem, Israel’s memorial to the Holocaust.


The tragedy of entgrenzte Wissenschaft or “morally transgressive science” continues to be the subject of historical research, anguished confession, and institutional and national reconciliation. Though the German phrase is generally associated with the Nazi era, scientific and moral transgressions continue to damage twin subjects—as with David Reimer, sexually reassigned as a girl, whereas his brother was raised as a boy.

The theory of behaviorism came almost like a backlash against eugenics. B. F. Skinner’s (1962) publication of Walden Two heralded a theory that attempted to liberate us from the hardwired-genetic preoccupation of scientists before and during World War II. Skinner was convinced that we developed our identity and much of our physical being in response to the environment; this argued that our development could be controlled by proper conditioning. Among the many mysteries of human experience, sexuality is also developed in a complex interrelationship between nature and nurture.

Its extreme was demonstrated by the tragic experience of David Reimer, a twin whose circumcision damaged his penis and whose parents were guided by Dr. John Money (1975), a reputable sexologist at Johns Hopkins University. Money’s research operated on the principle that gender and sexuality could be programmed. His advice was to have David sexually reassigned and raised as a girl while his brother continued his life as a boy (p. 66). Money’s reasoning, like that of many physicians after him, was that the trauma of having a relatively small penis and of not being able to urinate standing up like other boys would be worse than the battery of surgeries, hormone injections, and behavioral modifications of transitioning into a girl. Ethicists like Dreger (2004), above, argue that this is a socially motivated decision based on rigid and unrealistic notions of “normal” genders. The similarity and contrast of the twins’ experiences growing up rather proves that genes are fundamental to sexual identity, regardless of training, reinforcement, surgery, hormones, or therapy.

Money lost track of his subject after reporting that his transformation was a success (Colapinto, 1997; Money, 1975), and it was because of Money’s reputed success that infant sexual reassignment became accepted practice throughout the 20th century. It was not until Drs. Milton Diamond and Kevin Sigmundson (1997) published their follow-up review that Money’s fiasco became public. In the film, Sex: Unknown (2001), David Reimer begins to tear up as he says, “The medical community was under the impression that my case was a success story. And I was shocked when I heard that people thought that my case was a success story” (Cohen & Sweigart, 2001, film for television). This is an unusual example of the twin studies method since there are only two subjects—still, it proves the incontrovertible genetic factors that drive our gender identities and sexuality.


The technology to “fingerprint” DNA was developed by 1985. Its effect on twin studies can be imagined as accessing the genetic material of subjects greatly amplifies the possibilities in nature-nurture research. How do researchers find twins for their studies? According to the journal Twin Research and Human Genetics, there are over 50 twin registries worldwide (Busjahn & Hur, 2006, p. 705). The Danish Twin Registry was established in 1954 and is the oldest national registry. In national twin registries, twins are identified through medical, school, and other available public documentation. In many other countries, for example, the United Kingdom, twins volunteer their participation in the registry.

An example of the kind of documentation now possible is the Biological Psychology Department of the Vrijie Universiteit Amsterdam. The department has been maintaining a database of twins and their families, routinely collecting blood and urine samples over time and preserving the DNA in a biobank. Using both the kinship method and the ability to test hypotheses against biological samples from monozygotic and dizygotic twins, the scientists can track the genetic markers for phenotypes with far greater sophistication (Boomsma et al., 2006, p. 853).

Such studies, using registries all over the world, have illuminated the complex interactions of genetic and environmental factors in behavioral patterns, such as schizophrenia (Li, Sundquist, Hemminki, & Sundquist, 2009; Torrey, 1994), attention deficit disorder (Bennett et al., 2006), anxiety and depression (Hicks, DiRago, Iacono, & McGue, 2009; Wray et al., 2009), aggression (Brendgen et al., 2008), perfectionism and eating disorders (Tozzi et al., 2004) as well as drug use and alcoholism (Slutske et al., 2008; Whitfield et al., 2004). A cursory review of the medical literature proves the value of twin studies in determining heritability in a range of conditions including asthma (Thomsen, Ferreira, Kyvik, Fenger, & Backer, 2009), heart disease (Wienke, Holm, Skytthe, & Yashin, 2001; Zdravkovic et al., 2002), fertility (Kohler et al., 2006), osteoporosis (Tse, Macias, Meyer, & Hargens, 2009), obesity (Mustelin, Silventoinen, Pietilaenen, Rissanen, & Kaprio, 2009; Peeters et al., 2007), and immune function (de Craen et al., 2005). Infectious diseases are transmitted by germs in the environment, but susceptibility and the ability to heal also have genetic components (Jepson, 1998, p. 79). Longitudinal studies offer insight into physical and psychological development from infancy onward. As twin registries track the lives of their increasingly older subjects, we can identify and better understand the many factors that affect the health and happiness of the aged (Harvald et al., 2004, pp. 318-335).


In Plato’s Symposium, several dinner guests are discussing love and its origins. When it is Aristophanes’s turn to speak, he tells the story of how we were all originally made up of two heads, four legs, and four arms, each half facing outward. These were beings who created such havoc just trying to move around that Zeus lost his patience and cut them in half. This is why and how we love; each half-being is searching for his or her other, explained Aristophanes. Though Socrates naturally disabused him, Aristophanes’s image may explain our fascination with duality, the “glamour” (as Alessandra Piontelli, 2002, described it) that twins so often carry with them as they move through their lives.

The whole of twins research and the twin studies method is centered on these mysteries of similarity and difference and by what magical combination of chance, genetic wiring, and nurturance they occur. By comparing these variations in large populations of twins, we can speculate and then prove whether a particular characteristic is genetic in origin. The twin studies method has untangled the biology and behavior of mental illnesses, such as schizophrenia, as well as pointed to the genes that may contribute to alcoholism, heart disease, or infectious diseases. As new technologies in fertility, cloning, and stem cell research develop, and as siblings become harvestable for their cells and organs, the ethical conundrums identified in twins research may prompt the right kinds of questions about the experience of others in this new phase of science.