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One aspect stands out to distinguish the two groups, namely, the factor of sex. Identical twins are always of one sex; fraternal twins may or may not be. This in itself suggests a different mode of genesis for the two types of twins. It is well established that in all higher forms of animal life, the sex of a fertilized ovum is determined by the presence or absence of a particular chromosome, called the X chromosome. The fertilized ovum, which has two X factors (XX), develops into a female; that which has a single X factor, or an X factor with a corresponding Y factor develops into a male.
Identical twins are generally born with one placenta and enclosed in one chorion. This immediately suggests that they are the result of the germination of one ovum, which by some variation of cell division has become two separate germinal bodies, each perfect in itself and able to achieve perfect embryonic development. This is the generally accepted theory to which most of the evidence seems to point, although some authorities have suggested other possible modes of genesis.
If all twins were of fraternal origin, then boy-boy, mixed, and girl-girl pairs should occur approximately in the ratio 1:2:1 by the normal laws of chance. The number of twins of like sex would be approximately equal to the number of mixed pairs. By extensive observations, however, it has been found that about sixty-two or sixty-three percent of all twins are of like sex, a proportion which would be accounted for if about twenty-five percent of all twins, or about forty percent of all like-sex twins are identical twins.
Few individuals are in any strict sense bilaterally symmetrical. One side of a profile presents a different outline from the other; one ear may be slightly larger, longer, fuller; one leg heavier; the finger and palm prints on the one hand quite unlike those on the other; and so on for almost every paired structure in the body. The most generally accepted theory to account for the differences between identical twins is based on this normal asymmetrical tendency of the individual.
There is a continuous gradation in the resemblance between twins from almost complete duplication to rather great difference. Two groups of twins have been distinguished: (a) uniovular and (b) binovular, but within each group, gradations of resemblance are noted. Within the uniovular group, this generalization holds less than within the binovular, for, in the former group, there is a “bunching” towards the extreme of identical resemblance. Within the entire range of all twins taken as one group, this tendency is also noted.
When two individuals are separated in infancy, brought up as different, and still manifest such mental similarities, it is impossible to resist the conclusion that the physical make-up of the individual is very largely settled by the time he is born.
Although most laypeople find the study of identical twins reared apart most compelling, there are reasons for nonrandom selection and nonrandom assignment to environments that render the study of reared apart less useful than research on adopted children. If there were a study of identical twins reared in uncorrelated environments, genetic differences would be controlled, whereas both within-family and between-family environmental variables are free to vary. This would be an ideal study of genetic differences. Unfortunately for science, there are simply too few pairs of reared apart, too peculiarly sampled, to make these subjects useful to social science. Adopted children, on the other hand, provide almost as useful data as the rare identical twins reared apart, and they are far more available. Adopted children are not genetically descended from the family of rearing so that environmental differences between families are not confounded with genetic differences in the children if the adopted children are randomly placed by adoption agencies. Theoretically, regressions of adopted child outcomes or adoptive family characteristics will provide genetically unbiased estimates of true environmental effects in the population. Unfortunately, adoptive families are selected by agencies for being above average in many virtues, including socioeconomic status. Thus, they are always an unrepresentative sample of the population to which one would like to generalize. Although it is possible that the adoptive family coefficients on the background are good estimates of the population values, it is difficult to know without modeling the way in which the families were selected. An easier corrective for the possible bias of selected adoptive families is to have a comparison sample of biologically related children in the same adoptive families or a sample of biological families that are similarly selected.
Constance Holden (2000) also presented his famous work on the subject of identical twins reared apart.
“Holden reports on a study that emphasizes the importance of heredity In shaping human behavior. She tells us that investigators were often astonished at similarities between long-separated twins, similarities that are or (primarily attributed to common environmental Influences”.
Holden gives examples of few twins reared apart; the most interesting case is of famous British twins. Holden reports:
“Bridget and Dorothy are 39-year-old British housewives; identical twins raised apart who first met each other a little over a year ago. When they met to take part in Thomas Bouchard’s twin study at the University of Minnesota, the manicured hands of each bore seven rings. Each also wore two bracelets on one wrist and a watch and a bracelet on the other”.
Holden describes various features of the Minnesota study in which identical traits of twins were studied and analyzed.
The Minnesota study is unprecedented in its scope. Using a team of psychologists, psychiatrists, and medical doctors to probe and analyze every conceivable aspect of the twins’ life histories, medical histories and physiology, tastes, psychological inclinations, abilities, and intelligence.
The case of Jim twins clearly shows that traits of twins quite resemble regardless of different environments.
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“Take the “Jim twins,” as they have come to be known. Jim Springer and Jim Lewis were adopted as infants into working-class Ohio families. Both liked math and did not like spelling In school. Both had law enforcement training and worked part-time as deputy sheriffs. Both vacationed in Florida; both drove Chevrolets. Much has been made of the fact that their lives are marked by a trail of similar names. Both had dogs named Toy. Both earned and divorced women named Linda and had second marriages with women named Betty. They named their sons James Allan and James Alan. Respectively. Both like mechanical drawing and carpentry. They have almost identical drinking and smoking patterns. Both chew their fingernails down to the nubs”. (379)
Holden admits that twins also show the difference in their traits in some cases.
“The twins also have their differences. One wears his hair over his forehead; the other has It slicked back with sideburns. One expresses himself better orally, the other In writing. But although the emotional environments in which they were brought up were different, the profiles on their psychological inventories were much alike.”
Holden gives an example of Bridget and Dorothy, along with another twin sister Barbara and Daphne, who show great identical qualities.
“Other well-publicized twin pairs are Bridget and Dorothy, the British housewives with the seven rings, and Barbara and Daphne, another pair of British housewives. Both sets are now hi their late 30’s arid were separated during World War IL Bridget and Dorothy are of considerable interest because they were raised In quite different socioeconomic settings—the class difference turns out mainly to be reflected In the fact that the one raised In modest circumstances has bad teeth, Otherwise, say the investigators, they share “striking similarities.”
“The other British twins, Daphne and Barbara, are fondly remembered by the investigators as the “giggle sisters.” Both were great gigglers, particularly together, when they were always setting each other off. Asked If there were any gigglers in their adoptive families, both replied in the negative. The sisters also shared identical coping mechanisms in the face of stress: they Ignored it, managed to “read out” such stimuli; in keeping with this, both flatly avoided conflict and controversy—neither, for example, had any interest In politics. Such avoidance of conflict is “classically regarded as learned behavior,” says Bouchard. Although the adoptive families of the two women were not terribly different, “we see more differences within families than between these two.” (382)
Holden concludes that the study of identical twins reared apart gives very important knowledge about the impact of heredity and environment on the traits of twins.
“The twin study may also make it clear that estimating the relative contribution of heredity and environment to mental and psychological traits can never be boiled down to percentages. Sonic people, for example. They may have authoritarian personalities no matter what their upbringing; the authoritarianism of others may be directly traceable to their environment. Similarly, with Intelligence, some people may be smart or dumb regardless of outside influences, whereas the intelligence of others may be extremely malleable.”
By far the greatest interest attaches to a comparative study of identical twins reared together with identical twins reared apart; but until more data, accurate as to detail, are assembled on such twins separated in infancy, it is impossible to draw final conclusions on the effects of differences in the environment on the development of test intelligence. Workers must be cautioned against making deductions that, merely because the twins have been separated, their environments have been essentially “different.” It is quite possible for two separate foster homes, having relatively similar standards and requirements, to be highly “similar” (e.g., as in the case of Bridget and Dorothy). It is equally possible, on the other hand, for two different environments to obtain within the one physical home as illustrated in the case of Barbara and Daphne. To capture these subtle factors and operations of the environment, rather careful measuring devices must be standardized and employed.
If the environments of separated pairs–individuals or groups-were rated quantitatively, new interpretations would be inevitable; and the extent to which degrees of specific environment corresponded with intellectual variation could be found. In the meantime, we have to depend, much less accurately, on social history reports based in large part on highly uncertain personal memory, opinion, and anecdotal account.
The degree of similarity in test intelligence reported for identical twins reared together was go. For siblings reared together, the average is around, indicating a very much lower degree of resemblance, a result which has been subject to constantly changing interpretation ever since studies of this sort were inaugurated by Francis Galton over half a century ago. The earlier investigators, apart from the inadequacy of their tools of measurement, did not tend to allow for similarities of environment roughly proportional to the degree of genetic relationships and therefore reached conclusions on the heredity-environment problem by comparing correlations for twins with correlations for siblings. Furthermore, they lacked knowledge of the mechanism of heredity, and hence were led to draw conclusions by comparing correlations of general intelligence with those of the simplest physical traits such as eye color; and while their reasoning is impressive and convincing to the layman, it will not stand up under careful analysis, especially an analysis made in the light of modern knowledge of genetics. Reasoning from physical similarity to consequent mental similarity is also without justification.
If human matings took place only between individuals of the same stock, and only within stocks homologous for all their traits (that is, composed of a number of identical individuals), then we could expect a perfect correlation between sibs in respect of those traits which are hereditary and not affected by the environment. The index of correlation for each trait would be a measure of the extent to which the trait had been affected by such differences in the environment as exist between sibs.
But human matings rarely take place between individuals of the same stock, and no human stock is homologous except identical twins or triplets, each group of which is necessary of the same sex and cannot reproduce within the group. The purest family lines Twin studies indicate that this familial resemblance is due to heredity rather than a shared family environment. Again, genetic influence is substantial, and nonadditive genetic variance appears to be important. Twin concordance is positively related to the severity of the cases.
These studies also point to substantial influence of the environment; for example, concordance for first-degree relatives is a long way from 50%. One of the most important findings, however, is that a shared family environment is unimportant: Familial resemblance is just as great when biological relatives are adopted apart as when they live together in the same family. Studies of identical twins discordant for schizophrenia have not been successful in identifying the source of these within-family environmental influences.
So far, no specific environmental source of liability is known; the most likely environmental contributor, stress, may come from many sources and, apparently, may come during any stage of development. Prenatal or birth complications, early deprivations, broken homes, censuring parents, the death of someone close, failures in school, poor work or social relations, childbirth, a bad drug trip, as well as all kinds of good fortune may have effects on a predisposed individual that are obvious only in retrospect. In prospect, it will be impossible to prophesy the events themselves, let alone their effects.
Adoption studies add substantially to our knowledge about the genetics of intelligence in adulthood. One adoption study of IQ compared resemblance in adoptive families to resemblance in nonadoptive families in which the children were from 16 to 22 years of age. In other words, by the time they are adults, offspring resemble their parents in IQ primarily for hereditary reasons and only slightly for reasons related to the fact that they shared a family environment with their parents.
Much has been written about the relatively few cases of identical twins reared apart, in part because this particular adoption design is so easy to understand. If pairs of genetically identical individuals are reared in uncorrelated environments, their correlation directly estimates heritability. However, the use of this design is severely hampered by the rarity of such twin-only 69 pairs that have been reported.
Studies of twins reared apart are currently being conducted in the United States (the Minnesota Study of Twins Reared Apart; Bouchard, 1984) and in Sweden ( Pedersen, Friberg, Floderus-Myrhed, McClearn, & Plomin, 1984). These newer studies promise to add substantially to the quantity and quality of the literature on twins reared apart. A preliminary analysis of the Minnesota study data on 29 pairs of identical twins reared apart, whose average age at testing was yielded a twin correlation of.58 for Raven’s Progressive Matrices.
The most dramatic evidence for genetic influence on adult personality comes from adoption studies involving identical twins adopted apart. In two older studies, identical twins reared apart were as similar as identical twins reared together for extraversion and neuroticism, which is consistent with the hypothesis that growing up in the same family with a sibling has little effect on personality resemblance. In the ongoing Minnesota Study of Twins Reared Apart, the median correlation for 28 pairs of twins reared apart is.65 for Tellegen’s Differential Personality Questionnaire as compared to a correlation of.53 for identical twins reared together and.27 for fraternal twins reared together ( Bouchard, 1984). A second-order factor called “negative affect” is similar to neuroticism and yields a correlation of.64 for the separated identical twins.
- Bouchard, T. J., Jr.. Twins reared together and apart: What they tell us about human diversity. In S. W. Fox (Ed.), Individuality and determinism (pp. 147-178). New York: Plenum. ( 1984)
- Holden Constance, “Identical Twins Reared Apart, ” Science 2000.