The answer is quite simple, as long-time friends and partners can attest: shared experiences. As for the astonishing stories of one twin knowing the other is in danger, these anecdotes tend to have more mundane explanations when we dig into them.
The case of Gemma and Leanne Houghton sure seems paranormal. The two twins were teenagers and Gemma suddenly felt compelled to check on her sister. But as the deputy editor for Skeptical Inquirer magazine, Benjamin Radford, pointed out in an article on twins, Leanne had a history of seizures and her sister had been told to keep an eye on her.
No telepathy needed. But the most persistent myth about so-called identical twins is baked into the name. Monozygotic twins are not identical, and this realization has implications for scientific research.
We expect monozygotic twins to have identical DNA. Given that our genome consists of 3 billion letters, comparing the DNA of monozygotic twins to see if it really is identical used to be impossible. Shortcuts had to be employed, like testing for specific regions of the DNA known to vary a lot.
However, improvements in technology and in its affordability now allow scientists to take a much deeper look, and they are finding out that many monozygotic twins are not identical at the DNA level after all. When scientists recently read the DNA of twin pairs , they reported that, on average, so-called identical twin pairs differed by 5. Find a Pediatrician. Text Size. Page Content. The information contained on this Web site should not be used as a substitute for the medical care and advice of your pediatrician.
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Family Dynamics. Differences within pairs of identical twins reared together provides a direct estimate of nonshared environment as experienced by identical twins. Because we are developmentalists, we feel compelled to make the point that all components of variance can change during development. Estimates of genetic and environmental components of variance depend upon the age of the subjects sampled.
Genetic change during development is the focus of a new subdiscipline, developmental behavioral genetics 7. Nonshared and shared environmental components can also change during development.
Research is needed to trace the developmental course of shared and nonshared environmental variance. For example, there may be a general trend for nonshared environmental variance to increase with age as individuals expand their social and environmental networks beyond the family.
On the other hand, as this happens, there may be fewer forces contrasting children in the same family. Research throughout the lifespan - especially research past adolescence - will be needed to resolve such developmental issues. One striking example of developmental change in the relative influence of shared and nonshared environmental variance serves to indicate the potential usefulness of a lifespan perspective: For IQ, the shared environment component of variance diminishes dramatically after childhood, as discussed in the next section.
This section provides a brief summary of behavioral-genetic research in personality, psychopathology, and cognition that leads to the conclusion that the most important source of environmental variance is nonshared environment. This material is based on a recent review of behavioral-genetic research throughout the lifespan which can be consulted for additional studies and details 7. Although readers might take issue with the precise magnitude of one or another of the estimates, the forest should not be overlooked for the trees.
Our point, one that to our knowledge has not been disputed, is that nonshared environment is responsible for most environmental variation relevant to psychological development. Thus, our goal in the following section is not to provide an encyclopedic review of behavioral-genetic studies but rather to summarize the results to the extent needed to understand their message regarding the importance of nonshared environment.
The importance of nonshared environment was first highlighted by Loehlin and Nichols 8 whose twin analyses of personality data led to the following conclusion: Thus, a consistent - though perplexing - pattern is emerging from the data and it is not purely idiosyncratic to our study. In a review of 10 recent twin studies of personality 9 , the average twin correlations were. It might seem odd to report average correlations across a domain as diverse as personality.
Nonetheless, the twin results are generally similar across the dozens of traits measured by self-report questionnaires. A study of over 12, adult twin pairs in Sweden 12 revealed twin correlations of.
Similar results emerge for less central dimensions of personality as well. The identical and fraternal twin correlations, respectively, for these scales were. Another example involves twin results for a new personality questionnaire, the Differential Personality Questionnaire, which assesses nontraditional dimensions of personality. A twin study of over identical twin pairs and over fraternal twin pairs yielded the following sampling of correlations for identical and fraternal twins, respectively:.
The only personality trait that appears to show significant shared environmental influence is masculinity-feminity, which one might argue falls more in the category of attitudes than personality These twin studies used self-report questionnaires. Other assessment procedures, however, yield similar results.
The average identical twin correlation is about. The few twin studies that have used objective observations of personality yield somewhat less ubiquitous evidence for nonshared environmental variance than do paper-and-pencil questionnaire Nonetheless, estimates of nonshared environmental influence from these studies are still substantial - usually greater than estimates of shared environmental variance, even when error variance is taken into account.
Studies of nontwin siblings and other family relationships confirm the hypothesis that shared family environment accounts for a negligible amount of environmental variance relevant to personality development.
For example, one of the earliest studies found an average sibling correlation of. Four recently reported adoption studies of personality indicate that this modest familial resemblance is not due to shared family environment - the average adoptive sibling correlation is. Adoptive sibling correlations are also low in the first report of infant adoptive siblings, involving 61 pairs at 12 months and 50 pairs at 24 months tested as part of the Colorado Adoption Project Parental ratings of temperament yielded average adoptive sibling correlations of.
Behavioral-genetic data on psychopathology are also consistent with the conclusion that environmental variation is preponderantly of the nonshared variety. Research on schizophrenia is difficult to summarize briefly because concordance rates vary widely depending on the following: whether or not age correlations are used, the type of diagnostic criteria used, and the selection and severity of probands.
Concordance rates for identical twins are substantially higher than those for fraternal twins - indeed, higher than would be expected on the basis of a simple additive genetic model in which identical twins would be about twice as similar as fraternal twins.
Regardless of the complications this pattern of twin concordance causes for estimates of genetic influence, the results indicate that most schizophrenic identical twins do not have an affected cotwin. Because these are genetically identical pairs of individuals, nonshared environment must be the reason for these striking differences within pairs of identical twins.
Thus, sharing the same family environment with a schizophrenic relative does not increase familial concordance. Gottesman and Shields 25 also review attempts to isolate environmental sources of variance and conclude: 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 relationships, 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 25 []. Research on manic-depressive psychosis yields results similar to those for schizophrenia 3. Environmental influences on less severe forms of psychopathology, such as neuroses and alcoholism, also appear to be predominantly nonshared. In other words, the most important influences on psychopathology lie in the category of nonshared environment.
Much more often than not, affected children in families with more than one child will have unaffected siblings. Until recently, environmental variance that affects individual differences in IQ was thought to fall primarily in the category of shared environment. In 11 studies, the average IQ correlation for adoptive siblings is. Twin studies agree: The average IQ correlation in over 30 studies is.
Although these data appear to converge on the reasonable conclusion that shared environment accounts for a substantial portion of environmental variance relevant to IQ, doubts have begun to arise. For fraternal twins, who share environment to a greater extent than do nontwin siblings, the IQ correlation is about. The crucial piece of evidence in support of substantial shared environmental variance is the correlation of.
These studies have included adoptive siblings still living at home, with two exceptions. This unsettling finding implies that shared environment is important for IQ during childhood when children are living at home and then fades in importance after adolescence when children have left home.
The hypothesis that shared environmental influences have no lasting impact on IQ is supported by results of a recent study of adoptive and nonadoptive siblings The study included 52 pairs of adoptive siblings and 54 pairs of nonadoptive siblings ranging from 9 to 15 years of age, with the average age of 13 years.
A battery of cognitive ability measures was developed for administration over the telephone; this battery correlated with face-to-face testing near the reliabilities of the tests.
An unrotated first principal component, used as an index of IQ, yielded a reasonable correlation of. A similar pattern of results emerged for specific cognitive abilities. Thus, this study leads to the conclusion that shared environmental influence on IQ and specific cognitive abilities is of negligible importance by the end of early adolescence.
Because these estimates of shared environmental influences were obtained directly from adoptive sibling correlations, reasonable confidence can be attached to this conclusion. For example, the sample of 52 pairs of adoptive siblings permits detection of a true correlation of.
In summary, nonshared environmental influence is a major component of variance for personality, psychopathology, and IQ after childhood. Although one can quibble with the magnitude of our estimates, they would have to be substantially in error before they would affect our argument that most of the environmental variance is nonshared.
The purpose of this section is to consider some conceptual details of the distinction between shared and nonshared environment before discussing sources of nonshared environment. These details include other labels for shared and nonshared environment, the distinction between environmental components of behavioral variance and the relationship between specific environmental measures and behavior, the impact of nonshared environmental influence on the development of singletons, genotype-environment correlation and interaction, and model-fitting.
Shared and nonshared environmental influences were named by Rowe and Plomin in , although the distinction between environmental influences that contribute to the resemblance between relatives and those that do not has been implicit in quantitative genetics since its inception.
Many labels have been used to refer to these two components of environmental variance. Shared environmental influence has been called E2, between-family, and common environmental variance, labels that have been used to refer to nonshared environmental include El, within-family, individual, unique and specific environmental variance.
Rowe and Plomin suggested that the symbols El and E2 31 are probably best in that they carry no connotations, although they have the distinct disadvantage that they provide no mnemonic to remember which is which. Within- and between-family environment are the terms most often used. They are useful for those familiar with the terminology of analysis of variance which considers variance within and between groups.
Variance within families refers to differences among family members and variance between families describes resemblance among family members. For these reasons, we suggest that the most descriptive and straightforward terms to use are shared and nonshared. It should be noted that this discussion pertains to environmental components of behavioral variance, not to the relationship between specific environmental measures and behavioral measures. That is, the total impact of genetic variability on phenotypic variability will be detected regardless of the complexity of the genetic effects - for example, whether the genetic effects arise from variability in structural genes that code for polypeptides or from regulatory genes.
Similarly, quantitative genetics estimates the bottom line of environmental influence, regardless of the specific mechanisms by which environmental factors affect behavior.
Although this components of variance approach may be unsatisfying for those who would like to know which specific genes and which specific environmental factors are responsible for the components of variance, it seems to be a reasonable first step to ask about components of variance - without this tack, we would not have discovered that nearly all environmental variance is of the nonshared variety.
It is a major strength of the approach that it can reveal the presence of genetic and environmental influences even when these are not assessed directly. Attempts to isolate specific environmental factors will be presented later. A related issue, however, should be mentioned at this time.
Traditional environmental research attempts to relate measures of family environment to measures of behavior of one child per family. The yield from such research has been disappointing, especially if one considers the amount of variance explained Knowing this research, one might ask why such environmental factors as parental affection should be important within families when they account for little variance in behavior across families.
That is, if it makes little difference that some parents love their children more than other parents love their children, why should parental love make a difference within families if a parent loves one child more than another?
The answer is that there is no necessary relationship between the causes of differences between families and the causes of differences within families. That is, environmental factors that create differences within families can act independently of factors that cause differences between families. For example, a child really knows only his own parents; the child does not know if his parents love him more or less than other parents love their children.
A child is likely to be painfully aware, however, that parental affection toward him is less than toward his sibling. How does nonshared environment relate to singletons?
In general, reasons why two children in the same family differ are likely to yield clues as to the environmental source of variance for singletons as well. The easiest example involves nonsystematic events such as accidents and illnesses which are just as likely to befall singletons. However, systematic nonshared influences may also be found to affect singleton variance. For example, if certain characteristics of peer groups differ within pairs of siblings and contribute importantly to behavioral differences within sibling pairs, it is likely that these characteristics also contribute to variance for singletons.
Obviously, singletons do not have siblings with whom they interact; thus, this potential source of nonshared environment cannot contribute variance for singletons. Although it might seem at first that differential parental treatment of two children in the same family is irrelevant to singletons, it is possible that, once identified, such factors might contribute to the variance of singletons.
There is evidence that parents with more than one child treat the children similarly if we look at the children at the same age, which suggests that parental treatment is not an important source of nonshared environment Except for twins, however, siblings are not the same age, and when we examine contemporaneous parental treatment of children of different ages, we find that parents treat the children differently Differences in parental behavior during development can also affect singletons in that parents will treat their singleton children differently during the course of development.
Thus, studies of differences within pairs of siblings are likely to illuminate factors responsible for singleton variance as well as sibling variance. The important point in the present context is the obvious one: that the study of singletons cannot isolate factors that make two children in the same family different from one another.
Because this is the best clue we have as to the source of environmental variance relevant to psychological development, it makes sense to focus on environmental sources of differences between children in the same family. Two complicating factors in the estimation of quantitative genetic parameters are genotype-environment correlation and genotype-environment interaction Genotype-environment correlation refers to an increase in phenotypic variance that occurs when children experience environments correlated with their genetic propensities.
Phenotypic variance can also be due to genotype-environment interaction when children respond differently to the same environment because of genetic differences among them.
What are the effects of genotype-environment correlation and interaction on estimates of shared and nonshared environment? Consider a direct estimate of nonshared family environment: the extent to which the correlation for identical twins reared together is less than 1.
This estimate will not include either genotype-environment correlation or interaction because identical twins are identical genetically; thus, in terms of genetic propensities, identical twins will correlate and interact with the environment in a similar manner. Similarly, the direct estimate of shared family environment - the correlation for unrelated children reared together - will not include genotype-environment correlation or interaction because these children are genetically uncorrelated; thus, in terms of their genetic propensities, they will correlate and interact with the environment in ways that do not add to their resemblance.
However, estimates of nonshared or shared environment derived as the remainder of phenotypic variance after other components of variance are taken into account can be affected by genotype-environment correlation and interaction because of their effects on estimates of genetic variance Fitting models to adoption and twin data is a powerful way to estimate quantitative genetic parameters Although model-fitting techniques differ in their specifics, they all express family resemblance in terms of an underlying model consisting of several unobserved genetic and environmental parameters.
The approach is powerful because it makes assumptions explicit, it tests a specific model, and it can incorporate into a single analysis different types of data, such as family and adoption data, rather than analyzing each type of data separately. Model-fitting procedures, however, only find significant parameters when they are implicit in the basic data: for example, in a study of adoptive siblings, a reasonable model-fitting analysis will estimate significant shared family environmental influences only if the correlation for adoptive siblings is significant.
For this reason, and because of the relative inaccessibility of most models, we have emphasized the basic correlational data and merely note that model-fitting approaches confirm our conclusions.
What is happening environmentally to make children in the same family so different from one another? One gloomy prospect is that the salient environment might be unsystematic, idiosyncratic, or serendipitous events such as accidents, illnesses, and other traumas, as biographies often attest. In his autobiography, Darwin noted one example: The voyage of the Beagle has been by far the most important event in my life, and has determined my whole career; yet it depended on so small a circumstance as my uncle offering to drive me thirty miles to Shrewsbury, which few uncles would have done, and on such a trifle as the shape of my nose 37 [p.
It is possible that nonshared environmental influences could be unsystematic in the sense of stochastic events that, when compounded over time, make children in the same family different in unpredictable ways. Such capricious events, however, are likely to prove a dead end for research. More interesting heuristically are possible systematic sources of differences within families. Table 1 describes categories of environmental factors that could lead to observed differences between children in the same family.
These include such systematic sources of nonshared influence in the family as birth-order and gender differences of siblings, interactions between siblings, differential treatment by parents, and extrafamilial influences such as peers. Categories of environmental influences that cause children in the same family to differ.
Source : Adapted from Rowe and Plomin Fraternal twins are also called dizygotic twins. And the difference between fraternal and identical twins is that fraternal twins derive from two different eggs. Fraternal twins may be the same gender, they may have many of the same characteristics, but also may be very different from each other and, in fact, share half of their genes just like their sisters and brothers.
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