Are twins different from singletons during early childhood?

This paper examines the specificity of intrafamilial experiences to which twins are exposed during early childhood, and the impact of this family context on the psychological development of twins. An overview of the literature on the psychological development of twins indicates two broad categories of findings. The first category covers studies dealing with the developmental features that appear early in twins. Although there are numerous studies comparing the psychomotor and verbal development of twins to singletons, only a few have dealt with the emergence of personality and identity, and most have been conducted in clinical settings. The second set of studies deals with mother-twin relations, explored in recent years in terms of the psychosocial stress created by twin births. Few studies have examined the mother-twin relationship in the light of major theories on mother-child bonding (psychoanalysis, attachment, interactionist model). These developmental models have mainly focused on mother-infant dyads, while the construction of this specific mother-twin relationship needs to be set within the framework of a triadic situation. Rather than describing a specific ‘twin mentality’, it is argued here that the twin situation should be seen as a special case of adaptative development, both for the mother and the child, which has effects on the formation of the individual.     

Observed Mother– and Father–Child Interaction Differences in Families with Medically Assisted Reproduction‐Conceived Twins and Singletons

Increased medically assisted reproduction (MAR) use to treat infertility has resulted in a growing twin birth rate. Little is known about parent–child relationships for twin relative to singleton children in middle childhood. This study fills this gap by examining parent–child relationships in 57 families with eighty 6‐ to 12‐year‐old MAR twin and singleton children using observational data (warm and supportive communication, control, and hostility). Nested ANCOVAs indicate that while mothers exhibit similar interactional behaviors toward twins and singletons, fathers have less optimum behaviors toward twins relative to singletons. Twins displayed less engaged behavior with mothers and fathers relative to singletons. Given the vitality of parent–child relationships for family and child adjustment, future studies should examine determinants and outcomes of twin–singleton relationship differences to bolster twins’ and their families’ functioning.     

Sharing genes fosters identity fusion and altruism

Researchers have shown that the more genes twins share, the more they care about one another. Here, we examine a psychological mediator of such genetic influences, “identity fusion” (a visceral sense of oneness with them). Results supported this hypothesis. Relative to dizygotic twins, monozygotic twins reported stronger fusion and elevated desire to have contact and share experiences with their twin (Study 1), to forgive and grant favors to their twin after being disappointed by him/her (Study 2), and willingness to make sacrifices for their twin (Study 3). Fusion with the twin mediated the impact of zygosity on these outcomes. These findings demonstrate that genetic relatedness fosters a powerful feeling of union with one’s twin that predicts sharing, tolerance, and self-sacrificial behavior toward him or her.     

Problems in Psychiatric Genetic Research: A Reply to Faraone and Biederman

This article examines evidence cited in favor of the operation of genetic factors in attention-deficit hyperactivity disorder (ADHD). Like other psychiatric conditions, a belief in the genetic basis of ADHD is derived from the results of family, twin, and adoption studies. Because family studies are widely believed to be confounded by environmental factors, primary emphasis is placed on twin and adoption studies. ADHD twin studies depend on the validity of the equal environment assumption (EEA), which holds that the environments of identical (MZ) and fraternal (DZ) twins are the same. Here it is argued that however the EEA is defined, it cannot be accepted. Therefore, the greater similarity or concordance of MZ twins when compared to DZ twins is plausibly explained by environmental factors. Adoption studies constitute a third method for investigating the role of genetic factors in ADHD. It is argued that these studies are greatly flawed by factors including non blinded diagnoses and the failure to study the biological relatives of adoptees. After an examination of the total weight of evidence in favor of a genetic basis or predisposition for ADHD, it is concluded that a role for genetic factors is not supported and that future research should be directed toward psychosocial causes     

The effects of shared environment on adult intelligence: A critical review of adoption, twin, and MZA studies

There has been a vigorous debate for decades concerning the heritability of intelligence. In recent years, the debate has been focused on whether the components of IQ variability change with age and on separating environmental effects into shared and unshared components. Citing evidence from adoption studies, studies comparing identical and fraternal twins, and studies of identical twins raised apart, some prominent psychologists have concluded that the shared environment has a significant effect on the intelligence of children but little or no effect on the intelligence of adults. In this article, the evidence from such studies is reviewed. The article reaches the conclusion that while there is some evidence from adoption studies supporting the claim that shared environment has little or no effect on adult intelligence, that evidence is inconclusive and is inconsistent with evidence from twin studies and from studies of identical twins reared apart. (PsycINFO Database Record (c) 2012 APA, all rights reserved).     

Why aren't identical twins linguistically identical? Genetic, prenatal and postnatal factors

Results of twin studies clearly demonstrate that genetic factors play an important role in the rate of language acquisition and linguistic proficiency attained by normal and impaired children and adults [see Stromswold, K. (2001). The heritability of language: A review and meta-analysis of twin, adoption and linkage studies. Language, 77, 647-723.]. That said, twin-based heritability estimates for language rarely exceed .6 and monozygotic (MZ) twins (who are usually assumed to have identical genetic and environmental endowments) sometimes have very different linguistic profiles. In addition, twins are more likely to suffer linguistic delays and impairments than singletons. Postnatal factors, such as differences in linguistic input twins receive, are usually assumed to be the major reason for these findings. This paper discusses how genetic, epigenetic, and perinatal environmental factors can lower heritability estimates for language, cause MZ twins to be linguistically discordant, and increase the risk of language impairments in twins. We present results from our ongoing Perinatal Environment and Genetic Interaction (PEGI) study that suggest that perinatal environmental factors affect linguistic development more than postnatal factors, and that postnatal factors affect cognitive development more than perinatal factors. Because perinatal factors are overwhelming biological, whereas postnatal factors tend to be psychosocial (e.g., how and how much parents speak to their children), these results support nativist/biological theories of language and language development and call into question empiricist/emergentist theories. These results are also consistent with modularist theories of language. We end by suggesting new methods that can be used to tease apart the effects of prenatal and postnatal environment and to investigate how these factors interact with genetic factors.     

The equal environments assumption of classical twin studies may not hold

The classical twin method ‐ comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins ‐ in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far‐reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well‐known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio‐cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ—DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems.     

Left-handedness in twin families: support of an environmental hypothesis

Information on handedness, assessed as writing-hand, was collected from 197 MZ twin pairs and 203 DZ twin pairs and from their parents, spouses, and children. Associations for pairs of relatives were studied by 2 X 2 tables, computing chi squared-values and tetrachoric correlations. Correlations of about .3 were obtained for mother-offspring and sibling pairs, while for cousins the correlation was .25 (in a small sample). No other significant associations were found, not even for twins. These results suggest only a small, if any, genetic effect, and only a small environmental between family effect, most of which seems to be a maternal effect. The lack of cotwin correlations and correlations for one of the twins with her/his ordinary brothers/sisters suggest the existence of an effect specific to twins, since the correlation for pairs of ordinary siblings is significant. Conclusions drawn from twin studies alone may be biased. The frequency of left-hand writers has increased from 1% to 10% in the different age groups born during the last century in Norway. This age effect is present mainly in hand writing and not so much in handedness generally, suggesting a decrease in cultural repression against left-hand writing.     

Separated twins and the genetics of personality differences: a critique

This article discusses studies of separated twins, with special emphasis on the Minnesota Study of Twins Reared Apart (MISTRA), to determine whether they support the existence of an important genetic component in behavioral and personality differences. The methods and conclusions of the MISTRA team are discussed in the context of earlier studies of separated identical twins. I argue that volunteer-based studies are biased toward greater twin similarity. In addition, the MISTRA research team did not publish or share raw data and case history information. Reared-together and reared-apart monozygotic twins share important environmental similarities not controlled for by comparing personality correlations. I propose an alternative control group consisting of biologically unrelated pairs of strangers matched on all environmental factors common to pairs of separated monozygotic twins. I conclude that the evidence from studies of twins reared apart does not support the role of genetic factors in personality and behavioral differences.     

Genetic and Rearing Environmental Influences on Adult Personality: An Analysis of Adopted Twins Reared Apart

ABSTRACT We report a genetic and environmental analysis of California Psychological Inventory (CPI) scale scores gathered on a sample of 45 sets of monozygotic twins reared apart (MZA) and 26 sets of dizygotic twins reared apart (DZA) Analysis of twin intraclass correlations and the results of models fit to the twin data demonstrate that the heritability of most scales and five factors of the CPI is about 50 When compared to results from studies of adult MZ and DZ twins reared together few of the scales demonstrate any common family environmental influence Placement coefficients on the Family Environment Scale (FES) can explain only a minor portion of the correlations for twins reared apart The influence of specific rearing environmental factors on adult personality was evaluated by analyzing the relationship between the FES and the CPI in this adoptee sample One FES factor (Cohesion vs Conflict) does correlate substantially with the CPI factor of Consensuality and may account for up to 24% of the variance in that factor, but the retrospectively gathered     

Twin imitation for antisocial behavior: implications for genetic and family environment research.

When twin pairs influence each other's behavior, observed variance is greater for MZ twins than for DZ twins under at least 1 of 2 conditions: (a) the trait has some heritability and (b) MZ twins influence each other more than do DZ twins. Applied to a trait that has an underlying continuous distribution but is measured as a dichotomy, the presence of reciprocal twin influence predicts that if the base rate for the trait is not exactly 50%, then the prevalence of the trait should differ in MZ and DZ twin pairs. This prediction held for registered criminality in a large twin cohort. Methods of analysis that permit reciprocal twin interaction not only provide better statistical fits to the data but also yield estimates of heritability that agree with adoption data. The results suggest that the genetic influence on registered criminality may be more modest than previously thought.     

Maternal speech style with prelinguistic twin infants

The mother–infant communicative speech of a group of mothers of 4‐month‐old first‐born twin infants was compared to the speech of a group of mothers of first‐born singleton infants. Maternal groups were matched on age, education level, mother–infant attachment status and infant gender, and maternal depression was assessed as a control variable. Maternal speech was coded for focus, content, complexity and syntax of mothers' utterances. The findings of earlier studies with toddler age twins, that maternal speech style was more directive and less infant‐focused, were replicated in this prelinguistic period of infancy. Compared to mothers of singletons, mothers of twins used less infant‐focused speech, were less responsive to their infants' cues, and attributed less agency to their infants. Mothers of twins also used fewer questions and requests but did not differ from mothers of singletons in their use of negatives and imperatives. These early differences in the language learning environments of twin and singleton infants may be due to the reduced opportunities that mothers of twins have to establish dyadic communicative routines with their infants and to familiarize themselves with their infants as interactive partners, and may have implications for the early language development of twins. Copyright © 2002 John Wiley & Sons, Ltd.     

Burt's twins: a question of numbers

This article estimates the number of monozygotic twins raised apart (MZA) potentially available to Cyril Burt in his twin studies. It concludes that between 77,000 and 88,000 monozygotic (MZ) twin pairs would have been born and survived to the age of ten in the relevant period in England and Wales, and that between 750 and 1,750 surviving pairs are likely to have been separated during childhood. Burt's claim to have collected 53 MZA pairs is therefore not inherently implausible. However, when all the other constraints on Burt's studies are taken into account, his numbers are problematic. There are also serious difficulties with his claim to have studied 12 pairs born to middle-class parents. It is highly improbable that Burt could have located a majority of these cases through personal contacts as he claimed.     

Mothers' representations of their 13-month-old twins and child-raising attitudes

The purpose of this study was to describe maternal representations of the twin sibling relationship and of the children's differences and similarities. Mothers' mental representations were also analyzed with regard to two aspects of their child-raising attitudes: individualization of childcare routines, and the question of differentiation by “external signs of twinship” (dress, toys, and first names). Data from interviews with 44 mothers of twins conducted 13 months after the birth were analyzed in terms of 8 categories—complicity, rivalry/jealousy, agonistic relations, dominance, imitation, physical appearance, development, and personality. A multiple correspondence analysis of the data pointed out that the first dimension describing the mothers' representations concerns the contrast between the “unalike” representation of dizygotic twins and the “alike” representation of monozygotic twins. The second dimension corresponds to the casual, twinship-minimizing representations generated by the twin condition. Two other dimensions express the issues of aggression and dominance. The relationship between the child-raising behavior of the mothers and their representations of the twin couple turned out to be more complex than expected. The results are discussed in terms of the specificity of the psychological issues facing mothers of twins. Defense mechanisms used by some mothers to cope with the exceptional nature of the twin phenomenon are also suggested. © 1998 Michigan Association for Infant Mental Health     

Genetic mediation of the relationship between social support and psychological well-being.

This study assessed genetic and environmental influences on the association between social support and psychological functioning using the combined adoption/twin design from the Swedish Adoption/Twin Study of Aging. A subsample of 424 twin pairs, 50 years and older, was used: 64 pairs of identical twins reared apart, 95 pairs of identical twins reared together, 132 pairs of fraternal twins reared apart, and 133 pairs of fraternal twins reared together. Multivariate model-fitting analyses indicated that the relationship between the perceived adequacy of social support and psychological well-being (depression and life satisfaction) was mediated in part by genetic factors.     

Growing up and growing apart: a developmental meta-analysis of twin studies

Developmental change in twin similarity was examined with age contrasts in a meta-analysis of twin studies from 1967 through 1985. Intraclass rs were coded from 103 papers that included data for monozygotic or dizygotic twins, or for both, on personality or intelligence variables. Analyses indicated that there was a general tendency for some intraclass rs to decrease with age. In other words, as twins grow up, they grow apart. There were also developmental differences associated with components of variance for heritability, the shared environment, and the nonshared environment. Mechanisms through which the nonshared environment may operate are discussed.     

The nature of adult twin relationships: an attachment-theoretical perspective

Twin relationships have been hailed as one of the most unique and intimate kinds of relationships. Unfortunately, there is a paucity of empirical research that addresses the interpersonal nature of twin relationships. In this article, the authors argue that attachment theory may provide a useful framework for understanding the nature of twin relationships. The authors present data indicating that (a) twins are more likely than non-twin siblings to use their sibling as an attachment figure; (b) the developmental course of twin attachment differs from that of other attachments; and (c) certain factors, such as genetic relatedness, empathy, including the other in the self, and shared experiences, may impact the extent to which twins use one another as attachment figures.     

Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness

Game theorists have long been interested in why some partners are more successful at achieving mutual goals than others. The present study was conducted to determine if tacit coordination (non-negotiated consensus) varies as a function of genetic relatedness between social partners. The sample included 7–13-year-old monozygotic (MZ) twin pairs (n = 53), dizygotic (DZ) twin pairs (n = 85) and virtual twin (VT) pairs (n = 42; same-age unrelated siblings). Co-twin agreement on questions completed under individual and coordination conditions was compared across the three groups. MZ twins showed significantly greater overall agreement than DZ twins and VTs, using multiple criteria. The implications of these findings are considered with reference to behavioral-genetic and evolutionary theories of behavior.     

Don Jackson's "A critique of the literature on the genetics of schizophrenia": a reappraisal after 40 years

In 1960, Don Jackson published "A Critique of the Literature on the Genetics of Schizophrenia." Jackson's critique nearly relegated the classical twin method to obscurity as being hopelessly confounded by environmental factors. Jackson noted several trends in the schizophrenia twin data that were difficult to explain on genetic grounds. In fact, none of Jackson's 12 major points, examined in this article, have been satisfactorily answered by proponents of the twin method. The evidence in support of Jackson's most controversial claim-that the psychology of twinship itself might lead to a greater rate of schizophrenia among twins when compared with the single-born population--is inconclusive. However, although several leading twin researchers have dismissed Jackson's entire thesis on the basis of the inconclusiveness of this one claim, this idea was not central to Jackson's basic argument. The research undertaken for this article appears to confirm the validity of the most important points of Jackson's analysis and that therefore the classical twin method is of doubtful value as an indicator of genetic influences on schizophrenia and other diagnoses or psychological trait differences. Jackson's article demonstrated the implausibility of what came to be known as the MZ/DZ "equal environment assumption" of the twin method, implying that the twin method records nothing more than environmental differences between monozygotic and dizygotic twins.     

Aspects of narcissism and symbiosis, or, essential neurosis of twins

Following a brief introduction I address the relationships of twins from five different perspectives: the Intimate Connection, the Mirror Image and Complementarity, Object- and Self-Representation, Self and Object or Rivalry, and Intersubjective Communication. This approach attempts to understand twin relationships and the individual development of twins in terms of their intense mutual dependence, akin to infantile symbiosis, and in terms of narcissism. In their similarity to each other, twins may choose each other as love objects even as they see themselves in the other. That is, a twin may "love what he himself is" or "someone who was once part of himself." This "type of object-choice … must be termed 'narcissistic'" (Freud, 1914, pp. 90, 88). Such "cathexis of an undifferentiated self-object" is considered to be "primary narcissism" (Burstein, 1977, p. 103). Hoffer (1952) describes primary narcissism as "the lack of all qualities discriminating between self and not-self, inside and outside" (p. 33).