Brazilian Twin Studies: A Scoping Review

The current study was motivated by an interest in deepening understanding of Brazilian twin research, which is underrepresented internationally, in an effort to rectify this situation. Our aim was threefold: (1) to carry out a comprehensive investigation of Brazilian research on twins according to the area of knowledge; (2) to evaluate the representation of research in the field of psychology in comparison with other areas; (3) to evaluate characteristics of the research that may have contributed to its exclusion from the comprehensive meta-analysis of 50 years of twin research. A scoping review was performed according to PRISMA guidelines. Titles and abstracts were searched up to 2022 in six databases: CAPES, BDLTD, PePSIC, PubMed, Google Scholar, and SciELO, using selected keywords both in Portuguese and in English (e.g., 'twins' and 'Brazil'; 'twinning' and 'Brazil'; 'gemelaridade' [twinning], and 'gêmeos' [twins]). Three hundred and forty publications were included in the review. Approximately half (53.8‰) used the classic twin design to investigate the heritability of several traits, and the other half (46.2%) used other research designs. The scoping review showed that the number of publications doubled approximately every 10 years. Most publications were from the health area, with medicine accounting for approximately half of the studies, followed by psychology, odontology, and biology. We found that the interest in studying twins among Brazilian scientists is increasing over the years and there are reasons to be enthusiastic about the potential impact of this trend in the global scenario.

Reared-Apart and Reunited: Extraordinary Dynamics/Twin Research Reviews: Twin Bereavement and Lifespan Associations; Identifying Novel Mutations in Twins; Parenting Premature Multiples; Conjoined Twins in Cross-Cultural Context/In the News: Madeline Albright, Mother of Twins, Passes Away; Identical Twin Songwriters Compose New Musical; Visually Impaired Twin Skiers; Identical Twin Brothers Undergo Heart Transplants; Naming the Genain Quadruplets

The focus of this article is on insights gained from the extraordinary dynamics that took place between a pair of reunited monozygotic female twins from Greece. Topics include visualizing the co-twin prior to meeting, the unique love and attraction between the twins, a mother's response to discovering her child's cotwin and the challenges posed by insufficient kinship categories. The following section of this article surveys four recent twin research reports. They include Bereavement and Lifespan Associations; Novel Mutations in Twins; Parenting Premature Multiples; and Conjoined Twins. The final part of this article presents interesting and informative news about twins from popular sources. The topics covered are the passing of Madeline Albright (Secretary of State and mother of twins); identical twin songwriters; visually impaired twin and sibling skiiers; identical twin brothers who both received heart transplants; and the naming of the Genain quadruplets.

Reared-apart/reared-together Chinese twins and virtual twins: Evolving research program and general intelligence findings

China's One-Child Policy (1979-2015) limited rural families to two children and urban families to one child. This practice, which led to the abandonment of hundreds of thousands of female infants, is indirectly responsible for the separate placement of infant twins. The availability of this sample launched the first prospective study of these pairs. Participants include families and twins comprising 15 monozygotic (MZA) pairs and 7 dizygotic (DZA) pairs from countries throughout the world. The research program is described, and the initial wave of IQ scores for MZA and DZA twin pairs is examined in the first comprehensive report from this study. The twins' mean age at participation was 9.41 years (SD = 6.36), and their ages ranged from 3.19 to 24.98 years. Informative contrasts with adopted-together Chinese twins and virtual twins (same-age unrelated individuals reared together) highlight shared genetic and environmental effects on intellectual development. Applied directions based on findings from these novel samples are described.

Social closeness of MZ and DZ twin parents towards nieces and nephews

Twins' marriages to non‐twins yield genetically and socially informative kinships. Monozygotic (MZ) twins' genetic identity makes them ‘genetic parents’ of their nieces/nephews, and their nieces/nephews their ‘genetic children’. The present study is the first to apply twin‐family models to study social relatedness. Analyses of twin families (MZ: N = 248; DZ: N = 75) tested evolutionary‐based concepts concerning social closeness, perceived similarity and caretaking. Hypotheses based on Hamilton's inclusive fitness theory were supported: MZ twin aunts/uncles expressed greater social closeness towards their nieces/nephews than DZ twin aunts/uncles; and female twins from same‐sex pairs expressed greater closeness towards their nieces/nephews than male twins from same‐sex pairs. This study underlines benefits of associations between behavioural‐genetic and evolutionary psychological theories, methods and interpretations. Copyright © 2007 John Wiley & Sons, Ltd.

Twin study of laboratory-induced aggression

The aim of this study was to explore genetic and environmental contributions to laboratory-induced aggressive behavior. On a sample of 478 adult twins (316 monozygotic), the Competitive Reaction Time Task was used for aggression induction. The results showed that the initial, basic level of aggression could be explained by both shared (45%) and nonshared environmental factors (55%), while only nonshared environmental factors (100%) had a significant influence on changes in aggression as provocation increased. Genetic factors had no influence on laboratory-induced aggression. The results highlight the importance of environmental factors in shaping situation-specific aggressive responses to provocation.

The Twin Relationship Questionnaire

Abstract. Twins’ relationships evoke critical dilemmas for parents and teachers regarding raising and educating their twins. The current study provides comprehensive psychometric information about the Twin Relationship Questionnaire (TRQ; Fortuna, Goldner, & Knafo, 2010 ) as examined in nine samples, using mothers’ and fathers’ reports on 1,560 pairs of twins, tested in five measurement points from age 3 to age 9. On half of the subsamples, we conducted principal component analyses (PCA), whereas on the other subsamples, we conducted confirmatory factor analyses (CFA). Five factors were found in the PCA and confirmed by the CFA: conflict, closeness, dependence, dominance, and rivalry. All scales showed adequate internal consistencies and corrected item-total correlations. Correlation matrixes between scales demonstrate a triad of conflict, rivalry, and dominance scales, as opposed to the dyad of closeness and dependence. As evidence for the TRQ’s reliability and validity, we report the degree of agreement between mothers and fathers, the 10-month stability of mothers’ reports, and the associations of the TRQ subscales with experimentally assessed prosocial behaviors between the twins at age 6.5, and with the twins’ reports on their closeness at age 11. The replication of the findings across ages and raters reinforces the reliability and validity of the questionnaire.

Pairs of Genetically Unrelated Look-Alikes : Further Tests of Personality Similarity and Social Affiliation

Relationships of physical resemblance to personality similarity and social affiliation have generated considerable discussion among behavioral science researchers. A "twin-like" experimental design (involving genetically unrelated look-alikes, U-LAs) explores associations among resemblance in appearance, the Big Five personality traits, self-esteem, and social attraction within an evolutionary framework. The Personality for Professionals Inventory (PfPI), NEO/NEO-FFI-3, Rosenberg Self-Esteem Scale, and a Social Relationship Survey were variously completed by 45 U-LA pairs, identified from the "I'm Not a Look-Alike" project, Mentorn Media, and personal referrals. The mean U-LA intraclass correlations were negligible for all Big Five personality traits on the PfPI and NEO/NEO-FFI-3 (r_i = -.02 and - .04, respectively). In contrast, mean r_i values of .53 and .15 for monozygotic (MZA) and dizygotic (DZA) reared-apart twins, respectively, have been reported for these personality measures. The U-LA self-esteem correlation (r_i = -.18) was also below the correlations reported for MZ and DZ reared-together twins (r_{i =} .31 and .13, respectively). Finally, far fewer U-LAs expressed close social relationships (20%) than MZA (80%) and DZA (65%) twins. The present study extends earlier findings indicating that appearance is not meaningfully related to personality similarity and social relatedness. The criticism that MZ twins are alike in personality because their matched looks invite similar treatment by others is refuted. A more judicious interpretation is reactive genotype-environment correlation, namely that MZ twins' similar personalities evoke similar reactions from others. MZ twins' close social relations most likely derive from their perceptions of genetically based within-pair similarities that are lacking in U-LAs.

Twins Switched at Birth: Frequency, Life Histories, Twin Relationships, and Critical Issues/Twin Research Reviews: Classroom Placement; Mirror Syndrome; Unusual Twinning; Parenting Stress/Human Interest: Triplet Film; Educational Achievement; Prison Release; Co-Twin Impersonation; Facial Recognition Technology

Twins switched at birth are a rare subtype of twins who grow up apart. The frequency with which this event occurs, as well as selected life history events, twin relationships, and critical issues generated by the nine recorded cases are reviewed. This is followed by summaries of recent studies addressing classroom placement for twins, mirror syndrome in mothers, unusual twin types, and parenting stress. The article concludes with a review of human interest items from the general media, namely a triplet film, educational achievement, prison release, co-twin impersonation, and facial recognition technology.

Social Support and Strain Across Close Relationships: A Twin Study

Social relationships play a critical role in health and well-being throughout life. We analyzed the genetic and environmental variance co-variance structure for social support and strain across four sets of relationships including with one’s co-twin, spouse/partner, family and friends. The sample included 5288 Norwegian twins aged 40–80. Older people reported less support from their co-twin and friends and less strain from their family and friends. Genetic influences contribute importantly to variation across all the measures, with estimates ranging from 0 to 58%; variance due to shared environmental influences was most important for the twin-relationship, ranging from 0.11 to 0.42%. Social support was negatively correlated with social strain across all sets of relationships. With the exception of the co-twin relationship, these associations were primarily mediated by genetic and non-shared environmental effects.

A Possible Twin: The 1960s Twin Study Revisited/Twin Research: Twin-to-Twin Heart Transplantation; Distinguishing Monozygotic Twins; Twin Conceptions via Oocyte Donation; Factors Affecting Craniofacial Traits/In the Media: Triplet Delivery in the UK; Conjoined Twins and the Concept of Self; Colombian Twin Trainers; Skin Grafting to Save an Identical Co-Twin; Lack of Physical Flaws in Dolly the Cloned Sheep; Possible Conjoined Twins of Opposite-Sex; Passing of the Remaining Twin From the World's Longest Separated Pair

This article begins with the story of a 51-year-old Los Angeles, California man, Justin Goldberg, whose daughter caught a glimpse of his striking look-alike at a popular market. Many people have so-called doppelgängers, but this occurrence is especially intriguing - the individual in question, born in New York City in the mid-1960s to an unwed mother, was an adoptee placed by the Louise Wise Adoption Agency. This agency, under the guidance of a prominent psychiatrist, decided to place twins in separate homes. Some of these twin children were part of a controversial child development study that was hidden from them and their parents. Next, recent and current twin research on heart transplantation, distinguishing monozygotic co-twins, twin conceptions via oocyte donation and factors affecting craniofacial traits are summarized. The article concludes with highlights on twins in the media, specifically, a triplet delivery in the United Kingdom, self-concept and consciousness in conjoined twins, Colombian twin trainers, skin grafting to save an identical co-twin, lack of physical flaws in Dolly the cloned sheep, possible opposite-sex conjoined twins, and the passing of the remaining twin from the world's longest separated pair.

Social Comparison Orientation in Monozygotic and Dizygotic Twins

Twin research has offered evidence that monozygotic (MZ) twins are more socially close than dizygotic (DZ) twins, but has not paid much attention to the way twins compare themselves with their co-twin. The few studies in this area suggest that ‘horizontal comparisons’ (social comparison motivated by solidarity or communion with others) matter more for MZ twins than for DZ twins, at least when the co-twin is the social comparison standard. Consistent with this view, we predicted higher interest in MZ twins relative to DZ twins to select their co-twin rather than other people in general as the social comparison standard. The Social Comparison Orientation (SCO) scale, which measures the inclination to compare with others in a horizontal rather than vertical mode (comparing either upward or downward), was administered in 90 MZ pairs and 57 same-sex DZ pairs (63% female; average age 18.06 years) from the Netherlands Twin Register. MZ twin pairs showed significantly higher SCO scores than DZ twin pairs (with a large effect size) on the co-twin SCO, whereas the two groups did not differ from each other on the general SCO excluding the co-twin as social comparison standard. In MZ twin pairs, anxiety was associated with social comparison with others in general, not with their co-twin. For both scales, twin resemblance was explained by additive genetic variance. The present findings provide direct evidence that horizontal comparisons with the co-twin are of particular importance for MZ twins.

Morality: An Evolutionary Account

Refinements in Darwin's theory of the origin of a moral sense create a framework equipped to organize and integrate contemporary theory and research on morality. Morality originated in deferential, cooperative, and altruistic "social instincts," or decision-making strategies, that enabled early humans to maximize their gains from social living and resolve their conflicts of interest in adaptive ways. Moral judgments, moral norms, and conscience originated from strategic interactions among members of groups who experienced confluences and conflicts of interest. Moral argumentation buttressed by moral reasoning is equipped to generate universal and impartial moral standards. Moral beliefs and standards are products of automatic and controlled information-processing and decision-making mechanisms. To understand how people make moral decisions, we must understand how early evolved mechanisms in the old brain and recently evolved mechanisms in the new brain are activated and how they interact. Understanding what a sense of morality is for helps us understand what it is.

Twin Classics: Research That Always Inspires/Twin Studies: Elder Twin Relationships; Superfecundated Twinning in Chimpanzees; Conjoined Twinning and Embryo Transfer; Reduced Frequency of In Vitro Multiples/Professional and Human Interest: First Identical Twin Renal Transplant; Identical Triplet Wedding; Spanakos Twins: Boxers; Twins in Space; Political Twins

Ten classic works in twin research are described. These volumes are rich in data, interpretation, and impact, and contain life history material that adds depth and dimension to the quantitative findings. Next, research on social relationships in older twins, superfecundated twinning in chimpanzees; effects of embryo transfer on conjoined twinning and the reduced frequency of in vitro multiples is reviewed. Finally, there has been considerable public interest surrounding the first identical twin renal transplant, an identical triplet wedding; identical twin boxers, a twin living in space, and a politically active twin pair.

Genetic and experiential influences on behavior: Twins reunited at seventy-eight years

Twins living in different countries offer opportunities to explore associations between observed differences and experiential effects. This report compared the life histories, cognitive abilities, personality traits, psychomotor skills, medical characteristics, job satisfaction, social support and social relations of dizygotic (DZ) female twins reunited at 78, the world's longest separated set. The twins' advanced age also enabled a study of how co-twin differences in aging may be associated with current behavioral and social differences. Consistent with previous studies, these dizygotic reared apart (DZA) twins showed discordance across some, but not all, traits. Their different rearing situations and life histories may explain current differences in their responses to meeting their twin. This case highlights the importance of both genetic and rearing factors on behavior, but does not allow firm conclusions regarding the extent to which these sources explain individual developmental differences. However, such data contribute to the growing number of cross-culturally separated twins, generating novel hypotheses that may be assessed using larger samples.

Twins Reunited: Scientific and Personal Perspectives/Twin Research Studies: Multiple Birth Effects on IQ and Body Size; Life Style, Muscles, and Metabolism; Monochorionic Dizygotic Twin with Blood Chimerism; Amniocentesis for Twins/Twins in the Media: Identical Doctors; Freedom Fighter for Twins; Twin Scholarships; Auguste and Jean-Felix Piccard; Twins Born Apart

A reunion of 38-year-old female monozygotic twins took place in Daegu, South Korea, on January 14, 2014. Scientific and personal perspectives on this extraordinary event are provided. A review of timely twin research follows, covering the effects of multiple births on IQ and body size, lifestyle and physical fitness associations, a rare case of a dizygotic twin with blood chimerism and definitional issues surrounding amniocentesis-related loss in multiple birth pregnancies. Interesting and informative mention of twins in the media includes twin doctors, a twin freedom fighter, the availability of college scholarships for twins, a new book about the Piccard family (two of whose members were twins), and co-twins born before and after the new year. A follow-up to a previous mention of identical twin biatheletes is also provided.

Twin Reunions: The Science Behind the Fascination/Twin Research Reports: Altitude and Hypoxia; Twin Pregnancy Outcomes; Space Mission/Media Highlights: Chinese Twins Reunited; Twin Loss Discovered; Hidden Twins; Twin Euthanasia; Twin Savior

A description of the unique qualitative features of reunions between separated monozygotic twins is presented. The scientific implications of these observations are considered with reference to understanding human social behavior in general. This is followed by summaries of twin research on altitude and hypoxia, pregnancy outcomes, and space travel. Finally, recent accounts of twins in the media are noted; in particular, a rare reunion of adult monozygotic male Chinese twins, a novelist's personal discovery of twin loss, two renowned but ‘hidden’ twins, the moving story of an identical Belgian pair, and a twin savior.

Reflections on Twin Relationships: Twins Reared Apart and Twins of Opposite Sex / Research Reviews: Second Language Acquisition; Twin Research on Political Behavior; Twinning Rate in Cândido Godói / Human Interest: Twin ‘Cousins’ Separated at Birth; Chinese Quadruplets; Genetic Testing; KÀ (Cirque du Soleil)

The complexities of twin relationships posed by separate rearing and by opposite sex are considered. Unusual cases may highlight unique social-interactional processes and outcomes occurring in these pairs. Research reviews include recent twin studies on second language acquisition, political behavior, and multiple birth rates. Items of more general interest include twin ‘cousins’ reared apart, indistinguishable monozygotic quadruplets, a genetic testing dilemma, and a performance about separated twins.

Memories of the Child Development Center Study of Adopted Monozygotic Twins Reared Apart: An Unfulfilled Promise

This month's News, Views and Comments column differs from those that have appeared in previous issues. The first of the companion papers presented here offers the first in-depth historical overview of Dr. Peter Neubauer's controversial study of infant identical twins separated at birth, launched in the 1950s. The author, Dr. Lawrence Perlman, was a research assistant on the project while earning his doctoral degree in clinical psychology from New York University. His paper is followed by my own critical appraisal of issues raised by that study, some of which are still timely today. Readers are invited to forward their comments to me (nsegal@fullerton.edu) for possible consideration in future columns of Twin Research and Human Genetics.

Twins Switched at Birth: A Case from the Canary Islands / Research Reviews: Twin Study of Materialism; Twins and Clones; / In the News: ART and Premature Twins; A Pair of College Presidents; Twins in Real Estate; Tribute to a Twin

Monozygotic (MZ) twins switched at birth represent a rare class of twins who are reared apart and reunited (MZA). Background data and descriptive findings from a case study of such a pair, born thirty-six years ago in the Canary Islands, are presented. The next section reviews a twin study of materialism and an essay addressing relations between MZ twinning and human reproductive cloning. This article concludes with summaries of several newsworthy, twin-related items. They include the multiple birth consequences of artificial reproductive technology, the friendly rivalry between identical twin college presidents, a joint venture in real estate and a tribute to a deceased twin.

Reunited Twins: Spouse Relations / Twin Research Reports / Timely Topics

The present article explores the social attraction that may evolve on the part of reared apart twins' spouses toward their brothers- and sisters-in-law, that is, their spouses' newly found co-twin. This topic was inspired by the fascinating story of monozygotic reared apart (MZA) twins who were reunited in Perth, Australia in 2008, at age 50. It is followed by brief reviews of twin research articles concerning divorce rates among mothers of multiples and X-chromosome inactivation in MZ female twin pairs. The final section presents informative human interest stories involving twins.

Biologically Costly Altruism Depends on Emotional Closeness among Step but Not Half or Full Sibling

We studied altruistic behaviors of varying biological cost (high, medium, and low) among siblings of varying genetic relatedness (full, half, and step). In agreement with inclusive fitness theory, the relative importance of either reliable (such as co-residence) or heuristic (such as emotional closeness) kinship cues depended crucially on the costs of help. When help did not endanger the altruist's life, thus making reciprocation possible, emotional closeness was the strongest predictor of altruism; perceived physical and psychological similarity to the sibling amplified altruistic behavior via their association with emotional closeness. When help endangered the altruist's life, thus making reciprocation unlikely, the strongest predictor of altruism was the ancestrally valid kinship cue of co-residence duration. Emotional closeness predicted costly altruism only for step siblings; its effects were nonsignificant when siblings were genetically related. Our findings support the idea that emotional closeness promotes costly altruistic behavior by serving as a surrogate kinship cue when more reliable cues are missing.

Personality and birth order in monozygotic twins adopted apart: a test of Sulloway's theory; Research Reviews: twin births and cancer risk in mothers, male sexual dysfunction, twin study of ultimatum game behavior; Human Interest: 'The Land of Twins', twin-like reunion-I, twin-like reunion-II

A brief overview of Sulloway's (1996) theory of birth order and personality is presented. A reared apart twin approach for testing his personality findings regarding openness to experience and conscientiousness in first borns and later borns is described. This is followed by summaries of three recent twin studies. The topics include cancer risk in mother of twins, sexual dysfunction in males and responder behavior during ultimatum games. This article concludes with a discussion of twinning rates and rituals among the Yoruba of western Nigeria, and descriptions of two unusual reunions between siblings and twins.