Multiple regression analysis of twin and sibling data

DeFries and Fulker (1985) employed a multiple regression analysis of twin data in which a cotwin's score is predicted from the proband's score, the coefficient of relationship (R = 1.0 for monozygotic twins and 0.5 for dizygotic twins), and their interaction to yield direct estimates of heritability (h²) and the proportion of variance due to common or shared environmental influences (c²). The purpose of the report is to extend this model to allow for the analysis of data from more than two familial relationships simultaneously. Data from identical twins, fraternal twins, and siblings pairs, in which one member of each pair was selected based on low reading performance, were analyzed. To analyze the data simultaneously, additional coefficients were added to the regression equation in order to differentiate shared environmental influences of twins and siblings. When the model was applied to the combined cognitive data, significant estimates for heritability were obtained in three of the six tests analyzed. The results also indicated a lack of justification for the separate twin and sibling environmental parameters, and a discussion of more parsimonious models is included.     

A twin study of personality and temperament during middle childhood

In a study of 105 same-sex twin pairs, ages 7 to 10 years, maternal ratings on 23 bipolar scales yielded six factors designated as compliant morality, applied cognitive, sociability, emotionality, tough-mindedness, and activity-distractibility. Analyses by twin pairs (68 identical, 37 fraternal) indicated that correlations for identical pairs were significantly higher than fraternal pairs on all but one factor: tough-mindedness. Profile analyses for the six factors indicated that the profiles of scores across the factors were more concordant for the identical twin pairs. Correlations obtained from the individual scales produced differences between the identical and fraternal twin pairs as well. Overall, the data suggested that several components and the total organization of those components of personality and temperament are genetically influenced.     

Neuroticism and self-reported somatic health: a twin study

Using a twin sample (N?=?188; 53 monozygotic and 39 dizygotic twin pairs, and 4 single twins whose co-twin did not participate), this study sought (1) to estimate heritabilities of neuroticism and of somatic complaints rated on the basis of two different time frames ('the last week' vs. 'in general'); (2) to estimate the genetic association between neuroticism and the complaints indices and (3) to examine to what extent somatic complaints are aetiologically distinct from neuroticism. Using models with common additive genetic (A) and individual-specific environmental (E) factors, the heritabilities for neuroticism and complaints 'in general' and during 'the last week' were 0.46 (0.20-0.65), 0.44 (0.22-0.62), and 0.45 (0.22-0.63), respectively. Nearly 60% of the phenotypic correlation between neuroticism and somatic complaints were accounted for by A. Furthermore, a substantial part of the variance in somatic complaints was due to unique genetic and individual-specific environmental influences unrelated to neuroticism. These results suggest that somatic complaints are moderately heritable and that a considerable portion of the covariance between neuroticism and complaints measures is due to genetic factors. Yet, the findings also suggest that these two attributes are distinct entities with overlapping, but not identical, underlying genetic and environmental influences.     

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.     

Insecure attachment and personality disorder: a twin study of adults

This study used 239 twin pairs from a volunteer community sample to investigate how anxious and avoidant attachment are related to personality disorder (PD). Factor analysis showed that self‐reported anxious attachment and 11 PD scales from the Dimensional Assessment of Personality Problems loaded onto one factor (emotional dysregulation), and avoidant attachment and four PD scales loaded onto a second factor (inhibitedness). Biometric models indicated that 40% of the variance in anxious attachment was heritable, and 63% of its association with corresponding PD dimensions was attributable to common genetic effects. Avoidant attachment was influenced by the shared environment instead of genes. Correlations between avoidance and corresponding PD dimensions were attributable to experiences in the nonshared environment that influenced both variables. Copyright © 2006 John Wiley & Sons, Ltd.     

Irradiation-enhanced twin boundary migration in BCC Fe

The effects of irradiation on twin boundary migration in BCC Fe are studied by atomistic simulations. It is found that under the applied shear strain–stress, thermal spikes may create twinning dislocation loops (TDLs) at twin boundaries, so triggering twinning. Irradiation-generated clusters of point defect at twin boundaries may act as sources to nucleate TDLs. When a vacancy loop intersects with a twin boundary, the critical stress to activate a TDL is less than half of that required for a defect-free twin boundary.     

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.     

A rejecting baby in a twin set

The notion of a “rejecting” baby has not received much attention in the research literature. One member of a pair of twins which came to our attention provided a good example of a “rejecting” baby. This baby avoided eye contact with all people at close proximity. It is possible that the notion of “rejecting” babies should be attended to by parents, doctors and others involved with the welfare of young children. Perhaps they do not know what to look for in the early phases of this expression. It is possible these early omissions mask detection of later behavioral difficulties, by which time such children have acquired distinctive interactive styles increasingly resistent to modification.     

Delinquent peer affiliation and conduct problems: A twin study

Many putative environmental risks correlate with individuals' genotypes. The association between delinquent peer affiliation and conduct problems may occur because of shared genetic liability. Five hundred fifty three monozygotic and 558 dizygotic twin pairs, aged 11 to 18 years, were assessed for delinquent peer affiliation and conduct problems. The authors investigated whether genes contribute to both delinquent peer affiliation and the correlation between delinquent peer affiliations and conduct problems. Delinquent peer affiliation was influenced by genetic, shared environmental, and nonshared environmental factors; genetic factors also contributed to the correlation between delinquent peer affiliations and conduct problems, providing evidence for genotype-environment correlation. The magnitude of the genetic variance of conduct problems was contextually dependent on levels of delinquent peer affiliation and was greater at higher levels of delinquent peer affiliation.     

Jung's twin brother. Otto Gross and Carl Gustav Jung

This paper is a preliminary communication of several years of research into the life and work of the Austrian psychoanalyst and anarchist Otto Gross (1877–1920). Although he played a pivotal role in the birth of modernity, acting as a significant influence upon psychiatry, psychoanalysis, ethics, sociology and literature, he has remained virtually unknown to this day. Following a biographical sketch and an overview of his main theoretical contributions, the impact of Gross' life and work on the development of analytical theory and practice is described. His relationship with some of the key figures in psychoanalysis is presented, with particular emphasis on his connections to Jung. The paper concludes with an account of relevant contemporary interest in his work: the founding of the International Otto Gross Society, the first edition of The Collected Works of Otto Gross on the Internet, and the 1st and 2nd International Otto Gross Congresses which took place in Berlin in 1999 and at the Burghölzli Clinic, Zürich, in October 2000.     

Intellectual similarity of virtual twin pairs: Developmental trends

Virtual twins (VTs) are same-age unrelated siblings reared together from infancy who replicate twinship, but without the genetic relatedness. A 2005 report from the ongoing Fullerton Virtual Twin Study found an IQ intraclass correlation of .26 (p < .01, n = 113 pairs) and a within-pair difference of 13.22 IQ points. The average age of VTs in that study was 8.10 years (SD = 8.56, range: 4.01-54.84 years). An opportunity to retest members of 43 VT pairs, 1.70-8.96 years after their time 1 assessment, allowed additional analyses of genetic and environmental influences underlying general intellectual development. A decrease in the VT IQ correlation and an increase in the within-pair difference were indicated, consistent with increasing genetic and/or non-shared environmental influences and decreasing shared environmental influence on general intellectual development throughout childhood.     

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.     

A twin study of attachment style in young adults

The relative contribution of genetic and environmental influences to individual differences in attachment security is still incompletely understood. We assessed attachment style with the Experiences in Close Relationships questionnaire in a volunteer sample of 677 twins (43% male) ages 23-24 years drawn from the population-based Italian Twin Register, who belonged to 244 complete pairs (46% monozygotic) and 189 unmatched pairs. Genetic structural equation modeling was performed with the Mx program. Genetic effects accounted for 45% and 36% of individual differences in attachment-related anxiety and avoidance, respectively. Furthermore, the covariation between anxiety and avoidance was found to be mainly due to genetic factors, with heritability of the latent attachment security phenotype estimated at 62%. Unshared environmental factors explained the remaining proportion of variance. Although our findings are best regarded as preliminary given some study limitations, they suggest that both nature and nurture contribute to individual differences in adult attachment.     

A twin study of computer anxiety in Turkish adolescents

The present study investigated computer anxiety within a sample of Turkish twins aged 10-18. A total of 185 twin-pairs participated in the study. Of the twins, 64 pairs (34.6 percent) were monozygotic (MZ) and 121 pairs (65.4 percent) were dizygotic (DZ). Of the 121 DZ twins, 54 pairs (44.63 percent) were same-sex twins and 67 pairs (55.37 percent) were opposite-sex twins. Computer anxiety was assessed using Computer Anxiety Rating Scale-Turkish Version (CARS-TV), one of the three main scales of "Measuring Technophobia Instruments" developed by Rosen and Weil. The results of paired t test comparisons showed no significant differences in MZ and same-sex DZ twin-pairs' levels of computer anxiety. On the other hand, a significant difference was found in opposite-sex DZ twin-pairs' level of computer anxiety. Interesting enough, males appeared to be more computer anxious than their female co-twins. In the present study, using Falconer's formula, heritability estimate for computer anxiety was derived from correlations based on MZ and DZ twins' mean scores on CARS-TV. The results showed that 57 percent of the variance in computer anxiety was from genetics and 41.5 percent was from nonshared environmental factors. Shared environmental influence, on the other hand, was very small and negligible. Interpretations of results and potential directions for future research are presented.     

Factor structure and aetiological architecture of the BRIEF: A twin study

Executive function is a broad construct that encompasses various processes involved in goal‐directed behaviour in non‐routine situations (Banich, 2009). The present study uses a sample of 560 5‐ to 16‐year‐old twin pairs (M = 11.14, SD = 2.53): 219 monozygotic twin pairs (114 female; 105 male) and 341 dizygotic twin pairs (136 female, 107 male; 98 opposite sex) to extend prior literature by providing information about the factor structure and the genetic and environmental architecture of the Behavior Rating Inventory of Executive Function (BRIEF; Gioia et al., 2000, Child Neuropsychol., 6, 235; Gioia et al., 2000, Behavior rating inventory of executive function, Lutz, FL: Psychological Assessment Resources), a multifaceted rating scale of everyday executive functions. Phenotypic results revealed a 9‐scale, 3‐factor model best represents the BRIEF structure within the current sample. Results of the genetically sensitive analyses indicated the presence of rater bias/contrast effects for the Initiate, Working Memory, and Task‐Monitor scales. Additive genetic and non‐shared environmental influences were present for the Initiate, Plan/Organize, Organization of Materials, Shift, and Monitor and Self‐Monitor scales. Influences on Emotional Control were solely environmental. Interestingly, the aetiological architecture observed was similar to that of performance‐based measures of executive function. This observed similarity provided additional evidence for the usefulness of the BRIEF as a measure of ‘everyday’ executive function.