The contribution of genetic and environmental factors to the stability of obsessive–compulsive (OC) symptoms has not yet been established in adult population based samples. We obtained the Young Adult Self Report Obsessive–Compulsive Subscale in mono- and dizygotic twins from the population-based Netherlands Twin Register in 1991, 1995 and 1997 and the Padua Inventory Revised Abbreviated in 2002. Stability of OC symptoms was analyzed as a function of genetic and environmental components. Heritability of OC behavior was around 40% at each time-point, independent of the instrument used. OC behavior was moderately stable with correlations ranging between r = .2 (for 11-year intervals), .4 (for 4–5 year intervals) and .6 (for 2 year intervals). Genetic correlations across time were higher, varying between .4 and .9, indicating that the stability of OC symptoms is mainly due to stable genetic factors. This study showed a moderate heritability and stability for OC behavior in adults. Genetic stability across time is high.

The purpose of the present study was to determine if a general factor of personality (GFP) could be extracted from the six dimensions of the HEXACO model and four factors of trait emotional intelligence. Participants were 1,192 pairs of twins (666 MZ pairs, 526 DZ pairs) between the ages of 19 to 86 years, who completed the Trait Emotional Intelligence Questionnaire — Short Form and the HEXACO Personality Inventory — Revised. Principal components analysis yielded a strong GFP accounting for 33% of the variance, on which all variables with the exception of honesty-humility from the HEXACO showed moderate to large loadings. Behavioral genetic (BG) analyses revealed that individual differences in the GFP were entirely attributable to additive genetic and non-shared environmental factors — results that are in accord with previous BG analyses of a GFP. The present study adds to the body of evidence in support of a heritable GFP but an alternative perspective is also discussed.

The aim of this study was to estimate the rates of recovery, treatment-seeking, and natural recovery from pathological gambling (PG) in men and women in a community-based national survey, and to examine the role of gambling problem recognition in recovery from PG. Participants were 4,764 individuals from a community-based Australian national twin registry (104 with a lifetime history of PG) who were administered a structured psychiatric telephone interview. Women were more likely than men to recover from (56% versus 36%; odds ratio = 2.3) and to seek treatment for PG (32% versus 13%; odds ratio = 3.2). Most individuals who recovered from PG did so without treatment (82%), but this was higher among men than among women (92% versus 57%; odds ratio = 5.3). This is the first study to document sex differences in treatment-seeking and recovery from PG. These findings highlight the value in continuing to develop self-help and brief treatments for PG that will appeal to those who are unlikely to seek formal help.

We examine the impact of rearing by an alcoholic parent on risk for child behavior problems using data on 2492 offspring drawn from two ongoing studies of children of female and male same- and opposite-sex twin pairs. Results of regression models predicting child behavior problems from parent and co-twin lifetime history of alcohol use disorder (AUD) provide support for genetic but not environmental transmission of externalizing and a measure of total problem behaviors. Results for internalizing behavior were inconclusive with respect to transmission of risk.

Visual impairment is a leading cause of morbidity and poor quality of life in our community. Unravelling the mechanisms underpinning important blinding diseases could allow preventative or curative steps to be implemented. Twin siblings provide a unique opportunity in biology to discover genes associated with numerous eye diseases and ocular biometry. Twins are particularly useful for quantitative trait analysis through genome-wide association and linkage studies. Although many studies involving twins rely on twin registries, we present our approach to the Twins Eye Study in Tasmania to provide insight into possible recruitment strategies, expected participation rates and potential examination strategies that can be considered by other researchers for similar studies. Five separate avenues for cohort recruitment were adopted: (1) piggy-backing existing studies where twins had been recruited, (2) utilizing the national twin registry, (3) word-of-mouth and local media publicity, (4) directly approaching schools, and finally (5) collaborating with other research groups studying twins.

In this article we report on reading ability of twin children in kindergarten to Grade 2 as a function of whether members of the pairs are assigned to the same or different classrooms. All analyses were run using mixed model regressions to account for the interdependence between twin pairs. The samples, total N = 1505, are from Australia and the United States. We found a close-to-significant difference in favor of same-class children in kindergarten and Grade 1. However, when results were adjusted to take account of pre-existing differences in disruptive behavior and in preliteracy ability, the class assignment effects disappeared. We suggest that these pre-existing differences, particularly disruptive behavior, are influencing decisions about whether to separate twins or not and also affecting early reading performance, a conclusion supported by significant correlations between the behavioral measures, preliteracy, and school-based reading. We conclude that, on average, early literacy in twins is not directly affected by their assignment to the same or different classrooms.

Phenotypically discordant monozygotic twins offer the possibility of gene discovery through delineation of molecular abnormalities in one member of the twin pair. One proposed mechanism of discordance is postzygotically occurring genomic alterations resulting from mitotic recombination and other somatic changes. Detection of altered genomic fragments can reveal candidate gene loci that can be verified through additional analyses. We investigated this hypothesis using array comparative genomic hybridization; the 50K and 250K Affymetrix GeneChip® SNP arrays and an Illumina custom array consisting of 1,536 SNPs, to scan for genomic alterations in a sample of monozygotic twin pairs with discordant cleft lip and/or palate phenotypes. Paired analysis for deletions, amplifications and loss of heterozygosity, along with sequence verification of SNPs with discordant genotype calls did not reveal any genomic discordance between twin pairs in lymphocyte DNA samples. Our results demonstrate that postzygotic genomic alterations are not a common cause of monozygotic twin discordance for isolated cleft lip and/or palate. However, rare or balanced genomic alterations, tissue-specific events and small aberrations beyond the detection level of our experimental approach cannot be ruled out. The stability of genomes we observed in our study samples also suggests that detection of discordant events in other monozygotic twin pairs would be remarkable and of potential disease significance.

The associations between social support and depression, and between stress and depression have been the subject of considerable research, and although this has included longitudinal designs, these have rarely controlled for genetic effects that mediate these associations. The sample comprised 7,356 female and 4,882 male participants aged 18–95 from the Australian NHMRC Twin Registry (ATR). Of these, between 100 and 324 female pairs and between 41 and 169 male pairs, depending on the measure, were monozygotic (MZ) pairs discordant for depression. We use the co-twin control design in combination with prospective analyses to explore the association between a composite of predictors (perceived social support, stress, and support × stress) and depression. With familial effects included, both perceived support and stress were antecedents to, and sequelae of, depression, but no stress-buffering occurred. With familial effects controlled, stress was a sequela of a prior depressive episode, and neither lack of support nor stress were antecedents to depression, though their interaction approached significance for males. The male twin who later became depressed had previously reported lower perceived support in the face of multiple stressors compared to his co-twin who did not become depressed. We show that associations commonly observed with prospective designs are partly due to familial factors.

The Enrichment Study (ES) was designed to extend the Minnesota Twin Family Study (MTFS) by oversampling 11-year-old twins at especially high risk for substance use disorders by virtue of having a childhood disruptive disorder. The sample was ascertained from Minnesota birth records. To identify high-risk twins, we conducted telephone screening interviews for parent-reported symptoms of attention-deficit/hyperactivity disorder (ADHD) and conduct disorder (CD) as well as indications of academic disengagement. Twins who exceeded a predetermined threshold were invited to participate. To facilitate comparison with the previously ascertained MTFS participants, a random sample of 11-year-old twins was also recruited. As part of the ES study, 499 twin pairs, and their parents, visited the University of Minnesota, where each participant completed a clinical interview, psychophysiological evaluation, and thorough assessment of environmental risk. We were highly successful in recruiting at-risk twins; 52% of the screened male twins and 41% of the screened females met criteria for a diagnosis of ADHD, CD, or oppositional defiant disorder (ODD). At the pair level, 63% of the screened pairs had at least one member with a childhood disruptive disorder. This article provides an overview of the study design and includes a review of recent findings using this sample of twins.

Ahigher frequency of skewed X chromosome inactivation (XCI) is found in patients with autoimmune thyroid disease (AITD) than in controls. Although goitre is often present in AITD, a recent study failed to show an association between XCI and clinically overt nontoxic goitre. However, the etiology of overt goitre is complex, and the mechanisms influencing thyroid volume may involve fewer factors than the mechanisms underlying overt goitre. In order to examine the impact of XCI on thyroid volume in euthyroid females, we studied whether within cohort (n = 138) and within twin pair (n = 69) differences in XCI are correlated with differences in thyroid volume. XCI was determined by PCR analysis of a polymorphic CAG repeat in the first exon of the androgen receptor gene. Thyroid volume was determined by ultrasound. Neither in the within cohort nor in the within twin pair analysis could we demonstrate a statistically significant association between XCI and thyroid volume: Regression coefficient (β) = 0.023 (95% confidence interval, –0.062–0.108), p = 0.592 and β = 0.038 (–0.080–0.156), p = 0.521, respectively. Controlling for potential confounders such as zygosity, age, TSH, smoking habits and use of oral contraceptives did not change the findings. In conclusion, in a sample of euthyroid Danish female twins, we found no evidence of a relationship between XCI pattern and thyroid volume.

Nicotine and its primary oxidative metabolites are metabolized in part by glucuronidation. Genetic variation in UGT isoenzymes that catalyze glucuronidation activity suggests that variation in glucuronidation rate is in part genetically determined. The relative contribution of genetic and environmental sources to individual differences in the rate of glucuronidation of nicotine, cotinine, and trans-3'-hydroxycotinine was estimated in a twin study of nicotine pharmacokinetics. Glucuronidation rate was defined using measures that either accounted for variability in renal clearance or assumed the same relative renal clearance of parent drug and glucuronide conjugate across individuals. The former definition resulted in highly correlated nicotine and cotinine glucuronidation measures that were substantially influenced by the combined effect of additive (heritable) and non-additive (dominant and epistatic) genetic effects. These findings suggest that genetic variation in UGT isoenzymes that act in additive and interactive ways is an important determinant of individual variability in nicotine and cotinine metabolism via glucuronidation pathways.

Recent studies in Asian populations have identified variants in the EDAR and FGFR2 genes that arose following the divergence of Asians and Europeans and are associated with thick straight hair. To date no genetic variants have been identified influencing hair texture in Europeans. In the current study we examined the heritability of hair curliness in three unselected samples of predominantly European ancestry (NS1 = 2717; NS2 = 3904; NS3 = 5079). When rated using a three point scale (Straight/Wavy/Curly) males were ~5% more likely to report straight hair than females and there were suggestions in the data that curliness increased slightly with age. Across samples significant additive and dominant genetic influences were detected resulting in a broad sense heritability of 85–95%. Given the magnitude and the specificity of the EDAR effect on hair morphology in Asian populations we are hopeful that future association studies will detect similar genetic influences in European populations.

Loss of a twin has been largely overlooked by bereavement counselors and clinicians. This particular type of loss is also poorly understood among the public. Consequently, some bereaved twins have established a unique organization, Twinless Twins Support Group International, for the purpose of sharing this traumatic life experience with one another. The present article reviews events from the annual convention of Twinless Twins, held in July 2009, in Denver, Colorado. Selected findings from research on twin loss are also presented. Next, twin research concerned with the origins and implications of empathy, hormone replacement therapy and multiple birth conception will be reviewed. This article concludes with summaries of real life stories of twins' kidney disease, literary talent and retirement festivities.