Using behavioral genetic analyses, we investigated and present a possible relationship between adolescent alcohol use and six domains of common problem behaviors in a community-based sample of 633 twin pairs who were under the legal drinking age of 21 (mean age = 15.0 years). The underlying etiology of the six problem behavioral domains, classified as conduct problems, hyperactivity, school problems, low self-esteem, neuroticism, and social withdrawal, was previously described (Siewert et al., 2003) as two heritable and genetically distinct dimensions of problem behavior. We took the two best-fitting models from that study (one that proposed a generalized behavior problem factor along with an internalizing behavior factor, and one that proposed an externalizing behavior factor along with an internalizing behavior factor) and extended the analyses in this study to include an index of alcohol use. Our results suggest that there is a strong genetic relationship between adolescent alcohol use and a broad spectrum of both externalizing and internalizing behavioral problems. The individual who seems to be at risk for either generalized or specifically externalizing behavioral problems is also at risk for adolescent alcohol use. However, the individual who exhibits internalizing problem behaviors appears to be protected from adolescent alcohol use. We propose that adolescent alcohol consumption needs to be understood in the context of these genetically influenced externalizing and internalizing propensities.

We analyzed depressive and psychosomatic symptoms in relation to co-twin dependence in 419 twins at the age of 22 to 30 years. Depressive symptoms were assessed, as previously, with Children's Depression Inventory modified to be suitable for this age and reported as a total score and three subscales (low self-confidence, anhedonia and sadness) based on factor analysis as reported in a previous epidemiological study conducted in Finland. Items assessing nervous complaints and somatic symptoms were adapted from Finnish studies of juvenile health habits. Inter-twin dependence decreased with increasing age in both genders. Monozygotic twins, especially monozygotic females, reported most often to be dependent on their co-twin. When the symptom reporting was evaluated in relation to co-twin dependence, no relation was found between co-twin dependence and depressive symptom reporting. However, dependence-independence imbalance within twin pair was associated with elevated levels of depressive and psychosomatic symptoms, especially in twins who perceived themselves as dependent and the co-twin as independent. We conclude that there was no relation found between co-twin dependence and depressive symptom reporting in male and female twins except for the few imbalance cases, where most symptoms were reported by those dependent twins who felt their co-twin as independent.

Monozygotic (MZ) twinning is generally considered to be rare in species other than human. We inspected sex ratios in European zoo-bred ring-tailed lemurs (Lemur catta), revealing a significant excess of same-sex twins. Of 94 pairs, 60 (64%) were either both males or both females (p = .004). Application of the Weinberg differential rule argues that 27% of all twins in this species are MZ pairs. In this protected species, where twinning is commonplace (~50% of newborns are twins), the probable existence of frequent MZ twinning has ramifications for breeding programs aimed to maximize genetic diversity, and suggests that twin studies in a species other than human could have potential as a medical research tool.

There is now considerable evidence that host genetic factors are important in determining the outcome of infection with Mycobacterium tuberculosis (MTB). The aim of this study was to assess the role of several candidate genes in the variation observed in the immune responses to MTB antigens. In-vitro assays of T-cell proliferation, an in-vivo intradermal delayed hypersensitivity response; cytokine and antibody secretions to several mycobacterial peptide antigens were assessed in healthy, but exposed, West African twins. Candidate gene polymorphisms were typed in the NRAMP1, Vitamin D receptor, IL10, IL4, IL4 receptor and CTLA-4 genes. Variants of the loci IL10 (−1082 G/A), CTLA-4 (49 A/G) and the IL4 receptor (128 A/G) showed significant associations with immune responses to several antigens. T-cell proliferative responses and antibody responses were reduced, TNF-α responses were increased for subjects with the CTLA-4 G allele. The T-cell proliferative responses of subjects with IL10 GA and GG genotypes differed significantly. IL4 receptor AG and GG genotypes also showed significant differences in their T-cell proliferative responses to MTB antigens. These results yield a greater understanding of the genetic mechanisms that underlie the immune responses in tuberculosis and have implications for the design of therapeutic interventions.

The purpose of the present study is to clarify the mechanism of Japanese self-esteem (SE) in genetic and environmental influences using twin methodology. Eighty-one pairs of adolescent twins, including 50 pairs of monozygotic (MZ) twins and 31 pairs of dizygotic (DZ) twins, participated in this study. Self-esteem was assessed using the Rosenberg Self-Esteem Scale (RSES), translated into Japanese. As a result of using univariate twin analyses, model comparisons using the Akaike Information Criterion (AIC) indicated that the AE model was the best fit (AIC = −5.35). In the best-fitting AE model, the heritability (a2) of SE was revealed to be moderate, accounting for 49% of the variance; environmental influences (individual-specific environmental factors) explained 51% of the variance. These results are consistent with the findings of some behavioral genetics studies of SE in the West and show that there is no difference between Western and Japanese populations in the mechanism of SE considering genetic and environmental influences. The results also suggest the importance of considering both genetic and environmental factors in studies of Japanese SE.

An overview of the nature and success of organ transplantation in monozygotic (MZ) twins is presented. Recent timely examples demonstrate the unique donor–recipient relationship shared by MZ co-twins. Next, a recent analysis of MZ co-twin differences in the epigenetic profile and associations with differences in life history factors is presented. This work has been the focus of considerable attention by scientists and the media. The final section of this column summarizes information and events concerning twins' longevity, screen careers, older parenting of twins and the origin of the word ‘twin’.

Although twins often participate in medical research, few clinical trials are conducted entirely in twin populations. The purpose of this review is to demonstrate the substantial benefits and address the key challenges of conducting clinical trials in twin populations, or ‘twin-only trials’. We consider the unique design, analysis, recruitment and ethical issues that arise in such trials. In particular, we describe the different approaches available for randomizing twin pairs, highlight the similarity or correlation that exists between outcomes of twins, and discuss the impact of this correlation on sample size calculations and statistical analysis methods for estimating treatment effects. We also consider the role of both monozygotic and dizygotic twins for studying variation in outcomes, the factors that may affect recruitment of twins, and the ethics of conducting trials entirely in twin populations. The advantages and disadvantages of conducting twin-only trials are also discussed. Finally, we recommend that twin-only trials should be considered more often.

This paper intends to demonstrate that the conservative management of triplet pregnancy after delivery of one foetus is a feasible and reasonable approach. Three cases of triplet pregnancy with successful conservative management after miscarriage of one foetus, are presented and compared with cases in the literature. The route of delivery, as well as the role of tocolysis, cerclage, prophylactic antibiotic therapy and corticosteroids are discussed. Guidelines for conservative treatment are proposed. The deliveries of our three pregnancies were delayed by 63, 44 and 22 days respectively. Foetal and neonatal evolution are normal in five of the remaining foetuses. Only one intrauterine death is observed. No maternal complications with sequelae are reported. After abortion of the first triplet, contractions often persist and the birth of the two remaining foetuses may be unavoidable. Nevertheless, in our experience, confirmed by some reports in the literature, prolongation of the pregnancy after expulsion of the first foetus is possible. It can be achieved by cervical cerclage associated with tocolytic and antibiotic therapy. This management is not associated with significantly increased foetal-maternal morbidity. Twin Research (2000) 3, 71–75.

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.

Background: Hall (Embryologic development and monozygotic twinning. Acta Geneticae Medicae et Gemellologiae, Vol. 45, 1996, pp. 53–57) hypothesized that chromosomal aberrations can lead to monozygotic (MZ) twinning. However, twinning and chromosomal aberrations increase prenatal mortality and could reduce the prevalence of chromosomal aberrations in live-born twins. We compared prevalence proportion ratios (PPR) of chromosomal aberrations and trisomy 21 (T21) in live-born twins versus singletons born in Denmark during 1968–2009. Methods: We linked the Danish Twin Registry and a 5% random sample of all singletons to the Danish Cytogenetic Central Register and calculated PPR adjusted for maternal age for MZ, dizygotic (DZ), and all twins versus singletons. Zygosity was based on questionnaires or genetic markers. Results: No overall difference in risk of chromosomal aberrations or T21 in twins versus singletons was found. PPR in MZ and DZ twins was 0.87 (95% CI [0.60, 1.27]) and 1.05 (95% CI [0.88, 1.27]), respectively. For T21 there was a tendency to a lower prevalence in MZ twins compared to singletons (PPR: 0.29, 95% CI [0.07, 1.14]), whereas PPR was significantly increased in DZ twins (1.62, 95% CI [1.20, 2.19]). The observed proportion of MZ twin pairs among twin pairs with aberrations (0.22, 95% CI [0.16, 0.28]) was significantly lower than the proportion expected from the Weinberg method (0.32, 95% CI [CI, 0.26, 0.39]). Conclusion: Based on databases providing complete national coverage on twins with chromosomal aberrations, we found no overall difference in risk of chromosomal aberrations or T21 in twins versus singletons. Around conception twins may have an increased risk of chromosomal aberrations, but loss of especially MZ embryos could lead to similar risk among live-born twins and singletons.

The current research investigates the heritability of two of the most common response styles: acquiescence bias (tendency to agree or disagree with survey items regardless of the items’ actual content) and item keying (differential responding related to the use of regular- and reverse-keyed items). We estimated response styles from a common personality measure (HEXACO) and examined the heritability of each with univariate genetics analyses. The results show item keying effect was heritable but acquiescence bias was not. Neither response style was strongly influenced by the shared environment of the twins. Unique environmental effects were found to be substantial for response styles. The current findings have important implications for future research of response behaviors that are often overlooked by behavioral geneticists.

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.

Living in the same household exposes family members to shared environments and may be reflected in estimates of shared environment in twin analyses. The age at the separation of cotwins in a twin pair marks the end of such shared exposure, and the age of separation is commonly self-reported in studies. The objective of the study was to summarize the age at separation from residential records and use it to validate with self-reported separation status and age at the third and fourth wave of data collection in the FinnTwin12 cohort. Age at separation was generated from the address information, linking it to the Finnish Population information system since birth. Descriptive statistics by sex and zygosity are presented. The mean age at separation from residential records was 20.36 years old. Women separated earlier than men and dizygotic pairs earlier than monozygotic pairs. We also calculated the sensitivity and specificity with the self-reported separation status at waves 3 and 4, and interrater reliability with the self-reported separation age at wave 4. Age at separation from residential records had a relatively poor agreement with the self-report. This work enables us to use a more precise and objective measure for the shared environment in future twin studies.

The degree of assortative mating for psychological and physical traits in Asian societies is relatively unknown. The present study examined assortative mating for educational level, personality traits, religious affiliation, height, weight, and body mass index in a Korean sample. Age-adjusted spouse correlations were high for educational level (r = .63) and religious affiliation (r = .67), modest for most personality traits (rs = −.01 to .26), and trivial for height (r = .04), weight (r = .05), and body mass index (r = .11). These results were remarkably similar to those found from the western samples. Implications of the present findings in behavior genetic studies and human mating patterns were briefly discussed.

Co-twin control is a well-known methodological twin research design, but its variations and complexities are less well known. Various issues and illustrations are presented with reference to studies involving natural events, experimental interventions and rare happenings that underlie monozygotic (MZ) twins’ environmental differences. This discussion is followed by summaries of recent twin research pertaining to cancer risk in overweight twins, the physical risk to surviving twins after fetal loss of a co-twin, a 20-year update of twin concordance for Parkinson’s disease, and neuroanatomical differences in musically discordant MZ twin pairs. Several twin-related items that have attracted attention in the news are also summarized.

In both traditional and modern societies, twinship, as an unusual mode of reproduction, involves difficulties for social systems in maintaining consistent classification systems. It is proposed that the most prevalent response to twinship involves various ‘strategies of normalisation’ to defuse and contain the potential disruption. This proposition is illustrated and analysed in relation to ethnographic maternal drawn mainly (but not exclusively) from African communities in the twentieth century. Following a discussion of twin infanticide as the most extreme of the normalising strategies, the article concludes by identifying a number of paradoxes in the social construction of twinship. Twin Research (2000) 3, 142–147.

Sex differences in religion are well known, with females generally being more religious than males, and shared environmental factors have been suggested to have a large influence on religiousness. Twins from opposite-sex (OS) and same-sex (SS) pairs may differ because of a dissimilar psycho-social rearing environment and/or because of different exposures to hormones in utero. We hypothesized that OS females may display more masculine patterns of religiousness and, vice versa, that OS males may display more feminine patterns. We used a web-based survey conducted in Denmark, which is a secular society. The survey included 2,997 twins aged 20–40 years, identified through the population-based Danish Twin Registry. We applied la Cour and Hvidt's adaptation of Fishman's three conceptual dimensions of meaning: Cognition, Practice, and Importance, and we used Pargament's measure of religious coping (RCOPE) for the assessment of positive and negative religious coping patterns. Differences between OS and SS twins were investigated using logistic regression for each sex. The analyses were adjusted for dependence within twin pairs. No significant differences in religiousness and religious coping were found for OS and SS twins except that more OS than SS females were members of the Danish National Evangelical Lutheran Church and fewer OS than SS females were Catholic, Muslim, or belonged to other religious denominations. Moreover, OS males at age 12 had higher rates of church attendance than did SS males. This study did not provide evidence for masculinization of female twins with male co-twins with regard to religiousness. Nor did it show any significant differences between OS and SS males except from higher rates of church attendance in childhood among males with female co-twins.

A review of twin research on cleft lip and palate is presented. This information is accompanied by a look at the lives of young monozygotic (MZ) male triplets concordant for cleft lip, but discordant for the type and placement of the cleft and for the presence of cleft palate. Research on depression in mothers and fathers of twins conceived naturally and by artificial reproductive techniques follows. Current findings and implications of epigenetic differences in MZ twins are also summarized. Interesting life history events surrounding MZ twin chefs and table tennis players, as well as plans to construct the world's largest twin registry, are presented in the final section.

The seasonality of population data has been of great interest in demographic studies. When seasonality is analyzed, the population at risk plays a central role. In a study of the monthly number of births and deaths, the population at risk is the product of the size of the population and the length of the month. Usually, the population can be assumed to be constant, and consequently, the population at risk is proportional to the length of the month. Hence, the number of cases per day has to be analyzed. If one studies the seasonal variation in twin or multiple maternities, the population at risk is the total number of monthly confinements, and the study should be based on the rates of the multiple maternities. Consequently, if one considers monthly twinning rates, the monthly number of birth data is eliminated and one obtains an unaffected seasonality measure of the twin maternities. The strength of the seasonality is measured by a chi-squared test or by the standard deviation. When seasonal models are applied, one must pay special attention to how well the model fits the data. If the goodness of fit is poor, it can erroneously result in a statement that the seasonality is slight, although the observed seasonal fluctuations are marked.

The University of British Columbia (UBC) Twin Project is a registry of approximately 2,000 pairs of reared-together twins recruited across the major Canadian provinces. The focus of the project is an investigation of the behavioral genetics of personality and its disorders, and their relationship to other forms of mental illness. The goal is to find evidence for current diagnostic structures and classification systems, or certainly provide data for their reform. The primary measures employed are full-form self-report questionnaires of major psychological and psychiatric instruments covering personality, mood, general health, schizotypy, and more recently the anxiety disorders from symptoms resulting from exposure traumatic events, obsessive–compulsive behavior and beliefs, as well as coping strategies and health anxieties. No DNA has been collected. Data from the project have been used in several collaborative projects worldwide, and collaborative projects with other groups and interested researchers are welcomed.