Fetal death in a twin conception during second and third trimester is associated with increased risk of cerebral injury in the surviving twin. The aim of this study is to test the hypothesis that even early fetal losses as a ‘vanishing’ twin may be associated with an increased risk of cerebral impairment in the surviving twin. The study population comprised 362 pregnant women attending Liverpool Women's Hospital recruited between 1999 and 2001. Women were classified according to the first ultrasound scan into 3 groups: vanishing twin, twin and singleton. The vanishing twin group was further subdivided into ‘definite’ and ‘probable’. Children from these pregnancies were assessed at 1 year of age for their development and neurological function using the Griffiths Mental and Developmental Scales and Optimality score. Children from 229 pregnancies (63.2%) attended the assessment. Information on children from a further 21 (5.8%) pregnancies was obtained through a review of hospital records. Cerebral impairment was found in 2 children from the vanishing twin group, 2 from the twin group and none from the singleton group. When cases with definite vanishing twin are considered there is a significant difference between the vanishing twin and singleton group (relative risk 6.1; 95% confidence interval 1.5–8.3; p = .03). An additional study with an increased sample size would enable a more robust conclusion.

Since our last report on the voluntary Hungarian Twin Registry (HTR) in 2012, the number of pairs or multiplets included increased from 310 to 1044. Efforts to turn the registry into a population-based one are on the way. Nearly 128,000 twins living in Hungary (98,500 adults) will be mailed information on how to register on the new HTR website. Twins will be asked to invite their spouses and immediate family members. Meanwhile, strong cooperation through exchange programs has been developed with other foreign twin registries. Current research focuses on radiogenomics, musculoskeletal, cardiovascular and respiratory diseases, gut microbiome as well as basic molecular research and yielded new awards and further publications.

This study was conducted to review the overall short-term outcome of monoamniotic twins in Japan and to determine the prospective risk of fetal death so as to adequately counsel parents with monoamniotic twins. Study subjects were 101 women with monoamniotic twins who were registered with the Japan Society of Obstetrics and Gynecology Successive Pregnancy Birth Registry System and who had given birth at ≥22 weeks of gestation during 2002–2009. The gestational week at delivery (mean ± SD) was 31.8 ± 3.7. Fourteen women experienced intrauterine fetal death (IUFD). Short-term outcomes of co-twins born to the 14 women included 8 IUFDs, one early neonatal death within 7 days of life (END), and 5 survivors. Four other women experienced 5 ENDs. Thus, 13.9% (28/202) of infants died perinatally (22 IUFDs and 6 ENDs), 13.9% (14/101) of women experienced IUFD, and 82.2% (83/101) of women experienced neither IUFD nor END. Structural anomalies and twin-to-twin transfusion syndrome explained 17.9% (five infants) and 10.7% (three infants) of the 28 perinatal deaths, respectively. The prospective risk of IUFD was 13.9% (14/101) for women who reached gestational week 22−0/7, gradually decreasing thereafter but remaining at between 4.5% and 8.0% between gestational week 30−0/7 and 36−0/7.

We tested a hypothesis that there is no genetic correlation between general factors of intelligence and personality, despite both having been selected for in human evolution. This was done using twin samples from Australia, the United States, the Netherlands, Great Britain, and Croatia, comprising altogether 1,748 monozygotic and 1,329 same-sex dizygotic twin pairs. Although parameters in the model-fitting differed among the twin samples, the genetic correlation between the two general factors could be set to zero, with a better fit if the U.S. sample was excepted.

The Yoruba are an important ethnic group mainly occupying Southwestern Nigeria. Mainly for genetic reasons, this very large tribe happens to present the highest dizygotic twinning rate in the world (4.4 % of all maternities). The high perinatal mortality rate associated with such pregnancies has contributed to the integration of a special twin belief system within the African traditional religion of this tribe. The latter is based on the concept of a supreme deity called Olodumare or Olorun, assisted by a series of secondary gods (Orisha) while Yoruba religion also involves immortality and reincarnation of the soul based on the animistic cult of ancestors. Twins are therefore given special names and believed to detain special preternatural powers. In keeping with their refined artistic tradition, the Yoruba have produced numerous wooden statuettes called Ibejis that represent the souls of deceased newborn twins and are involved in elaborate rituals. Among Yoruba traditional beliefs and lore some twin-related themes are represented which are also found in other parts of the world. Basic features of the original Yoruba beliefs have found their way into the religious traditions of descendants of African slaves imported in the West Indies and in South America.

The Chinese National Twin Registry is the first and largest population-based twin registry in China. It was established in 2001. The primary goal of this program is the establishment of a population-based twin registry of 45,000 twin pairs from several regions representing north, south, urban, and rural areas in China. A secondary goal is to study genetic contributions to complex diseases, and to test associations of candidate genes with related phenotypes. Seven thousand, four hundred and twenty-three twin pairs have been enrolled in the registry, in which 1613 pairs have undergone detailed questionnaire assessments and physical examination. Based on the baseline registry, a twin cohort was established. Continued research includes studies on intermediate phenotypes of cardiovascular and cerebrovascular diseases and psychological studies in adult twins, studies on growth and development in adolescent twins, and so forth. The current state and future plans for the Chinese National Twin Registry will be discussed in this article.

The Georgia Cardiovascular Twin Study is a longitudinal study of biobehavioral antecedents of cardiovascular disease in youth and young adults, including around 500 twin pairs with roughly equal numbers of African Americans and European Americans. Focus of study includes the longitudinal change in relative influence of genetic and environmental factors (especially chronic stress) on development of risk factors for cardiovascular disease. Approaches include quantitative genetic modeling of phenotypic twin data as well as the examination of the influence of polymorphic variation in candidate genes and their potential interaction with environmental factors on these risk factors. Future work will expand the scope of the study to investigating the impact of chronic stress as measured by indices of the hypothalamus-pituitary-adrenal axis and the sympathetic nervous system on preclinical markers of cardiovascular disease, essential hypertension and type 2 diabetes.

The study endeavored to assess the epidemiology, postnatal survival, and growth pattern of multiple births in Sub-Saharan Africa (SSA). It was based on the data of 25 demographic health surveys conducted in the subcontinent since 2008. The records of 213,889 children born in the preceding 59 months of the surveys were included. The multiple birth rate was computed as the number of multiple confinements per 1,000 births. Factors associated with multiple births were identified using logistic regression and their survival pattern was assessed using the Kaplan–Meier method. The multiple birth rate was 17.1 (95% confidence interval: 17.7–16.6) and showed considerable variation across the 25 countries included in the study. Odds of multiple births were significantly increased with advanced maternal age, parity, and maternal height but not with wealth index, age at first birth, and month of birth. At the end of the fourth year of age, the cumulative survival probability was as low as 0.77 in multiple births as compared to 0.93 in their counterparts. The odds of neonatal, infant and under-five mortality were 5.55, 4.39, and 3.72 times increased in multiple births, respectively. Multiple births tend to be malnourished than singletons and the odds of wasting, stunting, and underweight were 1.31, 1.83, and 1.73 times raised, consecutively. Nevertheless, multiple births regain their weight-for-age (WFA) and height-for-age (HFA) deficits by the end of the fourth year of age. Counseling pregnant mothers with multiple gestation to give birth at a health institution and providing close medical follow-up during and after the neonatal period can improve the survival of multiple births.

Although the correlation between cognition and physical function has been well studied in the general population, the genetic and environmental nature of the correlation has been rarely investigated. We conducted a classical twin analysis on cognitive and physical function, including forced expiratory volume in one second (FEV1), forced vital capacity (FVC), handgrip strength, five-times-sit-to-stand test (FTSST), near visual acuity, and number of teeth lost in 379 complete twin pairs. Bivariate twin models were fitted to estimate the genetic and environmental correlation between physical and cognitive function. Bivariate analysis showed mildly positively genetic correlations between cognition and FEV1, rG = 0.23 [95% CI: 0.03, 0.62], as well as FVC, rG = 0.35 [95% CI: 0.06, 1.00]. We found that FTSST and cognition presented very high common environmental correlation, rC = -1.00 [95% CI: -1.00, -0.57], and low but significant unique environmental correlation, rE = -0.11 [95% CI: -0.22, -0.01], all in the negative direction. Meanwhile, near visual acuity and cognition also showed unique environmental correlation, rE = 0.16 [95% CI: 0.03, 0.27]. We found no significantly genetic correlation for cognition with handgrip strength, FTSST, near visual acuity, and number of teeth lost. Cognitive function was genetically related to pulmonary function. The FTSST and cognition shared almost the same common environmental factors but only part of the unique environmental factors, both with negative correlation. In contrast, near visual acuity and cognition may positively share part of the unique environmental factors.

In 2009, the first genome-wide association study (GWAS) for major depressive disorder (MDD) highlighted an association with PCLO locus on chromosome 7, although not reaching genome-wide significance level. In the present study, we revisited the original GWAS after increasing the overall sample size and the number of interrogated SNPs. In an analysis comparing 1,942 cases with lifetime diagnosis of MDD and 4,565 controls, PCLO showed a genome-wide significant association with MDD at SNP (rs2715157, p = 2.91 × 10−8) and gene-based (p = 1.48 × 10−7) level. Our results confirm the potential role of the PCLO gene in MDD, which is worth further replication and functional studies.

Depression and anxiety are highly comorbid due to shared genetic risk factors, but less is known about whether burnout shares these risk factors. We aimed to examine whether the covariation between major depressive disorder (MDD), generalized anxiety disorder (GAD), and burnout is explained by common genetic and/or environmental factors. This cross-sectional study included 25,378 Swedish twins responding to a survey in 2005–2006. Structural equation models were used to analyze whether the trait variances and covariances were due to additive genetics, non-additive genetics, shared environment, and unique environment. Univariate analyses tested sex limitation models and multivariate analysis tested Cholesky, independent pathway, and common pathway models. The phenotypic correlations were 0.71 (0.69–0.74) between MDD and GAD, 0.58 (0.56–0.60) between MDD and burnout, and 0.53 (0.50–0.56) between GAD and burnout. Heritabilities were 45% for MDD, 49% for GAD, and 38% for burnout; no statistically significant sex differences were found. A common pathway model was chosen as the final model. The common factor was influenced by genetics (58%) and unique environment (42%), and explained 77% of the variation in MDD, 69% in GAD, and 44% in burnout. GAD and burnout had additive genetic factors unique to the phenotypes (11% each), while MDD did not. Unique environment explained 23% of the variability in MDD, 20% in GAD, and 45% in burnout. In conclusion, the covariation was explained by an underlying common factor, largely influenced by genetics. Burnout was to a large degree influenced by unique environmental factors not shared with MDD and GAD.

The UBC Twin Project is an on-going study of personality and personality disorder that has collected personality data on approximately 1000 adult volunteer twin pairs. The primary purpose of the study is to develop and validate measures of the major forms of personality disorder as a quantitative alternative to the classificatory system first proposed by the DSM-III using the standard methods of personality and behavioural genetics research. To date, the study has explored the univariate and multivariate relationships between the major personality and personality disorder inventories, and their relationships to other psychopathologies including the mood and anxiety disorders and key psychological variables, such as cognitive ability, attachment and attitudes. The results have several implications for personality and personality disorders, most notably a rationalized diagnostic/nosological system to enhanced measurement instruments that index and reflect the influence of specific genetic and environmental influences.

Many studies that found associations between depression and nicotine dependence have ignored possible shared genetic influences associated with antisocial traits. The present study examined the contribution of genetic and environmental effects associated with conduct disorder (CD) and antisocial personality disorder (ASPD) to the comorbidity of major depression (MD) and nicotine dependence (ND). A telephone diagnostic interview, the Diagnostic Interview Schedule-III-R, was administered to eligible twins from the Vietnam Era Twin (VET) Registry in 1992. Multivariate genetic models were fitted to 3360 middle-aged and predominantly white twin pairs (1868 monozygotic, 1492 dizygotic pairs) of which both members completed the pertinent diagnostic interview sections. Genetic influences on CD accounted for 100%, 68%, and 50% of the total genetic variance in risk for ASPD, MD and ND, respectively. After controlling for genetic influences on CD, the partial genetic correlation between MD and ND was no longer statistically significant. Nonshared environmental contributions to the comorbidity among these disorders were not significant. This study not only demonstrates that the comorbidity between ND and MD is influenced by common genetic risk factors, but also further suggests that the common genetic risk factors overlapped with those for antisocial traits such as CD and ASPD in men.

Genes in the TGF9 signaling pathway play important roles in the regulation of ovarian follicle growth and ovulation rate. Mutations in three genes in this pathway, growth differentiation factor 9 (GDF9), bone morphogenetic protein 15 (BMP15) and the bone morphogenetic protein receptor B 1 (BMPRB1), influence dizygotic (DZ) twinning rates in sheep. To date, only variants in GDF9 and BMP15, but not their receptors transforming growth factor ß receptor 1 (TGFBR1), bone morphogenetic protein receptor 2 (BMPR2) and BMPR1B, have been investigated with respect to their roles in human DZ twinning. We screened for rare and novel variants in TGFBR1, BMPR2 and BMPR1B in mothers of dizygotic twins (MODZT) from twin-dense families, and assessed association between genotyped and imputed variants and DZ twinning in another large sample of MODZT. Three novel variants were found: a deep intronic variant in BMPR2, and one intronic and one non-synonymous exonic variant in BMPRB1 which would result in the replacement of glutamine by glutamic acid at amino acid position 294 (p.Gln294Glu). None of these variants were predicted to have major impacts on gene function. However, the p.Gln294Glu variant changes the same amino acid as a sheep BMPR1B functional variant and may have functional consequences. Six BMPR1B variants were marginally associated with DZ twinning in the larger case-control sample, but these were no longer significant once multiple testing was taken into account. Our results suggest that variation in the TGF9 signaling pathway type II receptors has limited effects on DZ twinning rates in humans.

Fifty-two Australian couples who had experienced the death of at least one member of a multiple birth (twin or higher order), with at least one survivor of that birth, were interviewed about their experiences at the time of the death, and since. This study compared parents' coping after the twins' deaths using the Beck Depression Inventory II, Perinatal Grief Scale, and unstructured interviews with some structured queries. Parents provided information on the influence of family, community and medical staff. According to retrospective reports, mothers experienced significantly more depression and grief than fathers at the time of loss. Both parents found the death of their twins grievous, but fathers, unlike mothers, were not encouraged to express their emotions. Although parents generally agreed about what helped them cope, fathers believed that they should be able to cope regardless of their grief. The strength of parents' spiritual beliefs had increased significantly since their loss, and there was some evidence that depressed and grieving mothers turned to spiritual support. Parents whose children died earlier reported levels of depression similar to those reported by parents whose children died later. To date, this is the largest study of grief in couples who have experienced the death of a twin and who have a surviving twin or higher order multiple.

The aim of this study was to quantify contributions of genetic and environmental factors to variation in timing of emergence of the primary teeth in a sample of monozygotic and dizygotic twins, using univariate model-fitting approaches. The sample comprised 94 pairs of monozygotic twins and 125 pairs of dizygous twins, all of European ancestry, aged from 2–6 years. Tooth emergence timing was based on parental report, with a subset of data validated by clinical assessment. Heritability estimates for tooth emergence timing were generally high, around 90%, however estimates for the lower right lateral incisor and the lower canines were around 50%. These findings confirm a strong genetic influence on observed variation in the timing of emergence of the human primary teeth.

In this study we examined the genetic architecture of variation in the pro-inflammatory state, using an extended twin-family design. Within the Netherlands Twin Register Biobank, fasting Tumor Necrosis Factor-α (TNF-α), Interleukin-6 (IL-6), C-Reactive Protein (CRP), and fibrinogen levels were available for 3,534 twins, 1,568 of their non-twin siblings, and 2,227 parents from 3,095 families. Heritability analyses took into account the effects of current and recent illness, anti-inflammatory medication, female sex hormone status, age, sex, body mass index, smoking status, month of data collection, and batch processing. Moderate broad-sense heritability was found for all inflammatory parameters (39%, 21%, 45%, and 46% for TNF-α, IL-6, CRP and fibrinogen, respectively). For all parameters, the remaining variance was explained by unique environmental influences and not by environment shared by family members. There was no resemblance between spouses for any of the inflammatory parameters, except for fibrinogen. Also, there was no evidence for twin-specific effects. A considerable part of genetic variation was explained by non-additive genetic effects for TNF-α, CRP, and fibrinogen. For IL-6, all genetic variance was additive. This study may have implications for future genome-wide association studies by setting a clear numerical target for genome-wide screens that aim to find genetic variants regulating the levels of these pro-inflammatory markers.

The Guangzhou Twin Registry, initiated in 2005, is a population-based registry of twins born between 1987 and 2000. To date, over 9700 pairs of twins, regardless of their health and medical history, were enrolled in the database using the Official Household Registry of Guangzhou City. The twins were subsequently verified by door-to-door visits based on the registry address. The primary goal of this registry is to develop a resource for genetic epidemiological studies on common diseases in the southern Chinese population. The initial focus is to distinguish the genetic and environmental determinants of eye diseases, in particular myopia and glaucoma. About 1000 pairs of twins living close to the Zhongshan Ophthalmic Center were invited for the first phenotyping examination, questionnaire administration and DNA collection in July and August 2006. An annual eye examination and other phenotype data collection have been scheduled for up to 5 years in order to investigate changes in phenotypes including the myopia progression, physical development and the changes of other eye-related phenotypes. Recruitment of adult twins aged 50 years and over is underway in the same city with the assistance of the government.

The Florida State Twin Registry began in 2002 through a pilot study of personality disorders and executive cognitive functioning in adult twins. Since 2006, the registry has grown substantially as part of the Learning Disability Research Center at Florida State University that recently began its second funding cycle through the National Institute of Child Health and Development. An update on the Florida State Twin Registry sample, focus, and measures is provided, as well as future directions.

Maximum likelihood estimation techniques are widely used in twin and family studies, but soon reach computational boundaries when applied to highly complex models (e.g., models including gene-by-environment interaction and gene–environment correlation, item response theory measurement models, repeated measures, longitudinal structures, extended pedigrees). Markov Chain Monte Carlo (MCMC) algorithms are very well suited to fit complex models with hierarchically structured data. This article introduces the key concepts of Bayesian inference and MCMC parameter estimation and provides a number of scripts describing relatively simple models to be estimated by the freely obtainable BUGS software. In addition, inference using BUGS is illustrated using a data set on follicle-stimulating hormone and luteinizing hormone levels with repeated measures. The examples provided can serve as stepping stones for more complicated models, tailored to the specific needs of the individual researcher.