The importance of including developmental and environmental measures in genetic studies of human pathology is widely acknowledged, but few empirical studies have been published. Barriers include the need for longitudinal studies that cover relevant developmental stages and for samples large enough to deal with the challenge of testing gene–environment–development interaction. A solution to some of these problems is to bring together existing data sets that have the necessary characteristics. As part of the National Institute on Drug Abuse-funded Gene-Environment-Development Initiative, our goal is to identify exactly which genes, which environments, and which developmental transitions together predict the development of drug use and misuse. Four data sets were used of which common characteristics include (1) general population samples, including males and females; (2) repeated measures across adolescence and young adulthood; (3) assessment of nicotine, alcohol, and cannabis use and addiction; (4) measures of family and environmental risk; and (5) consent for genotyping DNA from blood or saliva. After quality controls, 2,962 individuals provided over 15,000 total observations. In the first gene–environment analyses, of alcohol misuse and stressful life events, some significant gene–environment and gene–development effects were identified. We conclude that in some circumstances, already collected data sets can be combined for gene–environment and gene–development analyses. This greatly reduces the cost and time needed for this type of research. However, care must be taken to ensure careful matching across studies and variables.

Neuroimaging studies have indicated abnormalities in cortico-striato-thalamo-cortical circuits in obsessive–compulsive disorder patients, but results have not been consistent. Since there are significant sex differences in human brain anatomy and obsessive–compulsive symptomatology and its developmental trajectories tend to be distinct in males and females, we investigated whether sex is a potential source of heterogeneity in neuroimaging studies on obsessive–compulsive symptoms. We selected male and female twin pairs who were concordant for scoring either high or low for obsessive–compulsive symptoms and a group of discordant pairs where one twin scored high and the co-twin scored low. The design included 24 opposite-sex twin pairs. Magnetic resonance imaging scans of 31 males scoring high for obsessive–compulsive symptoms, 41 low-scoring males, 58 high-scoring females, and 73 low-scoring females were analyzed and the interaction of obsessive–compulsive symptoms by sex on gray matter volume was assessed using voxel-based morphometry. An obsessive–compulsive symptom by sex interaction was observed for the left middle temporal gyrus, the right middle temporal gyrus, and the right precuneus. These interactions acted to reduce or hide a main effect in our study and illustrate the importance of taking sex into account when investigating the neurobiology of obsessive–compulsive symptoms.

Disordered gambling (DG) will soon be included along with the substance use disorders in a revised diagnostic category of the Diagnostic and Statistical Manual of Mental Disorders DSM-5 called ‘Substance Use and Addictive Disorders’. This was premised in part on the common etiologies of DG and the substance use disorders. Using data from the national community-based Australian Twin Registry, we used biometric model fitting to examine the extent to which the genetic liabilities for DG and alcohol use disorder (AUD) were shared, and whether this differed for men and women. The effect of using categorical versus dimensional DG and AUD phenotypes was explored, as was the effect of using diagnoses based on the DSM-IV and the proposed DSM-5 diagnostic criteria. The genetic correlations between DG and AUD ranged from 0.29 to 0.44. There was a significantly larger genetic correlation between DG and AUD among men than women when using dimensional phenotypes. Overall, about one-half to two-thirds of the association between DG and AUD was due to a shared genetic vulnerability. This study represents one of the few empirical demonstrations of an overlap in the genetic risk for DG and another substance-related addictive disorder. More research is needed on the genetic overlap between DG and other substance use disorders, as well as the genetic overlap between DG and other (non-substance-related) psychiatric disorders.

The aim of this study was to estimate genetic and environmental influences on the longitudinal evolution of leisure-time physical activity habits from adolescence to young adulthood. Data were gathered at four time points, at mean ages 16.2, 17.1, 18.6, and 24.5 years. At baseline, the sample comprised 5,216 monozygotic and dizygotic twins, born 1975–1979, and, at the last follow-up point, of 4,531 monozygotic and dizygotic twins. Physical activity volume was assessed as frequency of leisure-time physical activity and participants were categorized into three groups: inactive, moderately active, and active. Genetic and environmental influences were estimated using a multivariate, longitudinal Cholesky decomposition with a ‘multifactorial liability threshold’ approach. The results suggest that, in both sexes the heritability of leisure-time physical activity remained moderate (~43–52%) during adolescence, declining to ~30% in young adulthood. Shared environmental influences increased from adolescence (~18–26%) to young adulthood (43% in men and 49% in women). Specific environmental influences remained relatively stable during the total follow-up (~20–30%). New genetic, shared, and specific environmental influences at every follow-up point were suggested by the low correlations across occasions. In conclusion, the study demonstrated gender differences in genetic influences in the evolution of leisure-time physical activity habits from adolescence to young adulthood. However, shared environmental influences, especially in women, were crucial in explaining longitudinal changes in leisure-time physical activity. These outcomes emphasize the need of gender-specific measures to promote physical activity habits during young adulthood.

It is well known that human behavior and individual psychological traits are moderately to substantially heritable. Over the past decade, an increasing number of studies have explored the genetic and environmental influence on religiousness. These studies originate predominantly from countries generally considered more religious than the very secular northern European countries. Comparisons of the results are complicated by diverse definitions of religiousness, but several studies indicate that the influence of the family environment is most predominant in early life, whereas genetic influences increase with age. We performed a population-based twin study of religiousness in a secular society using data from a Web-based survey sent to 6,707 Danish twins born 1970–1989, who were identified in the Danish Twin Registry. We applied Fishman's three conceptual dimensions of religiousness: cognition, practice, and importance. In all polygenic models and biometric analyses, we controlled for gender and age. The study sample comprised 2,237 same sex twins, a response rate of 45%. We found high correlations within both monozygotic and dizygotic twin pairs in most items of religiousness, indicating a large influence from shared environmental factors. Personal religiousness such as praying to God, believing in God, and finding strength and comfort in religion were more influenced by genetic factors than were social forms of religiousness such as church attendance. We found a small tendency for increasing genetic influence with increasing age for some religious items, but not for all.

This replication study examines relations between alexithymia and trait emotional intelligence (trait EI) at the phenotypic, genetic, and environmental levels. A sample of 1,444 same-sex twin pairs (850 MZ pairs and 594 DZ pairs) completed the Toronto Alexithymia Scale-20. A subset of 494 same-sex twin pairs (287 MZ pairs and 207 DZ pairs) had earlier completed the Trait Emotional Intelligence Questionnaire. Individual differences in alexithymia were attributable to genetic, non-shared environmental, and shared environmental factors. All but one of the facets of alexithymia were negatively and significantly correlated with the factors of trait EI, and these phenotypic correlations were entirely attributable to correlated genetic and correlated non-shared environmental factors. These bivariate results provide a valuable replication of those of Baughman et al. (Twin Research and Human Genetics, Vol. 14, 2011, pp. 539–543), which was conducted with substantially smaller samples of twins.

Genome-wide association studies (GWAS) of attention-deficit/hyperactivity disorder (ADHD) offer the benefit of a hypothesis-free approach to measuring the quantitative effect of genetic variants on affection status. Generally the findings of GWAS relying on ADHD status have been non-significant, but the one study using quantitative measures of symptoms found SLC9A9 and SLC6A1 were associated with inattention and hyperactivity–impulsivity. Accordingly, we performed a GWAS using quantitative measures of each ADHD subtype measured with the Strengths and Weaknesses of ADHD and Normal Behaviour (SWAN) scale in two community-based samples. This scale captures the full range of attention and kinetic behavior; from high levels of attention and appropriate activity to the inattention and hyperactivity–impulsivity associated with ADHD within two community-based samples. Our discovery sample comprised 1,851 participants (mean age = 22.8 years [4.8]; 50.6% female), while our replication sample comprised 155 participants (mean age = 26.3 years [3.1]; 68.4% females). Age, sex, age × sex, and age2 were included as covariates and the results from each sample were combined using meta-analysis, then analyzed with a gene-based test to estimate the combined effect of markers within genes. We compare our results with markers that have previously been found to have a strong association with ADHD symptoms. Neither the GWAS nor subsequent meta-analyses yielded genome-wide significant results; the strongest effect was observed at rs2110267 (4.62 × 10−7) for symptoms of hyperactivity–impulsivity. The strongest effect in the gene-based test was for GPR139 on symptoms of inattention (6.40 × 10−5). Replication of this study with larger samples will add to our understanding of the genetic etiology of ADHD.

Breastfeeding is considered the best and most natural way of feeding infants during the first months of life. Breastfeeding has multiple short- and long-term benefits for the health of the mother and babies, and from an evolutionist standpoint, it would be a behavior worth preserving throughout time. The aim of the present study was to explore the relative influence of genetic and environmental factors in this behavior. Three hundred and ninety pairs of adult female twins provided information about whether they breastfed their children and for how long. Three variables were analyzed: initiation and duration for the first baby, and mean duration for the complete offspring. Polychoric correlations were consistently higher for monozygotic twins, supporting a role for genetic factors (0.49 vs. 0.22 for initiation; 0.44 vs. 0.22 for duration in the first newborn; and 0.52 vs. 0.31 for duration on average). Model-fitting analyses found that in the best-fitting model, variance was explained by additive genetic and non-shared environmental factors, with estimated heritabilities ranging from 0.39 to 0.52 in the measures studied. The rest of the variance would be due to unique environmental factors. We conclude that genetic factors have a significant impact on the complex behavior of breastfeeding.

The associations between height and reproductive success in humans have attracted long-time scientific interest, but in addition to rather mixed previous results, little is still known about the background mechanisms of these associations. We analyzed the association of adult height with age at first birth and lifetime reproductive success using a twin study design that is able to optimally take into account family background and estimate the contributions of genetic and environmental factors. Information on live births as of June 2009 for 7,830 Finnish twins born 1950–1957 was extracted from the national population register. We found evidence for non-linear associations in men, as men in the third sex-specific height quintile had the highest probability of having one to two children, or three or more children at individual level analyses, and also to have any children when analyzing twin pairs discordant for height. Furthermore, the probability of having a spouse was highest in the third height category in men. Short stature was associated with earlier age at first birth in females, explained by correlated genetic factors, but not with lifetime number of children or having a spouse. Our results suggest that average stature may give some advantage for reproduction in males. In females, genetic factors explained the association between short stature and young age at first birth, which may suggest the role of hormonal factors.

Twin studies are extremely useful for investigating hypotheses of genetic influence on a range of behavioral and physical traits in humans. Studies of physical traits, however, are usually limited to size-related biological characteristics because it is inherently difficult to quantify the morphological counterpart – shape. In recent years, the development of geometry-preserving analytical techniques built upon multivariate statistical methodologies has produced a new discipline in biological shape analysis known as geometric morphometrics. In this study of hand shape analysis, we introduce the reader already familiar with the field of twin research to the potential utility of geometric morphometrics and demonstrate the cross-discipline applicability of methods. We also investigate and compare the efficacy of the 2D:4D ratio, a commonly used marker of sexual dimorphism, to the fully multivariate approach of shape analysis in discriminating between male and female sex. Studies of biological shape variation utilizing geometric morphometric techniques may be completed with software freely available on the Internet and time invested to master the small learning curve in concepts and theory.

An anonymous survey of Australian Fellows of the Royal Australian and New Zealand College of Obstetricians and Gynaecologists was conducted with the aim of understanding current practice and attitudes toward population-based carrier screening for inherited conditions in the setting of routine pregnancy care. Of 1,121 Fellows invited to complete the online questionnaire by e-mail, 237 (21%) responded, and of these 156 were practicing obstetricians and completed the whole survey. Of the respondents, 83% expressed support for population-based carrier screening for at least some conditions, with 97% supporting carrier screening for β-thalassaemia, and 83% supporting carrier screening for cystic fibrosis (CF). A small proportion of obstetricians reported offering carrier screening as part of routine pregnancy care (20% for β-thalassaemia, 8% for CF, 5% for fragile X syndrome, and 2% for spinal muscular atrophy). The main practical barriers identified for screening were cost, time constraints, and availability of supporting services. Addressing these issues is crucial for the successful implementation of population-based carrier screening programs in Australia and internationally.

Recently, there has been a significant increase in the rate of multiple births in most developed countries. However, few population-based studies have been conducted in China regarding the epidemiology of twin births in recent years. We performed a descriptive analysis of twin births from 1993 to 2005 using data from a population-based perinatal care program in southeast China. The twin birth rate in southeast China was 0.65%, and the twin birth rates from 1993 to 2005 fluctuated between 0.60% and 0.70%. During the three periods of 1993–1996, 1997–2000, and 2001–2005, the twin birth rate increased from 0.57% to 0.71% in urban areas (p = .005) and from 0.59% to 0.68% in mothers who had an education level of high school or higher (p = .046). After 2000, the twin birth rate of primiparae 30 years of age and older significantly increased from 0.72% to greater than 1.20%. We concluded that the twin birth rates in southeast China from 1993 to 2005 stayed constant in the overall population but increased in certain subgroups of women, presumably due to increased use of fertility treatment and the development of assisted reproductive technology.

Objective: The aim of this study was to compare the neurodevelopmental outcome at 12–18 months’ corrected age between multiples and singleton preterm infants. Methods: We designed a prospective study of preterm infants (≤32 weeks gestation) born and hospitalized in the neonatal intensive care unit between November 2008 and November 2009, whose assessments were performed at 12–18 months’ corrected age. Neurodevelopmental impairment was defined as the presence of any one of the following: moderate or severe cerebral palsy, severe bilateral hearing loss or bilateral blindness, mental developmental index score, or psychomotor developmental index score less than 70. Results were compared for both multiples and singleton infants. Results: One hundred and fifty-nine multiples and 211 singleton infants were assessed at 12–18 months’ corrected age. The neurodevelopmental outcome including all parameters at 12–18 months’ corrected age in multiples was not significantly different from singleton preterm infants. Conclusions: Multiple gestation in preterm infants is not associated with an increased risk of neurodevelopmental impairment at 12–18 months’ corrected age compared with singleton preterm infants. For further information, long term and high participation in neurodevelopmental follow-up and evaluation at pre-school age will be needed.

Fetal movements and fetal heart rate (FHR) are well-established markers of fetal well-being and maturation of the fetal central nervous system. The purpose of this paper is to review and discuss the available knowledge on fetal movements and heart rate patterns in twin pregnancies. There is some evidence for an association or similarity in fetal movement incidences or FHR patterns between both members of twin pairs. However, the temporal occurrence of these patterns seems to be for the most part asynchronous, especially when stricter criteria are used to define synchrony. The available data suggest that fetal behavior is largely independent of sex combination, fetal position, and presentation. Conversely, chorionicity appears to have some influence on fetal behavior, mainly before 30 weeks of gestation. There is preliminary evidence for the continuity of inter-individual differences in fetal activity and FHR patterns over pregnancy. Comparisons between studies are limited by large methodological differences and absence of uniform concepts and definitions. Future studies with high methodological quality are needed to provide a more comprehensive knowledge of normal fetal behavior in twin pregnancy.

Background: Twin pregnancies are at increased physiological and psychosocial risks. Objective: To investigate the prevalence of major depression in twin pregnancies and correlate with stress and social support. Method: The study included 51 pregnant women under specialized prenatal care who were evaluated by a Portuguese version of the semi-structured questionnaire Primary Care Evaluation of Mental Disorders (PRIME-MD) for Major Depression, and the Prenatal Psychosocial Profile (PPP) for evaluation of stress and social support. Results: Major depression was found in 33.3% of pregnant women, and prevailing symptoms were fatigue or loss of energy (100%), insomnia or hypersomnia (82.4%), changes in appetite (82.4%), decreased interest in daily activities (82.4%), and psychomotor agitation or retardation (82.4%). Among pregnant women who were diagnosed depressive, 76.5% also had a high level of stress and 47.1% complained about lack of social support. Statistical significance was found when correlating depression with perception of negative aspects of having twins and belief in significant body changes during pregnancy (p = .005 and .03, respectively). Marital status, occupation, and pregnancy planning were not significantly associated with the diagnosis of depression. Conclusion: Major depression occurs in one-third of pregnant women expecting twins and is associated with higher levels of stress and lack of social support. A multidisciplinary approach in these cases is fundamental to minimize further risks and complications.

This study aimed to explore if natural dizygotic (DZ) twinning is associated with earlier menopause and lower anti-Mullerian hormone (AMH) values. We investigated if advanced biological reproductive aging, which can be responsible for the multiple follicle growth in familial twinning, is similar to mechanisms that occur in normal ovarian aging, reflected by earlier menopause in mothers of DZ twins and lower levels of AMH. A total of 16 mothers of DZ twins enrolled with the Netherlands Twin Register (average age at first assessment: 35.9 ± 3.0 years) and 14 control mothers (35.1 ± 3 years) took part in a prospective study. Fifteen years after entry into the study, which included follicle-stimulating hormone (FSH) assessment, AMH was measured in stored serum samples and menopause status was evaluated. Average AMH levels were not significantly different between DZ twin mothers and controls (2.1 ± 2.4 μg/L vs. 1.9 ± 1.9 μg/L). Among the 16 mothers of twins, 7 had an elevated (FSH) value over 10 U/L at first assessment. Their AMH levels were lower than the nine twin mothers with normal FSH values: 0.6 ± 0.4 versus 3.4 ± 2.6 μg/L (p = .01). Of the mothers of twins, eight mothers had entered menopause at the second assessment compared with only one control mother (p = .07). Thus, slightly more DZ mothers were in menopause than the control mothers, although this difference was not significant. The subgroup of DZ twin mothers who had an increased FSH concentration 15 years ago had a limited ovarian reserve as reflected by lower AMH levels. These data indicate that advanced ovarian aging can be a feature in familial DZ twinning, particularly with elevated early follicular phase FSH.

The infant mortality rate (IMR) among single and twin births from 1999 to 2008 was analyzed using Japanese Vital Statistics. The IMR was 5.3-fold higher in twins than in singletons in 1999 and decreased to 3.9-fold in 2008. The reduced risk of infant mortality in twins relative to singletons may be related, partially, to survival rates, which improved after fetoscopic laser photocoagulation for twin — twin transfusion syndrome. The proportion of neonatal deaths among total infant deaths was 54% for singletons and 74% for twins. Thus, intensive care of single and twin births may be very important during the first month of life to reduce the IMR. The IMR decreased as gestational age (GA) rose in singletons, whereas the IMR in twins decreased as GA rose until 37 weeks and increased thereafter. The IMR was significantly higher in twins than in singletons from the shortest GA (<24 weeks) to 28 weeks as well as ≥38 weeks, whereas the IMR was significantly higher in singletons than in twins from 30 to 36 weeks. As for maternal age, the early neonatal and neonatal mortality rates as well as the IMR in singletons were significantly higher in the youngest maternal age group than in the oldest one, whereas the opposite result was obtained in twins. The lowest IMR in singletons was 1.1 per 1,000 live births for ≥38 weeks of gestation and heaviest birth weight (≥2,000 g), while the lowest IMR in twins was 1.8 at 37 weeks and ≥2,000 g.

A foundation for studying multiple myeloma, launched by a monozygotic twin, is described. Twin studies of that disease are also examined. A summary of selected research studies and news events follows, with specific reference to a new critique of twin research, sexual attraction in separated opposite-sex twins, twinning rates following assisted reproductive technology and family histories of multiple implantation. Human interest topics cover a gathering of researchers from the Minnesota Study of Twins Reared Apart, morning sickness and twin pregnancy, sexuality in conjoined twins, a famous hidden twin, and a film about switched at birth babies.