Religious attendance has been shown to correlate negatively with alcohol use. We investigated whether this relationship is driven by genetic or environmental factors. Data on frequency of church attendance and frequency of alcohol use were obtained from twins and their families in the Virginia 30 000 study. A comprehensive bivariate model of family resemblance was fitted to the data using Mx. This model is described in detail. Results indicate that genetic factors primarily account for the relationship between alcohol and church attendance in males, whilst shared environmental factors, including cultural transmission and genotype-environment covariance, are stronger determinants of this association in females.

Human populations living during the Holocene underwent considerable microevolutionary change. It has been theorized that the transition of Holocene populations into agrarianism and urbanization brought about culture-gene co-evolution that favored via directional selection genetic variants associated with higher general cognitive ability (GCA). To examine whether GCA might have risen during the Holocene, we compare a sample of 99 ancient Eurasian genomes (ranging from 4.56 to 1.21 kyr BP) with a sample of 503 modern European genomes (Fst = 0.013), using three different cognitive polygenic scores (130 SNP, 9 SNP and 11 SNP). Significant differences favoring the modern genomes were found for all three polygenic scores (odds ratios = 0.92, p = 001; .81, p = 037; and .81, p = .02 respectively). These polygenic scores also outperformed the majority of scores assembled from random SNPs generated via a Monte Carlo model (between 76.4% and 84.6%). Furthermore, an indication of increasing positive allele count over 3.25 kyr was found using a subsample of 66 ancient genomes (r = 0.22, pone-tailed = .04). These observations are consistent with the expectation that GCA rose during the Holocene.

This study examined the genetic and environmental effects on variances in weight, height, and body mass index (BMI) under 18 years in a population-based sample from China. We selected 6,644 monozygotic and 5,969 dizygotic twin pairs from the Chinese National Twin Registry (CNTR) aged under 18 years (n = 12,613). Classic twin analyses with sex limitation were used to estimate the genetic and environmental components of weight, height, and BMI in six age groups. Sex-limitation of genetic and shared environmental effects was observed, especially when puberty begins. Heritability for weight, height, and BMI was low at 0–2 years old (less than 20% for both sexes) but increased over time, accounting for half or more of the variance in the 15–17 year age group for boys. For girls, heritabilities for weight, height and BMI was maintained at approximately 30% after puberty. Common environmental effects on all body measures were high for girls (59–87%) and presented a small peak during puberty. Genetics appear to play an increasingly important role in explaining the variation in weight, height, and BMI from early childhood to late adolescence, particularly in boys. Common environmental factors exert their strongest and most independent influence specifically in the pre-adolescent period and more significantly in girls. These findings emphasize the need to target family and social environmental interventions in early childhood years, especially for females. Further studies about puberty-related genes and social environment are needed to clarify the mechanism of sex differences.

The present study examined the genetic and environmental contributions to the temporal stability of verbal, non-verbal and general intelligence across a developmental period spanning childhood and adolescence (5–18 years). Longitudinal twin data collected in four different studies on a total of 1,748 twins, comprising 4,641 measurement points in total, were analyzed using genetic adaptations of the simplex model. The heterogeneity in the type of instrument used to assess psychometric intelligence across the different subsamples and ages allowed us to address the auxiliary question of how to optimally utilize the existing longitudinal data in the context of gene-finding studies. The results were consistent across domains (verbal, non-verbal and general intelligence), and indicated that phenotypic stability was driven primarily by the high stability of additive genetic factors, that the stability of common environment was moderate, and that the unique environment contributed primarily to change. The cross-subscale stability was consistently low, indicating a small overlap between different domains of intelligence over time. The high stability of additive genetic factors justifies the use of a linear combination of scores across the different ages in the context of gene-finding studies.

Shorter telomere length (TL) has found to be associated with lower birth weight and with lower cognitive ability and psychiatric disorders. However, the direction of causation of these associations and the extent to which they are genetically or environmentally mediated are unclear. Within-pair comparisons of monozygotic (MZ) and dizygotic (DZ) twins can throw light on these questions. We investigated correlations of within pair differences in telomere length, IQ, and anxiety/depression in an initial sample from Brisbane (242 MZ pairs, 245 DZ same sex (DZSS) pairs) and in replication samples from Amsterdam (514 MZ pairs, 233 DZSS pairs) and Melbourne (19 pairs selected for extreme high or low birth weight difference). Intra-pair differences of birth weight and telomere length were significantly correlated in MZ twins, but not in DZSS twins. Greater intra-pair differences of telomere length were observed in the 10% of MZ twins with the greatest difference in birth weight compared to the bottom 90% in both samples and also in the Melbourne sample. Intra-pair differences of telomere length and IQ, but not of TL and anxiety/depression, were correlated in MZ twins, and to a smaller extent in DZSS twins. Our findings suggest that the same prenatal effects that reduce birth weight also influence telomere length in MZ twins. The association between telomere length and IQ is partly driven by the same prenatal effects that decrease birth weight.

Arange of environmental risk factors, with childbirth the most notable, have been associated with the development of pelvic organ prolapse and urinary incontinence. However, indications of genetic influence (positive family histories, ethnic differences) have prompted research into the heritability of measures of pelvic organ descent and joint mobility, which have also been associated with prolapse and incontinence. Genes appear to influence about half of the variation in these measures and, furthermore, the pelvic organ measures are associated with elbow hyperextension at a phenotypic level (r ≈ .2). We examined these measures in young, nulligravid women to determine if their association is due to a common genetic source. Data were collected from 178 Caucasian female co-twins and non-twin sisters, 50 of whom returned to be retested, which allowed reliability to be estimated and unreliable variance to be isolated in the multivariate analyses. Structural equation modeling was used to estimate genetic associations between latent elbow and bladder mobility factors for which heritabilities were estimated to be 0.80 and 0.64 respectively. The association between these factors appeared to be mediated by common genes (genetic r =.48, non-shared environmental r = −.06), with genes influencing latent elbow mobility accounting for 14% of the variation in latent bladder mobility. We speculate that genes influencing connective tissue structure may underlie this association.

Emerging theory and empirical work suggest that the ‘Bruce Effect’, or the increase in spontaneous abortion observed in non-human species when environments become threatening to offspring survival, may also appear in humans. We argue that, if it does, the effect would appear in the odds of twins among male and female live births. We test the hypothesis, implied by our argument, that the odds of a twin among male infants in Norway fell below, while those among females rose above, expected levels among birth cohorts in gestation in July 2011 when a deranged man murdered 77 Norwegians, including many youths. Results support the hypothesis and imply that the Bruce Effect operates in women to autonomically raise the standard of fetal fitness necessary to extend the gestation of twins. This circumstance has implications for using twins to estimate the relative contributions of genes and environment to human responses to exogenous stimuli.

The aim of this study was to compare facial 3D analysis to DNA testing in twin zygosity determinations. Facial 3D images of 106 pairs of young adult Lithuanian twins were taken with a stereophotogrammetric device (3dMD, Atlanta, Georgia) and zygosity was determined according to similarity of facial form. Statistical pattern recognition methodology was used for classification. The results showed that in 75% to 90% of the cases, zygosity determinations were similar to DNA-based results. There were 81 different classification scenarios, including 3 groups, 3 features, 3 different scaling methods, and 3 threshold levels. It appeared that coincidence with 0.5 mm tolerance is the most suitable feature for classification. Also, leaving out scaling improves results in most cases. Scaling was expected to equalize the magnitude of differences and therefore lead to better recognition performance. Still, better classification features and a more effective scaling method or classification in different facial areas could further improve the results. In most of the cases, male pair zygosity recognition was at a higher level compared with females. Erroneously classified twin pairs appear to be obvious outliers in the sample. In particular, faces of young dizygotic (DZ) twins may be so similar that it is very hard to define a feature that would help classify the pair as DZ. Correspondingly, monozygotic (MZ) twins may have faces with quite different shapes. Such anomalous twin pairs are interesting exceptions, but they form a considerable portion in both zygosity groups.

We analyzed birth order differences in means and variances of height and body mass index (BMI) in monozygotic (MZ) and dizygotic (DZ) twins from infancy to old age. The data were derived from the international CODATwins database. The total number of height and BMI measures from 0.5 to 79.5 years of age was 397,466. As expected, first-born twins had greater birth weight than second-born twins. With respect to height, first-born twins were slightly taller than second-born twins in childhood. After adjusting the results for birth weight, the birth order differences decreased and were no longer statistically significant. First-born twins had greater BMI than the second-born twins over childhood and adolescence. After adjusting the results for birth weight, birth order was still associated with BMI until 12 years of age. No interaction effect between birth order and zygosity was found. Only limited evidence was found that birth order influenced variances of height or BMI. The results were similar among boys and girls and also in MZ and DZ twins. Overall, the differences in height and BMI between first- and second-born twins were modest even in early childhood, while adjustment for birth weight reduced the birth order differences but did not remove them for BMI.

It has been suggested that the risk of adverse perinatal outcomes in twin pregnancies is exacerbated by concomitant gestational diabetes mellitus (GDM). This study aimed to assess the risk incurred by twin pregnancy and by a diagnosis of GDM, separately, on the development of poor perinatal outcomes. A retrospective cohort study was conducted on all pregnant women at a tertiary center between 2016 and 2017. The impact of GDM and twin pregnancies on perinatal outcomes — birth weight above the 90th centile for gestational age, cesarean delivery, clinical neonatal hypoglycemia, and premature delivery (before 37 weeks’ gestation) — was assessed using univariate and multivariate analyses. Overall, 13,527 women were eligible for the study; 11,915 were uncomplicated singleton pregnancies; 1379 of these had GDM; 194 were twin pregnancies, and 39 of these had GDM. Univariate analyses showed that twin pregnancies were associated with a higher risk of all perinatal outcomes except macrosomia. In the multivariate analyses, twin pregnancy was a much higher predictor of cesarean delivery (OR 8.40, 95% CI [6.25, 11.49], p < .0001) and preterm birth (OR 58.82, 95% CI [31.25, 125], p < .0001) compared to GDM but GDM was a higher predictor of neonatal hypoglycemia (OR 4.87, 95% CI [3.74, 6.29], p < .0001). Twin pregnancy is more strongly associated with all adverse perinatal outcomes except macrosomia. GDM does not increase risk of adverse perinatal outcomes except for neonatal hypoglycemia.

In a recent article McArdle and Prescott (2005) showed how simultaneous estimation of the bio-metric parameters can be easily programmed using current mixed-effects modeling programs (e.g., SAS PROC MIXED). This article extends these concepts to deal with mixed-effect modeling of longitudinal twin data. The biometric basis of a polynomial growth curve model was used by Vandenberg and Falkner (1965) and this general class of longitudinal models was represented in structural equation form as a latent curve model by McArdle (1986). The new mixed-effects modeling approach presented here makes it easy to analyze longitudinal growth-decline models with biometric components based on standard maximum likelihood estimation and standard indices of goodness-of-fit (i.e., χ2, df, εa). The validity of this approach is first checked by the creation of simulated longitudinal twin data followed by numerical analysis using different computer programs (i.e., Mplus, Mx, MIXED, NLMIXED). The practical utility of this approach is examined through the application of these techniques to real longitudinal data from the Swedish Adoption/Twin Study of Aging (Pedersen et al., 2002). This approach generally allows researchers to explore the genetic and nongenetic basis of the latent status and latent changes in longitudinal scores in the absence of measurement error. These results show the mixed-effects approach easily accounts for complex patterns of incomplete longitudinal or twin pair data. The results also show this approach easily allows a variety of complex latent basis curves, such as the use of age-at-testing instead of wave-of-testing. Natural extensions of this mixed-effects longitudinal approach include more intensive studies of the available data, the analysis of categorical longitudinal data, and mixtures of latent growth-survival/ frailty models.

The Duke Twins Study of Memory in Aging is an ongoing, longitudinal study of cognitive change and dementia in the population-based National Academy of Sciences-National Research Council (NAS-NRC) Twin Registry of World War II Male Veterans. The primary goal of this study has been to estimate the overall genetic and environmental contributions to dementia with a specific focus on Alzheimer's disease. An additional goal has been to examine specific genetic and environmental antecedents of cognitive decline and dementia. Since 1989, we have completed 4 waves of data collection. Each wave included a 2-phase telephone cognitive screening protocol, followed by an in-home standardized clinical assessment for those with suspected dementia. For many participants, we have obtained postmortem neuro-pathological confirmation of the diagnosis of dementia. In addition to data on cognition, we have also collected information on occupational history, medical history, medications and other lifetime experiences that may influence cognitive function in late life. We provide an overview of the study's methodology and describe the focus of recent research.

While it is widely assumed that twinship involves high levels of intimacy to the detriment of other relationships, an examination of twinship using measures of intimacy has not yet been conducted. Participants were 214 individual twins (128 MZ, 62 DZ same-sex, and 24 DZ mixed-sex) between 16–73 years of age. Twins completed measures of intimacy in reference to their co-twin, their closest same-sex friend, and their closest other-sex friend. As expected, the highest level of intimacy was reported for the co-twin. Contrary to expectation, MZ twins did not report significantly higher levels of intimacy with their co-twin, or significantly lower levels of intimacy with their closest friends when compared to DZ same-sex or DZ mixed-sex twins. Furthermore, twins who reported high levels of intimacy with their co-twin did not report significantly lower levels of intimacy with their closest friends. Implications and directions for future research are discussed.

The age of attainment for four motor developmental traits, such as turning over, sitting up without support, pulling up to a standing position and walking without support, was examined in 822 children, including 626 siblings from families with 2 to 6 children, 68 pairs of dizygotic twins and 30 pairs of monozygotic twins. Correlation analysis, carried out separately for each type of sibship, showed the highest pairwise correlations in monozygotic twins and the lowest correlation in non-twin siblings for all motor milestones. Variance component analysis was used to decompose the different independent components forming the variation of the studied trait, such as genetic effect, common twin environment, common sib environment and residual factors. The results revealed that the major proportion of the total variance after adjustment for gestation age for the attainment of each motor skill, except pulling up to standing position, is explained by the common twin environment (50.5 to 66.6%), whilst a moderate proportion is explained by additive genetic factors (22.2 to 33.5%). Gestational age was found to be an important predictor of appearance of all motor milestones, affecting delay of 4.5 to 8.6 days for the attainment of the motor abilities for each week of earlier gestation. The age of attainment of the standing position was affected only by shared sibs environment (33.3% of the total variance) and showed no influence of either genetic or common twin environment. Phenotypic between trait correlations were high and significant for all studied traits (range between 0.40 and 0.67, P < 0.01 in all instances). Genetic cross correlations, however, were not easily interpreted and did not show clear variance trends among the different groups of children.

We investigated the effects of classroom separation on twins' behavior, progress at school, and reading abilities. This investigation was part of a longitudinal study of a nationally-representative sample of twins (the E-risk Study) who were assessed at the start of school (age 5) and followed up (age 7). We examined three groups of twins: pairs who were in the same class at both ages; pairs who were in separate classes at both ages; and pairs who were in the same class at age 5, but separated by age 7. When compared to those not separated, those separated early had significantly more teacher-rated internalizing problems and those separated later showed more internalizing problems and lower reading scores. Monozygotic (MZ) twins showed more problems as a result of separation than dizygotic (DZ) twins. No group differences emerged for externalizing problems, ADHD or prosocial behaviors. The implications of the findings for parents and teachers of twins, and for school practices about separating twins, are discussed.

A growing body of literature asserts the importance of friendships for a child's emotional wellbeing, behavioural adjustment and developmental attainment. However, few studies have examined friendships in the twin situation. This paper examines the literature on friendships among children who are twins and asks two questions: (1) What are the implications of the twin situation for friendship formation? (2) How can the twin situation inform theoretical knowledge concerning friendship formation in the general population? The discussion examines the implications of the twin situation for the size and nature of twin children's friendship pool and questions whether children who are twins are more likely to be adaptive or maladaptive in their social behaviour. The paper argues for the need to study the process and mechanisms in friendship formation among twin children because the findings might inform not only studies of friendship but also behavioural genetic research.

Population-based studies in twins have been of insufficient size to explore the relationship between risk of cerebral palsy and intrauterine growth. Earlier studies in singletons have suggested an optimum size at birth for minimum cerebral palsy risk between the 75th and 90th percentiles of weight for gestational age. We aggregated data from nine European cerebral palsy registers for 1976 to 1990. Using sex-specific fetal growth standards for twins, a z score of weight-for-gestation was derived for each of the 373 twin cases. The rates of cerebral palsy in each z-score band were compared to the rate in the a priori reference band of 0.67 to less than 1.28 (equivalent to the 75th to less than 90th percentiles). In twins born at 32 weeks' gestation or more (92% of all twins), cerebral palsy rates were higher for both light and heavy-forgestation babies compared to an optimum (i.e., minimum risk) in the reference band. However, the rate ratio for heavy babies (90th percentile or greater) did not reach conventional (95% confidence intervals [CI]) statistical significance (rate ratios = 1.76; 90% CI 1.02–3.03). For twins born at less than 32 weeks, the significantly higher risk for cerebral palsy was observed consistently in all z-score bands less than average compared to the reference band. This multi-center study demonstrates that for twins born at 32 weeks' gestation or more, an increased risk of cerebral palsy is associated with deviations from optimal intrauterine growth at about 1 standard deviation above mean weight, as was earlier reported for singletons. For twins born at less than 32 weeks' gestation, this pattern is only demonstrable for babies weighing below the optimum weight-for-gestation.

The identification of genetic factors that confer susceptibility to the epilepsies has to date been the focus of genetic efforts in this field. Few studies have assessed the genetic contribution to disease course in epilepsy, yet an understanding of the genetic influences on epilepsy outcome is key to developing new therapeutic strategies. The aim of this study was to assess the genetic contributions to epilepsy outcome in twin pairs concordant for epilepsy. We studied 37 epilepsy concordant twin pairs (27 monozygotic, 10 dizygotic) in whom there were no recognized environmental contributions (e.g., acquired brain injury) to epilepsy, and in whom the most likely cause for epilepsy was a shared genetic susceptibility. Clinical outcome was determined using the binary measure of Seizure Status (seizure remission or recurrence) and on a six-category ordinal Outcome Scale. Epilepsy outcome was independent of age of seizure onset, age at assessment and major epilepsy syndrome diagnosis. The proportion of twin pairs concordant for Seizure Status was 0.81 (22/27) for monozygous and 1.0 (10/10) for dizygous pairs, p = 0.3. Within-pair correlation in outcome (Outcome Scale) was 0.60 (95% CI: 0.32, 0.78) in monozygous and 0.78 (0.48, 0.92) in dizygous pairs. These data provide no evidence for genetic influences on epilepsy outcome independent of those that contribute to disease susceptibility. The observed high correlations for outcome suggest that, for epilepsy, susceptibility genes also have a major influence on outcome.

The temporal variation in the stillbirth rates (SBR), measured as the number of stillborn per 1000 total births, among singletons, twins and triplets was studied on Swedish birth data for the period 1869 to 2001 and comparisons with data from other populations were made. Among both single and multiple births there were marked, almost monotonously decreasing trends in the stillbirth rates. Among singletons the stillbirth rate decreased from 29.5 per 1000 in the period 1869 to 1878 to 3.4 in the period 1991 to 2001. Among twins the stillbirth rate decreased from 94 per 1000 in 1869 to 1878 to a minimum of 8.2 in 1991 to 2001 and among triplets from 166 per 1000 to a minimum of 19.8. The relative declining pattern in the SBRs was almost the same, being 88% among singletons, 91% among twins and 88% among triplets. In the 1980s and 1990s the definition of the stillbirth rate was changed in many countries, including Finland, but no changes in the definition of stillbirths have been made in Sweden. The effect of the artificial reproduction techniques, including in vitro fertilization, on the rates of multiple maternities is also discussed. It was noted especially that they had a more marked effect on the triplet than on the twinning rate.

Self-report scales for symptoms of anxiety and depression are frequently used for screening and research purposes. A moderate phenotypic association between disorders measured by diagnostic interviews and symptoms of anxiety and depression measured by self-report scales has been shown, but little is known about the overlap in these phenotypes' genetic and environmental variance. In the present study, we used twin modeling to identify common genetic and environmental liabilities underlying the phenotypic association between the self-report Symptom Checklist-5 (SCL-5) and lifetime internalizing disorders derived from the Composite International Diagnostic Interview (CIDI). The sample consisted of 7,992 young adult twins from the Norwegian Institute of Public Health Twin Panel (NIPHT), who all responded to a questionnaire. A subset of 2,793 individuals later underwent structured interviews. The best fitting model showed a strong genetic correlation of 0.82 (95% confidence interval; 0.61–1.0) between current self-report symptoms of anxiety and depression, and lifetime internalizing disorders, which suggests an almost complete overlap in genetic liability. The correlation between environmental factors was much lower: 0.16 (0.00–0.34, 95% CI). This implies that brief self-report scales capture genetic variance that is highly overlapping with the genetic variance common to internalizing disorder diagnoses. It thus follows that SCL-5 and similar instruments may be used as screening instruments for genetic risk factors that influence liability to internalizing disorders. In addition, existing data on self-report symptoms of anxiety and depression can be used with increased confidence to specify models including effects from genes coding for internalizing disorders.