It has been suggested that twinning may influence handedness through the effects of birth order, intra-uterine crowding and mirror imaging. The influence of these effects on handedness (for writing and throwing) was examined in 3657 Monozygotic (MZ) and 3762 Dizygotic (DZ) twin pairs (born 1893–1992). Maximum likelihood analyses revealed no effects of birth order on the incidence of left-handedness. Twins were no more likely to be left-handed than their singleton siblings (n = 1757), and there were no differences between the DZ co-twin and siblingtwin covariances, suggesting that neither intra-uterine crowding nor the experience of being a twin affects handedness. There was no evidence of mirror imaging; the co-twin correlations of monochorionic and dichorionic MZ twins did not differ. Univariate genetic analyses revealed common environmental factors to be the most parsimonious explanation of familial aggregation for the writing-hand measure, while additive genetic influences provided a better interpretation of the throwing hand data.

The human ACTN3 gene encodes the protein α-actinin-3, a component of the contractile apparatus in fast skeletal muscle fibers. In 1999, we identified a common polymorphism in ACTN3 (R577X) that results in absence of α-actinin-3 in more than one billion people worldwide, despite the ACTN3 gene being highly conserved during human evolution. In 2003, we demonstrated that ACTN3 genotype influences elite athletic performance, and the association between ACTN3 genotype and skeletal muscle performance has since been replicated in athletes and non-athlete cohorts. We have also studied the evolution of the R577X allele during human evolution and demonstrated that the null (X) allele has undergone strong, recent positive selection in Europeans and Asian populations. We have developed an Actn3 knockout mouse model that replicates α-actinin-3 deficiency in humans and has already provided insight into the role of α-actinin-3 in the regulation of skeletal muscle metabolism, fibre size, muscle mass and contractile properties. In particular, mouse muscle lacking α-actinin-3 uses energy more efficiently, with the fast fibers displaying metabolic and contractile properties of slow oxidative fibers. While this favors endurance activities, the trade off is that the muscle cannot generate the rapid contractions needed to excel in sprinting. We propose that the shift towards more efficient aerobic muscle metabolism associated with α-actinin-3 deficiency also underlies the adaptive benefit of the 577X allele. Our future studies will focus on the effect of ACTN3 genotype on response to exercise and ageing, and the onset and severity of muscle disease phenotype.

Twin studies that examine the effect of specific environmental risk factors on psychiatric disorders assume that there are no differences in prevalences of these risk factors between twins and singletons. Violation of this assumption signifies that the results from twin studies might not generalize to singletons. Another assumption, not only often underlying twin studies but also epidemiological research, is that life- events are not influenced by familial factors. We tested differences in prevalences of experienced life events in a Dutch sample of 2086 monozygotic (MZ) twins, 2090 dizygotic (DZ) twins and 1307 of their siblings. Self-reported data on life events (illness of self, illness of a significant other, spouse/romantic relationship, divorce/break-up of a relationship, death of a significant other, traffic accident, robbery, violent assault, sexual assault) were available from a survey- study. We further investigated whether familial resemblance was present for the exposure to these life events and, if so, whether this resemblance was due to genetic or common environmental factors. No differences were found in the prevalences of life events between MZ twins, DZ twins and their siblings. There was evidence for familial aggregation of all life events, except for traffic accidents in women. Results indicated genetic control on the presence of a spouse or involvement in a relationship. Familial resemblance of illness and death of a significant other was mainly due to common environment. For the other life events, it was not possible to distinguish between genetic and common environmental effects.

The purpose of this study was to examine changes in the contribution of genetic and environmental influences to leisure time physical activity among male and female twins over a 6-year follow-up. At baseline the sample comprised 4,280 monozygotic and 9,276 dizygotic twin individuals, and at follow-up 4,383 monozygotic and 9,439 dizygotic twin individuals. Participants were aged 18–54 years at baseline. Genetic modeling results showed that genetic influences on leisure time physical activity declined from baseline (44%) to follow-up (34%). Most of the genetic influences identified at baseline were present at followup (rg = 0.72). Specific environmental influences increased from baseline (56%) to follow-up (66%) while at follow-up new environmental time-specific influences were observed (re = 0.23). The model with sex differences showed a higher estimate of genetic influences for men than women both at baseline (men 47% vs. women 42%) and at follow-up (men 38% vs. women 31%). The additive genetic correlation for this phenotype was greater for men (rg = 0.79) than women (rg = 0.64). The specific environmental influences were corresponding; at baseline men 53% and women 56% and at follow-up men 62 % and women 69%. The environmental correlations between the two time points were similar for men (re= 0.21) and for women (re= 0.24). In conclusion, in a sample of healthy twins most of the genetic influences on leisure time physical activity expressed at baseline were present at 6 years of follow-up. New specific environmental factors underlying follow-up leisure time physical activity were observed.

Twin studies of diabetes mellitus can help elucidate genetic and environmental factors in etiology and can provide valuable biological samples for testing functional hypotheses, for example using expression and methylation studies of discordant pairs. We searched the volunteer Australian Twin Registry (19,387 pairs) for twins with diabetes using disease checklists from nine different surveys conducted from 1980–2000. After follow-up questionnaires to the twins and their doctors to confirm diagnoses, we eventually identified 46 pairs where one or both had type 1 diabetes (T1D), 113 pairs with type 2 diabetes (T2D), 41 female pairs with gestational diabetes (GD), 5 pairs with impaired glucose tolerance (IGT) and one pair with MODY. Heritabilities of T1D, T2D and GD were all high, but our samples did not have the power to detect effects of shared environment unless they were very large. Weight differences between affected and unaffected cotwins from monozygotic (MZ) discordant pairs were large for T2D and GD, but much larger again for discordant dizygotic (DZ) pairs. The bivariate genetic analysis (under the multifactorial threshold model) estimated the genetic correlation between body mass index (BMI) and T2D to be 0.46, and the environmental correlation at only 0.06.

Estimates suggest up to 15% of multiples grow up as singleton survivors. Few studies have reported how bereaved multiple birth mothers with a surviving multiple cope with their bereavement. Using the population-based Western Australian Twin Child Health study database and other sources, we interviewed 66 bereaved mothers with at least one surviving multiple. For many, this contact was the first acknowledgement of their status as multiple birth mothers since their loss. The Beck Depression Inventory 2nd Edition (BDI) showed significant reduction in depression between the time of loss and our interview. For mothers as a group there was a high correlation between current and retrospective BDI, and retrospective BDI and all three Perinatal Grief Scales (PGS). There was a significant correlation between the three grief factors on the PGS. When subdivided, this held for mothers who suffered a loss at or before the neonatal period, but not for those whose loss occurred later. Bereaved mothers of multiples scored significantly higher on the PGS than the PGS norm for bereaved mothers of singletons, which we attribute to others not acknowledging their grief, and/or recruitment differences. There were no significant differences in PGS scores related to cause, the time since death, or sibling number or age. Spiritual beliefs and finding meaning in loss were positively related to scores for adjustment and acceptance. Although traumatised, most mothers accommodated their losses meaningfully in their lives. Their own support recommendations are included.

Alcohol abuse and dependence are among the most common psychiatric conditions identified in epidemiological surveys of the general population. The aim of this article is to examine the psychometric properties of Diagnostic and Statistical Manual of Mental Disorders, (4th ed.; DSM-IV; American Psychiatric Association, 1994) criteria for alcohol abuse and dependence using latent class analysis (LCA). Six thousand two hundred and sixty-five young Australian twins (median age 30 years) were interviewed by telephone between 1996 and 2000 using a modified version of the Semi-Structured Assessment for the Genetics of Alcoholism (SSAGA). DSM-IV symptoms of alcohol abuse and dependence were collected by structured diagnostic interview and analyzed using methods of LCA. LCA revealed a 4-class solution for women that classified individuals according to the severity of their alcohol- related problems: no/few problems (66.5%), heavy drinking (23.9%), moderate dependence (7.6%) and severe dependence (2.0%). Among men the preferred solution included 5 classes corresponding to no/few problems (46.4%), heavy drinking (34.3%), moderate dependence (12.2%), severe dependence (3.0%) and abuse (4.0%). Evidence of a male-specific class of alcohol-related problems corresponding to abuse partially supports the DSM conceptualization of alcohol use disorders but suggests that this conceptualization — and measurement — may need to be refined for women. Identification of a male- specific abuse class also has important implications for interventions and treatment as these individuals experienced significant alcohol-related problems and comprised approximately 21% of all men classified with an alcohol use disorder.

Previous genetic investigations of variation in normal sleep have focused on measures that describe sleep over longer periods of time. We undertook a study with the aim of evaluating whether heritability can be found in single-night sleep traits. A classical twin study design of monozygotic and dizygotic twins, enriched with siblings of twins was employed. The study included adult twin pairs and their siblings (N = 813 subjects from 342 families). A subsample of 66 individuals participated twice. For a single night, bedtime, awakening time and subjective sleep quality were assessed using a diary. The diary also assessed smoking, alcohol and coffee consumption, and the subjective evaluation of stress. Resemblance between family members was used to estimate the heritability of bedtime, awakening time, sleep problems and sleep quality as a function of sex. Most sleep measures showed familial clustering, but results differed for men and women. Heritability for bedtime and sleep problems was seen in women; and for awakening time in men. We conclude that heritability can be demonstrated for bedtime and subjective evaluation of even a single night of sleep. The contribution of the genetic make-up is sex specific. In women variance in awakening time is so affected by environmental circumstances, that the genetic contribution to the variance becomes negligible. In contrast, for males, variance in the evening bedtime is so affected by environmental circumstances, that the genetic contribution to the variance becomes negligible.

Although genetic factors are recognised as major contributors to otitis media, the presence of sex differences in heritability needs clarification. The aim of this study was to estimate the relative contribution of genetic and environmental effects in otitis media liability with particular focus on sex differences. Data from a cohort of Norwegian twins born between 1967 and 1979 with repeated measures on recurrent childhood otitis media were analysed. Altogether the sample included 4247 twin pairs. The tetrachoric correlations for monozygotic twins were .71 and .65 for males and females respectively. In dizygotic twins the correlations were .35 and .25 for males and females, respectively, and was .34 in opposite sexed pairs. The contribution of genetic and environmental effects was analyzed using structural equation modeling. The best fitting model showed that additive genetic effects explained 72% and 61% of the variance in males and females, respectively. The remaining variance was attributed to individual environmental effects. A model specifying equal heritability estimates for males and females yielded an almost equivalent fit. We found substantial genetic effects for liability to otitis media. There is no evidence that different sets of genes influence liability in males and females, but there may be sex differences in the relative importance of genetic effects.

This population-based study determined the impact of co-twin gender on twin intrauterine growth in addition to their infant gender, maternal height, maternal age and parity on intrauterine growth rate of singletons and twins. All singletons and twins born in Western Australia during the period of 1980 to 1995 were considered for the study. The multiple linear regression models showed that 76% of the variance in the mean birthweight was explained by the selected variables for twins and 51% for singletons. Twins grew more slowly than singletons from 26 weeks gestation. Among twins, opposite-sex twin pairs grew consistently faster than like-sex twins. Primiparous twin pairs grew more slowly than subsequently born twins. These regression equations can be used to assess the appropriateness of intrauterine growth in twin pairs of various gender combinations.

This study examined genetic and environmental contributions to the covariance between aggression and hyperactivity-impulsivity as rated by twins' teachers and parents. Sex-differences in these genetic and environmental contributions and rater bias/sibling interaction effects were of interest as well. Part of an ongoing nation-wide twin-family study of behavioral development and health habits, the sample consisted of 1636 Finnish twin pairs ascertained from five consecutive and complete twin birth cohorts. Data were collected at ages 11–12, using teacher and parental rating forms of the Multidimensional Peer Nomination Inventory. Bivariate analyses were performed using structural equation modeling allowing sex-limitation effects. Results show that, in addition to significant genetic and environmental influences specific to each behavior, aggression and hyperactivity-impulsivity share common genetic and environmental etiology. Results provide evidence that both genetic and environmental factors are important in creating the observed correlation between aggression and hyper-activity-impulsivity.

This study reports on the determination of zygosity in Chinese adult twins by simple questionnaire and physical features comparison. The subjects were 511 twin pairs from two cities and their town areas, consisting of 371 monozygotic (MZ) and 140 same-sex dizygotic (DZ) pairs, identified by ABO blood group and multiplex polymerase chain reaction of several polymorphic short tandem repeat markers. The twins themselves responded to 8 questionnaire items, 4 items on twin similarity, and 4 items on the frequency of mistaking one twin for another by parents, relatives, teachers and strangers when they were 6 to 13 years old. Research assistants responded to 20 items regarding twins' physical features at the moment of interview. A parsimonious model established using stepwise logistic regression analysis of the 28 items showed that the total accuracy of zygosity diagnosis was 90.1%. The accuracy was 89.2% when using only the items dealing with the confusion of twins and 85.4% using only similarity. In the questionnaire, ‘facial appearance’, ‘mistaken by teachers’ and ‘mistaken by strangers’ had stronger discriminating power between MZ and DZ twins. Two physical features — ‘eyelid’ and ‘middigital hair’ — were informative to some extent. There was no statistically significant sex and area difference in the validity of such questionnaire and physical features comparison-based classification. In conclusion, questionnaire-based zygosity assessment in this Chinese adult twin sample could still be regarded as a valid and valuable classification method. Physical features comparison, however, could only provide limited information for zygosity determination.

There are a growing number of large-scale initiatives for twin registers of children. The Australian Twin ADHD Project (ATAP) is used to illustrate two key limitations which may arise with such studies, namely (1) the importance of including or possibly excluding families in which one or both twins have significant developmental disability, and (2) the selective failure to recruit and/or the selective attrition of families in which parents and children share behavioral difficulties. Initially ATAP excluded 1 in 6 of families whose twins were enrolled in the volunteer-based Australian Twin Registry (ATR), and as more children with significant problems were identified, these families were sequentially excluded. With longitudinal data over ten years, two points about retention were identified: the difficulty of retaining the twins in late adolescence, and the loss of the families whose twins had more ADHD symptoms. We discuss strategies for limiting the loss of families and for ensuring comparability of data across registers with similar interests but different methods of recruitment and exclusion.

Previous analyses in a large population-based sample of female twins indicated that three dimensions of religiosity – personal devotion, personal conservatism and institutional conservatism – were, in different ways, significantly related to current depressive symptoms and substance use and lifetime psychiatric and substance use disorders. Furthermore, personal devotion, but neither personal conservatism nor institutional conservatism, buffered the depressogenic effects of stressful life events (SLEs). We here explore further these results, using linear, logistic and Cox regression models. Eight personality and six demographic variables had distinct patterns of association with the three dimensions. Personal devotion was positively associated with years of education, age, and optimism and negatively correlated with neuroticism. Personal conservatism was negatively associated with education, income, age, mastery and positively correlated with neuroticism. Institutional conservatism was negatively correlated with self-esteem and parental education. Covarying for these 14 variables produced little change in their association with psychiatric and substance use outcomes. The impact of the dimensions of religiosity differed as a function of the SLE category. High levels of both personal devotion and institutional conservatism protected against the depressogenic effects of death and personal illness. High levels of personal conservatism were associated with increased sensitivity to relationship problems. These results suggest that the association between religiosity and low risk for symptoms of depression and substance use may be in part causal. The relationship between dimensions of religiosity and response to SLEs is complex but probably of importance in clarifying the nature of the coping process.

The children of twins (COT) design has been proposed as an alternative to the adoption study to resolve the direct effects of parental treatment from secondary parent–child association due to genetic factors. The basic analytical approach compares the parent–offspring correlation with the correlation between children and the monozygotic (MZ) twins of their parents. We show that a significant difference between these correlations does not imply direct environmental causality when the measured parental treatment in question is dyadic, that is, influenced by both parents even when mating is random. Nongenetic causal effects yield very similar patterns of correlation to secondary genetic effects on dyadic treatment variables. The fact that many candidate environments, such as parental divorce, are dyadic gives reason to question the interpretation of their correlations with behavior in the children of twins.

Neurofibromatosis type 1 (NF1) is a common autosomal dominant disorder caused by heterozygotic inactivation of the NF1 tumor suppressor gene at 17q11.2. The associated phenotypes are highly variable, and modifying genes have been proposed to explain at least in part the intriguing expressivity. Given that haploinsufficiency of the NF1 gene product neurofibromin is responsible for some of the clinical manifestations, variations in expression of the wildtype NF1 allele might modify the phenotype. We therefore investigated epigenetic molecular modifications that could result in variable expression of the normal NF1 allele. To exclude confounding by DNA sequence variations, we analyzed monozygotic twin pairs with NF1 who presented with several discordant features. We fine-mapped the methylation pattern of a nearly 1 kb NF1 promoter region in lymphocytes of 8 twin pairs. All twin pairs showed significant intra-pair differences in methylation, especially of specific promoter subregions such as 5'UTR, exon 1 and intron 1 (+7 to +622), transcription factor binding sites and promoter elements like NF1HCS. Furthermore, we detected significant intra-pair differences in cytosine methylation for the region from -249 to -234 with regard to discordance for optic glioma with a higher grade of methylation in glioma cases. In conclusion, our findings of epigenetic differences of the NF1 promoter in leukocytes within mono zygotic twin pairs may serve as a proof of principle for other tissues. The results point towards a role of methylation patterns of the normal NF1 allele for expression differences and for modification of the NF1 phenotype.

The East Flanders Prospective Twin Survey (EFPTS) is a registry of multiple births in the province of East Flanders, Belgium. Since its start in 1964, over 10,000 twin-pairs have been registered. EFPTS has several unique features: it is population-based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are recruited at birth; basic perinatal data are recorded; chorion type and zygosity are established; since 1969, placental biopsies have been taken and frozen at –20°C for future research. Since its origin, the EFPTS has included placental data and allows differentiation of three subtypes of monozygotic twins based on the time of the initial zygotic division: the dichorionic–diamniotic pairs (early, with splitting before the fourth day after fertilization), the monochorionic–diamniotic pairs (intermediate, splitting between the fourth- and the seventh-day postfertilization) and the monochorionic–monoamniotic pairs (late, splitting after the eighth day postfertilization). Studies can be initiated taking into account primary biases, those originating ‘in utero’. Such studies could throw new light on the consequences of early embryological events and the gene–environment interactions as far as periconceptional and intrauterine environment are concerned.

This project sought to examine 2 competing hypotheses: first, that twins are likely to be more prosocial by virtue of growing up with another same-age peer in the home, or second, that they are less prosocial because they have become more comfortable interacting with a same-age peer who is both genetically and environmentally similar to themselves and therefore they are less comfortable with other children who are dissimilar. Two studies were conducted to compare twins to singletons on measures of prosocial and aggressive behaviors. In Study 1, 5-year-olds (N = 91 twins and N = 152 singletons) engaged in a peer play situation with an unfamiliar, same-age, same-sex peer, and they were rated on items assessing prosocial and aggressive behaviors. Results showed that twins were less prosocial but not more aggressive than were singletons. In Study 2, which was a supplemented follow-up study of twins in Study 1, 10- to 15-year-old twins (N = 98) and singletons (N = 84) were rated by their parents on prosocial and aggressive behaviors. No significant differences were found between the groups on prosocial behavior, but twins were rated as more aggressive than singletons. Thus, in early childhood twins appear to exhibit fewer prosocial behaviors with unfamiliar peers, but this prosocial deficit was not aligned with parent-reported prosocial behaviors in adolescence. In adolescence, twins were rated by parents as more aggressive. These studies suggest that twins may be at risk for poorer social interactions in early and middle childhood.

This review addresses the physiology of monochorionic diamniotic (MC/DA) twins and the potential for twin–twin transfusion syndrome (TTTS). It focuses on the underlying cardiovascular pathophysiology of TTTS and the cardiovascular impact of TTTS for both the recipient and the donor twin. It explains the principles for assessment and monitoring of these cardiovascular changes and how these may be used to guide pregnancy management. Finally, it describes the effect of treatment on the altered hemodynamics and how this can influence pregnancy and perinatal management, as well as longer-term follow-up.

Molecular genetic research has provided some evidence for the association between depression and metabolic disorders. We sought to determine if molecular findings are reflected in twin analyses testing if common genetic and environmental risk factors contribute to the co-occurrence of diabetes and depression. Data to derive depression and diabetes were collected from 1,237 male-male twins who participated in the 2005 Vietnam Era Twin Study of Aging (VETSA). The 1,237 twins were comprised of 347 MZ pairs, 3 MZ singletons, 267 DZ pairs and 6 unpaired twins. Depression was defined as a score below 46 on the Short Form-36 mental component summary score. Diabetes was defined by self report, use of anti-diabetic medications and insulin. Twin models were fit to estimate the correlation of genetic and environmental contributions to depression and diabetes. Consistent with other studies these data support the association between depression and diabetes (OR = 1.7; 95%CI: 1.1–2.7). Genetic vulnerability accounted for 50% (95%CI: 32%–65%) of the variance in risk for depression and 69% (95%CI: 52%–81%) of the variance in risk for diabetes. The genetic correlation between depression and diabetes was r = 0.19 (95%CI: 0–0.46) and the non-shared environmental correlation was r = 0.09 (95% CI: 0–0.45). Overall there is little evidence that common genetic and environmental factors account for the co-occurrence of depression and diabetes in middle aged men. Further research in female twins and larger cohorts is warranted.