DNA methylation differences between identical twins could account for phenotypic twin discordance of behavioral traits and diseases. High throughput epigenomic microarray profiling can be a strategy of choice for identification of epigenetic differences in phenotypically different monozygotic (MZ) twins. Epigenomic profiling of a pair of MZ twins with quantified measures of psychometric discordance identified several DNA methylation differences, some of which may have developmental and behavioral implications and are consistent with the contrasting psychometric profiles of the twins. In particular, differential methylation of CpG islands proximal to the homeobox DLX1 gene could modulate stress responses and risk taking behavior, and deserve further attention as a potential marker of aversion to danger. The epigenetic difference detected at DLX1 of ∼1.2 fold change was used to evaluate experimental design issues such as the required numbers of technical replicates. It also enabled us to estimate the power this technique would have to detect a functionally relevant epigenetic difference given a range of 1 to 50 twin pairs. We found that use of epigenomic microarray profiling in a relatively small number (15–25) of phenotypically discordant twin pairs has sufficient power to detect 1.2 fold epigenetic changes.

We examine the heritability of psychological resilience among US adults aged 25 to 74 years. Using monozygotic and same sex dizygotic twin pairs from the National Survey of Mid-Life Development in the United States (MIDUS) we show that positive affect is equally heritable among men (h2 = .60) and women (h2 = .59). We then estimate the heritability of positive affect after controlling for an exhaustive list of social and inter-personal stressors, and we operationalize the residual for positive affect as resilience. According to this specification, the heritability of resilience is higher among men (h2 = .52) compared to women (h2 = .38). We show that self-acceptance is one of the most important aspects of psychological functioning that accounts for the heritability of resilience among both men and women. However, compared to women, men appear to derive additional benefits from environmental mastery that may enable otherwise sex-neutral resilient tendencies to manifest.

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.

The importance of using multiple polymorphic genetic markers to determine unambiguously whether a twin pair is monozygotic (MZ) or dizygotic (DZ) has long been recognized. Concordance among a set of markers is used as evidence of monozygosity, as it would be improbable for DZ twins to be concordant at a large number of polymorphic loci. Several sources give a formula for the probability of two DZ twins sharing the same genotype at a locus, assuming knowledge of allele frequencies but not of either twin's genotype; this probability can be used to determine whether a set of markers will reliably distinguish between MZ and DZ status in a randomly selected twin pair. If the shared genotype is known, however, the likelihood-ratio test (LRT) of the null hypothesis of dizygosity against the alternative hypothesis of monozygosity takes into account the observed genotype and, by the Neyman-Pearson lemma, is the most powerful test of its size. The LRT is equivalent to conditioning on the genotype of one of the twins, and computing the probability, assuming DZ status, of the other twin sharing that genotype. The resulting p values are frequently lower than those produced by the unconditional probability, especially if rare alleles are observed. The unconditional probability can be recapitulated from conditional probabilities by averaging across all of the conditioned sibling's possible genotypes. To illustrate properties of the LRT applied to multiple markers, the probability distribution of the LRT p value is computed from allele frequencies of twelve unlinked markers published in Elbaz et al. (2006) and compared with the p value computed from unconditional probabilities.

One thousand and seventy three pairs of adult monozygotic (MZ) twins and 895 pairs of same sex adult dizygotic (DZ) twins from the United Kingdom (UK) completed the Humor Styles Questionnaire: a 32-item measure which assesses two positive and two negative styles of humor. MZ correlations were approximately twice as large as DZ correlations for all four humor styles, and univariate behavioral genetic model fitting indicated that individual differences in all of them can be accounted for entirely by genetic and nonshared environmental factors, with heritabilities ranging from .34 to .49. These results, while perhaps not surprising, are somewhat at odds with a previous study that we conducted in North America (Vernon et al., in press) in which genetic factors contributed significantly to individual differences in the two positive humor styles, but contributed far less to the two negative styles, variance in which was instead largely due to shared and nonshared environmental factors. We suggest that differences between North American and UK citizens in their appreciation of different kinds of humor may be responsible for the different results obtained in these two studies.

We expand our previous deterministic power calculations by calculating the required sample size to detect C in ACE models. The theoretical expected value of the maximum log-likelihood for the AE model was derived using two optimisation methods and these gave near-identical results. Theoretical predictions were verified by computer simulation and the results agreed very well. We have developed a user-friendly web-based tool, TwinPower, to perform power calculations to detect either A or C for the classical twin design. This new tool can be found at http://genepi.qimr.edu.au/cgi-bin/twinpower.cgi

Accumulated evidence suggests that social support is influenced by genetic and environmental factors. However, there are little data that examine this issue from Asian samples. We reported results from a preliminary study that examined familial effects on social support in a Chinese adult twin sample. We administered a 10-item social support instrument that measures three dimensions of social support (i.e., objective support, subjective support, and utilization of support) developed for the Chinese population. Two hundred forty-two same-sex twin pairs, where both members of the pair completed the personal interview, were included in the final analysis. Structural equation modeling was used to estimate additive genetic (A), shared environmental (C), and nonshared environmental (E) effects on each dimension of social support. Familial factors (A+C) explained 56.63% [95% CI = 45.48–65.72%] and 42.42% [95% CI = 29.93–53.25%] of the total phenotypic variances of subjective support and utilization of support, respectively. For the objective support, genetic effects did not exist, but common environmental effect explained 37.56% [95% CI = 26.17–48.28%] of the total phenotypic variances. Neither gender nor age effects were seen on any dimension of social support. Except for objective support, genetic factors probably influence variation in subjective support and utilization of support. Shared environmental factors may influence all dimensions of social support.

The present study examined the effects of chorionicity of twins on variations of height, weight, and body mass index (BMI) during childhood in the classical twin design. Mothers of 81 pairs of monochorionic monozygotic (MCMZ), 47 pairs of dichorionic monozygotic (DCMZ), and 457 pairs of dizygotic (DZ) twins drawn from the South Korean Twin Registry reported their children's height and weight. Twins' age ranged from 1.9 to 8.7 yrs, with a mean of 4.0 yrs and SD of 1.7 yrs. We computed maximum likelihood twin correlations and performed model-fitting analyses. In correlational and model-fitting analyses, we treated age and sex as covariates to control their main effects. Maximum likelihood MCMZ, DCMZ, and DZ twin correlations were, respectively, .96, .92, and .74, for height, .88, .91, and .57 for weight, and .93, .92, and .61 for BMI. The pattern of these twin correlations suggested very modest chorion effects on body measures. Model-fitting analyses confirmed the observations from twin correlations. Whereas genetic and shared environmental influences were significant for all three body measures, chorion effects attained statistical significance only for height (4%), and those for weight and BMI were zero. These findings indicate that genetic and environmental estimates for height, weight, and BMI during childhood are biased little by the chorion type of MZ twins, supporting the validity of the equal prenatal environment assumption in the classical twin design.

The increased incidence of multiple deliveries in Spain, in addition to changes in age at maternity and parity, is attributed to assisted reproductive treatments, but the relative contribution of the latter to this rise remains uncertain, due to the scarce information provided by clinics practicing those treatments. Population based data (1984–2004), including information on mother's age, nationality, marital status, date of delivery, and the characteristics of each (parity, single or multiple), and sex of newborns were provided by the Spanish Institute of Statistics. Twinning and triplet deliveries relate to maternal age, parity, and nationality. For younger ages (≤ 19, 20–24, 25–29) rates remained constant over time, but for older women (30–34, 35–39, ≥ 40) rates increased after 1994. From 1984 to 2004 the percentage of twins of opposite sex increased from 24.31 to 36.58 per cent. Since 1997, Spanish and non-Spanish mothers differentiate with respect to multiple maternity at ages over 30. In addition to unmarried Spanish women, immigrants constitute a reliable reference group that determines the convenience of segregating information on multiple deliveries respecting origin. The proportion of twins and triplets of opposite sex, maternal age, and parity patterns observed are concordant with a differential access to reproductive treatments depending on the woman's age. The present norm regulating the maximum number of fertilizations per cycle and the demand for these treatments explain the high incidence of multiple deliveries in Spain. A modified logistic curve predicts a stabilization of multiple deliveries, which will probably continue to be high in Spain.

The purpose of this study was to investigate whether there is any association between preeclampsia and eNOS, DDAH, and VEGF gene polymorphisms, and also to search for a possible association between haplotypes in eNOS, DDHA, and VEGF genes and the risk for preeclampsia. DNA was extracted from whole blood of 223 preeclampsia patients and 237 healthy pregnant women. The genotypes were analyzed by a single base primer extension assay using a SNaPShot assay kit. Results were analyzed with the Student's t-test, Chi-square test, and Logistic regression analysis. Haplotype analyses were performed using Haploview 3.2 version. There were no significant differences in genotype or allele frequencies of eNOS, DDAH, and VEGF gene polymorphisms between preeclampsia patients and controls. No increase in the risk of preeclampsia for those genes was observed under any model of inheritance and there were no statistically significant associations between any haplotypes and preeclampsia risk. Polymorphisms in eNOS, DDAH, and VEGF gene do not seem to be risk factors for preeclampsia.

Previous studies have described the relationships between cervical column morphology and craniofacial morphology. The aims of the present study were to describe cervical column morphology in 38 pairs of adult monozygotic (MZ) twins, and compare craniofacial morphology in twins with fusions with craniofacial morphology in twins without fusion. Visual assessment of cervical column morphology and cephalometric measurements of craniofacial morphology were performed on profile radiographs. In the cervical column, fusion between corpora of the second and third vertebrae was registered as fusion. In the twin group, 8 twin pairs had fusion of the cervical column in both individuals within the pair (sub-group A), 25 pairs had no fusions (subgroup B), and in 5 pairs, cervical column morphology was different within the pair (subgroup C), as one twin had fusion and the other did not. Comparison of craniofacial profiles showed a tendency to increased jaw retrognathia, larger cranial base angle, and larger mandibular inclination in subgroup A than in subgroup B. The same tendency was observed within subgroup C between the individual twins with fusion compared with those without fusion. These results confirm that cervical fusions and craniofacial morphology may be interrelated in twins when analysed on profile radiographs. The study also documents that differences in cervical column morphology can occur in individuals within a pair of MZ twins. It illustrates that differences in craniofacial morphology between individuals within a pair of MZ twins can be associated with cervical fusion.

We test the hypothesis that parents who conceived twins by assisted reproduction technology (ART), but experienced loss of one twin, have a different parental–child relationship compared with ART parents following a singleton pregnancy. We used the 1994–2005 ART database of the Centre for Infertility of the Arcispedale Santa Maria Nuova in Reggio Emilia, Italy to identify families of 53 singleton births after the ‘vanishing’ twin syndrome. The controls comprised 106 families who conceived and delivered singletons, matched for gestational age (≥ 28 ≤ 31, ≥ 32 ≤ 36, ≥ 37 weeks), maternal age (< 35, ≥ 35 ≤ 40, > 40 years), child's age (1–3, 4–6, and 7–11 years) and child's gender. We completed 3 tests: a sociodemographic questionnaire, the QUIT — Italian Questionnaires of Temperament — motor scale, and the Child Vulnerability Scale. We found that children in the study had significantly more difficulties at the beginning of nursery school (p = .002) and kindergarten (p = .0005), with more frequent anxiety of separation from the parents (nursery school, p = .009; kindergarten, p = .001). We found a lower mean QUIT motor score for the 7- to 11-year-old children when compared to the general Italian normative values, suggesting that parents perceived their children as having more motor difficulties. In contrast, analysis of the Child Vulnerability Scale showed that significantly more parents (15.1%) from the controls perceived their child as vulnerable compared to those from the study group (3.8%), p = .034. We conclude that despite the perceived motor difficulties and the difficulties in the process of individuation–separation that appear at the beginning of the different educational circumstances, parents of singletons following the ‘vanishing’ twin syndrome perceive their children as ‘invincible’, and thus less vulnerable compared to controls.

This book was conceived at the 2005 methodology workshop in Boulder, Colorado. Its purpose was to provide background reading and a working text for students and researchers being introduced to the principles and practice of linkage and association analysis in the context of twin and family studies of complex traits, including behavioral and psychological traits. It is edited by four of the most interesting and dynamic young researchers in the field, and authored by them and a roster of 25 additional world class experts in statistical, behavioral, and psychiatric genetics.

Evolutionary developmental biology (evo–devo) has become an established field of research, especially since the spectacular results obtained in the 1990s regarding cross-species molecular homologies of (Hox) genes acting early during embryogenesis in insects, vertebrates, and beyond. Amundson summarizes some of these results, which justify a central assertion of evo–devo, namely that one must understand how bodies are built in order to understand how the process of building bodies can be changed, that is, how evolution can occur. But Amundson's book is not about these discoveries, but about the history of evo–devo.

A brief overview of Sulloway's (1996) theory of birth order and personality is presented. A reared apart twin approach for testing his personality findings regarding openness to experience and conscientiousness in first borns and later borns is described. This is followed by summaries of three recent twin studies. The topics include cancer risk in mother of twins, sexual dysfunction in males and responder behavior during ultimatum games. This article concludes with a discussion of twinning rates and rituals among the Yoruba of western Nigeria, and descriptions of two unusual reunions between siblings and twins.