Caffeine is by far the most commonly used psychoactive substance. Caffeine is consumed regularly as an ingredient of coffee. Coffee consumption and coffee preference was explored in a sample of 4,495 twins (including 1,231 pairs) registered with the Netherlands Twin Registry. Twin resemblance was assessed by tetrachoric correlations and the influence of both genetic and environmental factors was explored with model fitting analysis in MX. Results showed moderate genetic influences (39%) on coffee consumption. The remaining variance was explained by shared environmental factors (21%) and unique environmental factors (40%). The variance in coffee preference (defined as the proportion of coffee consumption relative to the consumption of coffee and tea in total) was explained by genetic factors (62%) and unique environmental factors (38%).

The present study is the first behavioral genetic investigation of relationships between the Dark Triad of personality — Machiavellianism, narcissism, and subclinical psychopathy — and moral development. Participants were 154 monozygotic twin pairs and 82 same-sex dizygotic twin pairs. Higher scores on Machiavellianism and psychopathy were positively correlated with low levels of moral development; high psychopathy scores also correlated negatively with high levels of moral development. Individual differences in lower levels of moral development were attributable to genetic and nonshared environmental factors but, very interestingly, individual differences in the highest levels of moral development showed no genetic basis but were entirely attributable to shared and nonshared environmental factors. Finally, correlations between the Dark Triad and moral development variables showed no genetic basis while correlations among the moral development variables were variously attributable to correlated genetic and correlated environmental factors.

Web-based studies have become increasingly common in the social sciences, but have been rare in genetic epidemiology in general and twin studies in particular. We here review the methods, validity checks and preliminary correlational data from an on-line questionnaire collected from 2005–2008. During this time period, 44,112 individuals completed the questionnaire. This sample was 65.3% female, 85.4% 18 years or older, 72.0% Caucasian and had a mean educational level of 12.2 years. The sample included 609 twin, 333 sibling and 201 parent-offspring pairs as well as 342 dating partners, 313 ‘significant other’ pairs, 327 spouses and 2,316 friend pairs. A range of checks suggested low levels of invalid data. Correlations for personality, substance use and misuse, lifetime major depression, social attitudes, educational status, and height and weight were broadly similar to those obtained previously using conventional assessment methods. Web-based studies are a relatively easy and inexpensive way to ascertain large numbers of individuals, although obtaining twin pairs is more difficult, and female and monozygotic pairs are overrepresented. The sample is diverse and pair resemblance is generally similar to that obtained using interviews or mailed questionnaires.

A growing literature suggests that personality traits may be endophenotype markers for psychiatric illnesses. Although the phenotypic relationships between obsessive–compulsive disorder (OCD) and high neuroticism and low extraversion have been well documented, underlying genetic and environmental contributions to these associations have not been explored previously. Five hundred and twenty-four monozygoitc (MZ) and 228 dizygotic (DZ) pairs of adolescent and young adult twins (aged 13–24 years) drawn from the South Korean Twin Registry completed the Maudsley Obsessive Compulsive Inventory (MOCI) and the Neuroticism and Extraversion scale of the Eysenck Personality Scale by mail. The total score of MOCI (MOCIT) was significantly and positively correlated with Neuroticism (r = .44), but only weakly and negatively related to Extraversion (r = –.10). A trivariate Cholesky model was applied to the data. The additive genetic correlations in the best-fitting model were .51 between Neuroticism and MOCIT and –.17 between Extraversion and MOCIT, suggesting that additive genetic factors that lead to high neuroticism and low extraversion overlap with those genetic factors influencing high OC symptoms. These findings add to the cumulative evidence of the shared genetic etiology for the associations between a personality profile of high neuroticism and low extraversion and mental illnesses.

Appendicitis usually afflicts the young, but there is a large tail in the distribution of onset age. The genetics of this disease are still not well understood. A heritability analysis and genome wide linkage analysis of a large twin dataset was undertaken. Treating age of onset of appendicitis as a censored survival trait revealed a heritability of 0.21, and found evidence of linkage to Chromosome 1p37.3.

All Norwegian twin pairs born 1967–1974 and still living in Norway in 1992 were invited to a health questionnaire study (Q1). 2,570 pairs (65%) participated. These cohorts and the twin cohorts born 1967–1979 were invited to a new questionnaire study (Q2) in 1998. This time 3,334 pairs (53%) participated. Almost all pairs having participated in the 1998 study were invited to an interview study of mental health (MHS), taking place 1999–2004. 1,391 complete pairs (44%) participated. The questionnaire studies included extensive data on somatic health with fewer items on mental health and demography. Health-related and demographic information available from the Medical Birth Registry on all invited twins was applied to predict participation to the first study. A few registry variables indicating poor health predicted nonparticipation in Q1. Health information and demography from Q1 were tested as predictors of participation in the follow-up study (Q2). Monozygosity, female sex, being unmarried, having no children, and high education predicted participation, whereas few indicators of poor mental and somatic health and unhealthy lifestyle moderately predicted nonparticipation in Q2. No health indicators reported in Q2 predicted further participation. Standard genetic twin analyses of indicators of various mental disorders from Q2, validated by diagnostic data from the MHS, did not indicate differences in genetic/environmental covariance structures between participants and nonparticipants in MHS. In general the results show a moderate selection towards good mental and somatic health. Attrition from Q2 to the MHS does not appear to affect twin analyses of mental health related variables.

Both zygosity and chorionicity provide important information in twin research. The East Flanders Prospective Twin Survey (EFPTS) determines zygosity and chorionicity at birth and therefore provides a gold standard for the testing of diagnostic parameters that can be used to determine chorionicity. The aim of the present study was to investigate whether birthweight discordancy can be used as an indicator of chorionicity. The study sample consisted of 4,060 live-born twin pairs from the EFPTS. We studied MZ twins, using univariate and multivariate logistic regression analyses to calculate odds ratios (OR) and 95% confidence intervals (CI) of being MC in relation to discordancy level. Diagnostic parameters, including sensitivity and specificity, were calculated. A two-fold cross-validation was carried out and a bootstrap distribution with 10,000 samples was created to estimate the standard deviations. For discordancy levels of below 10%, 10–15%, 15–20%, 20–25% and above 25%, the ORs (95% CI) were 1.16 (0.91–1.47), 1.38 (1.05–1.80), 2.13 (1.51–3.01), 2.73 (1.73–4.29) and 2.81 (2.81–4.35) respectively. There were no gender differences. Sensitivity was 42.2% (SD 5.6%), specificity was 72.8% (SD 6.3%), positive predictive value was 72.8% (1.5%) and the negative predictive value was 39.2% (0.7%). In conclusion, although a higher discordancy level resulted in higher ORs of being an MC twin, birthweight discordancy level can only be used to some weak extent as a proxy for chorionicity, highlighting the need to assess and record chorionicity data in obstetrical units.

Maternal infection with parvovirus B19 during pregnancy can cause aplastic anemia in the fetus and may lead to nonimmune fetal hydrops and fetal demise. Twin pregnancies complicated by infection due to parvovirus B19 are very rare clinical events. We present a dichorionic, diamniotic, dizygotic twin pregnancy after in vitro fertilization with parvovirus B19 infection and viral transmission to both twins, but different outcomes. At 19 weeks gestation, hydrops fetalis was diagnosed for male twin A, female twin B did not show any abnormalities. At 22 weeks gestation an acute parvovirus B19 infection was detected and twin A was diagnosed with intrauterine fetal death (IUFD) by ultrasound at 23 weeks gestation. Viral DNA was detected in maternal blood as well as in placenta and liver tissue of this twin. Twin B was born at 35 weeks gestation asymptomatically and no signs of hydrops or other congenital anomalies but viral DNA was detected by PCR in serum. At the age of 2 years, both IgG titres against B19 and parvovirus DNA amplification copies were still positive in plasma of the surviving twin, but no clinical signs were detectable. It is remarkable that both twins were infected with parvovirus B19 early in pregnancy but showed a discordant clinical outcome. Our case report describes the rare occurrence of an intrauterine fetal death (IUFD) of one twin and the asymptomatic infection of the other in a twin pregnancy.

Current ultrasound techniques can accurately determine the chorionicity of twins, but not zygosity. We previously proposed that the zygosity of spontaneously conceived twins can be determined at early ultrasound, where 2 corpora lutea infers dizygosity, and 1 implies monozygosity. Here we did a case series, comparing zygosity predicted using this method with definitive DNA genotyping of twins after birth. We retrospectively identified 14 ultrasound reports of spontaneous twin pregnancies at 6(+0 days) to 13+6 weeks' gestation, where both ovaries were seen and the number of corpora lutea documented. We visited all twin pairs, obtained buccal smears, and determined zygosity by genotyping 9 independent microsatellite markers. All 8 cases where 2 corpora lutea were seen were dizygotic pregnancies. One further case where 3 corpora lutea were seen was also dizygotic. All 3 sets of monozygotic twins had 1 corpus luteum. There were 2 cases incorrectly assigned, where 1 corpus luteum was seen in dizygotic pregnancies. We conclude if 2 corpora lutea are seen at a first trimester ultrasound of spontaneously conceived dichorionic twins, they appear to be almost certainly dizygotic. However, if 1 corpus luteum is seen in dichorionic twins, zygosity cannot be determined with certainty since it is either monozygotic, or dizygotic where a second corpus luteum has been missed.

Theorems, proofs, laws and rules are commonly named according to the presumed investigator, but often earlier investigators have contributed substantially to the findings. One example of this is Hellin's law, which was named after Hellin, although he was not the first to discover it. In research on twinning and higher multiple maternities, the law has played a central role because it is approximately correct, despite showing discrepancies that are difficult to explain or eliminate. Several improvements to this law have been proposed. In this study, we re-examine some old papers to provide an overview of the scientists who have contributed to the genesis and the improvements of this law. In addition, we consider more recent contributions in which Hellin's law has been discussed and evaluated. It has been mathematically proven that Hellin's law does not hold as a general rule. However, most studies are based on empirical rates of multiple maternities, ignoring random errors. Such studies can never confirm the law, but only serve to identify errors too large to be characterized as random.

During the history of research on multiple maternities, Hellin's law has played a central role as a rule of thumb. It is mathematically simple and approximately correct, but shows discrepancies that are difficult to explain or to eliminate. It has been mathematically proven that Hellin's law does not hold as a general rule. Varying improvements to this law have been proposed. In this paper, we consider how Hellin's law can be used and tested in statistical analyses of the rates of multiple maternities. Such studies can never confirm the law, but only identify errors too large to be characterized as random. It is of particular interest to determine why the rates of higher multiple maternities are sometimes too high or too low when Hellin's law is used as a benchmark. Excesses of triplet and quadruplet maternities are particularly unexpected and challenging. Our analyses of triplet and quadruplet rates indicated that triplet rates are closer to Hellin's law than quadruplet rates. According to our analyses of the twinning rate and the transformed triplet rate and quadruplet rate for Sweden (1751–2000), both triplet and quadruplet rates showed excesses after the 1960s. This is mainly caused by artificial fertility-enhancing reproduction technologies. Regression analyses of twinning and triplet rates yield rather good fits, but deficiencies in the triplet rates are commonly present. We introduced measures of concordance between triplet rates with Hellin's law. According to these measures, historic data showed deficiencies in triplet rates, but recent data revealed excesses, especially found among older mothers. The excesses obtained are in good agreement with other studies of recent data.

Mistaken identity does not just affect identical twins, it can also affect the lives of unrelated individuals. The present article reviews several compelling cases in which confusion by others significantly altered the life circumstances of twins and non-twins. Next, current research on ovarian transplants, kidney donation, false beliefs and emotion understanding are reviewed. The concluding sections survey some unusual twin pairs — and pay tribute to the late Dr. Daniel G. Freedman, known for his landmark studies of fear of strangers in infant twins.