Research comparing the auditory systems of opposite-sex co-twins, homosexual females and bisexual females is reviewed. Suggestions for how the findings may apply to other populations are provided. Next, recent twin studies of disputed memories and male sexual behaviors are summarized, as are recent events of scientific interest to twin researchers. Finally, a tribute to the late Russian twin researcher Dr. Inna Vladimirovna Ravich-Scherbo is presented.

A simple method for estimating the gene frequency p and the penetrance value K from data on polymorphic monogenic characteristics on monozygotic twin pairs is presented. In spite of the method here presented having limited value because the results it yields cannot be evaluated on their own, the estimates of p and K it provides can be indirectly tested by comparing them to the ones obtained in familial aggregates through classical segregation analysis or by using the latter to calculate the expected proportions of dominant–dominant, dominant–recessive and recessive– recessive monozygotic twin pairs. When the method is applied to data on tongue-rolling ability published in the literature, a good agreement is observed between twin and familial estimates, thus indicating that the method is reliable and that it can be used as an ancillary way of corroborating or otherwise evidence of monogenic autosomal dominant mechanism inferred from the analysis of familial data. Twin Research (2000) 3, 294–298.

In this study, the temporal trends in the rates of multiple maternities in Denmark were analyzed and an association between the rates of multiple maternities and the socioeconomic variations in Denmark was obtained. The findings are compared with the results obtained in studies of Swedish rates. Obtained temporal differences in the twinning rates (TWRs) were similar to the results presented by Fellman and Eriksson (2014). Temporal variation in rates of multiple maternities in Sweden (1751–2000). JP Journal of Biostatistics, 11, 2, 143–156.

Genes associated with educational attainment may be related to or interact with adolescent alcohol, tobacco and cannabis use. Potential gene–environment interplay between educational attainment polygenic scores (EA-PGS) and adolescent alcohol, tobacco, and cannabis use was evaluated with a series of regression models fitted to data from a sample of 1871 adult Australian twins. All models controlled for age, age2, cohort, sex and genetic ancestry as fixed effects, and a genetic relatedness matrix was included as a random effect. Although there was no evidence that adolescent alcohol, tobacco or cannabis use interacted with EA-PGS to influence educational attainment, there was a significant, positive gene–environment correlation with adolescent alcohol use at all PGS thresholds (ps <.02). Higher EA-PGS were associated with an increased likelihood of using alcohol as an adolescent (ΔR2 ranged from 0.5% to 1.1%). The positive gene–environment correlation suggests a complex relationship between educational attainment and alcohol use that is due to common genetic factors.

The giant panda, native to mountains of south-west China, is one of the world's rarest bear species and is subject to considerable conservation effort. In captivity, the proportion of twins accounts for 54% of the total number of births. To date, little is known about zygosity in panda populations — specifically, the proportion of monozygotic and dizygotic twins. In this study, we used 10 microsatellite markers for reliable zygosity testing, and the probability of monozygotic twins was 99.963% when all 10 markers were concordant. Out of 43 studied twin pairs, no MZ twins were found, indicating that there may be no identical panda twins (or the incidence is very low). We speculate that the fertilized eggs of giant pandas do not have the capability to split into two identical embryos, or that this ability is very poor, which is likely due to delayed implantation that is common in bear species. The results of this study deepen our understanding of giant panda breeding, yield insight into panda twins’ likely mechanism of formation, and reduce the uncertainty of individual identity in wild population surveys.

The focus of this article is on insights gained from the extraordinary dynamics that took place between a pair of reunited monozygotic female twins from Greece. Topics include visualizing the co-twin prior to meeting, the unique love and attraction between the twins, a mother’s response to discovering her child’s cotwin and the challenges posed by insufficient kinship categories. The following section of this article surveys four recent twin research reports. They include Bereavement and Lifespan Associations; Novel Mutations in Twins; Parenting Premature Multiples; and Conjoined Twins. The final part of this article presents interesting and informative news about twins from popular sources. The topics covered are the passing of Madeline Albright (Secretary of State and mother of twins); identical twin songwriters; visually impaired twin and sibling skiiers; identical twin brothers who both received heart transplants; and the naming of the Genain quadruplets.

Dysmenorrhea is associated with epilepsy. Existing evidence is mostly limited to observational studies, which are liable to confounding and bias. This study investigated the causal relevance of dysmenorrhea on epilepsy using Mendelian randomization (MR). We extracted instrumental variants for dysmenorrhea and epilepsy from published genomewide association study data, focusing on individuals of East Asian descent. A comprehensive suite of MR estimations and sensitivity analyses was performed to ensure the robustness of the findings. Each outcome database was analyzed separately in both directions. For dysmenorrhea and epilepsy, 7 and 3 genetic variants respectively were selectively extracted as instrumental variants. The results suggest that dysmenorrhea is causally associated with an elevated risk of epilepsy (inverse variance weighted [IVW]: OR = 1.26; 95% CI [1.07, 1.47]; p = 4.42 × 10−3); conversely, no strong evidence was found to corroborate that epilepsy exerts a causal effect on the incidence of dysmenorrhea (IVW: OR = 1.04; 95% CI [0.82, 1.33]; p = .72). These findings provide novel insights into the causal relationship between dysmenorrhea and epilepsy, which may have implications for clinical decision-making in patients with epilepsy and dysmenorrhea.

The objective of this study was to investigate how different obesity measures link to circulating metabolites, and whether the connections are due to genetic or environmental factors. A cross-sectional analysis was performed on follow-up survey data at the Chinese National Twin Registry (CNTR), which was conducted in four areas of China (Shandong, Jiangsu, Zhejiang and Sichuan) in 2013. The survey collected detailed questionnaire information and conducted physical examinations, fasting blood sampling and untargeted metabolomic measurements among 439 adult twins. Linear regression models and bioinformatics analysis were used to examine the relation of obesity measures, including body mass index (BMI), waist circumference (WC) and waist-to-hip ratio (WHR) with serum metabolite levels and related pathways. A co-twin control study was additionally conducted among 15 obesity-discordant monozygotic (MZ) pairs (intrapair BMI difference >3 kg/m2) to examine any differences in metabolites controlling for genetic factors. Eleven metabolites were associated with BMI, WC and WHR after controlling for genetic and shared environmental factors. Pathway analysis identified pathways such as phenylalanine metabolism, purine metabolism, valine, leucine and isoleucine biosynthesis that were associated with obesity. A wide range of unfavorable alterations in the serum metabolome was associated with obesity. Obesity-discordant twin analysis suggests that these associations are independent of genetic liability.

Every statistical model is based on explicitly or implicitly formulated assumptions. In this study we address new techniques of calculation of variances and confidence intervals, analyse some statistical methods applied to modelling twinning rates, and investigate whether the improvements give more reliable results. For an observed relative frequency, the commonly used variance formula holds exactly with the assumptions that the repetitions are independent and that the probability of success is constant. The probability of a twin maternity depends not only on genetic predisposition, but also on several demographic factors, particularly ethnicity, maternal age and parity. Therefore, the assumption of constancy is questionable. The effect of grouping on the analysis of regression models for twinning rates is also considered. Our results indicate that grouping influences the efficiency of the estimates but not the estimates themselves. Recently, confidence intervals for proportions of low-incidence events have been a target for revived interest and we present the new alternatives. These confidence intervals are slightly wider and their midpoints do not coincide with the maximum-likelihood estimate of the twinning rate, but their actual coverage is closer to the nominal one than the coverage of the traditional confidence interval. In general, our findings indicate that the traditional methods are mainly satisfactorily robust and give reliable results. However, we propose that new formulae for the confidence intervals should be used. Our results are applied to twin-maternity data from Finland and Denmark.

Izabella and her partner sought pre-implantation genetic diagnosis (PGD) because Izabella had retinoblastoma due to a deletion in chromosome 13 and they want to have children not at genetic risk of retinoblastoma. Fortunately, Izabella's tumor was unilateral and was treated successfully and she is well. Izabella's chromosome abnormality is mosaic with 70% of lymphocytes having the deletion. This mosaicism may not be present in Izabella's ovaries. The couple went through PGD on two occasions and 13 embryos were tested. None had the deleted chromosome 13. IVF and PGD failed to produce a pregnancy. The couple wished to know what the experience provides as to the risk to their offspring: in particular, does it indicate a risk low enough to be acceptable if they go ahead with a natural pregnancy instead of another resort to PGD? Also, the couple did not want prenatal diagnosis. The situation therefore requires an estimate of the probability that an embryo will have the deletion. Counseling is problematic because there is no obvious way of selecting a prior probability from which to compute a Bayesian estimate of risk. Two solutions are offered, depending on the amount of information available about genes transmitted from the maternal grandparents.

Studies concerning twins with a sociological focus are scarce in Hungary as well as international research, although the number of twin births has increased dramatically worldwide. The raising and education of twins are tasks demanding special attention from both the family and institutions. In our study we examine these aspects, looking back from adulthood, using the narrow scope of the available data from research based on the ‘Hungarostudy 2021’ database (N total: 7000; n twins: 106). Our results, corresponding to the hypotheses of educational sociology, demonstrate how the relationships between family size and school career and increasing number of siblings reduces the chances of high educational attainment. A regression analysis confirmed that both the number of siblings and a later position in the birth order reduces the chance of obtaining a higher education. For the second child in a family, the chance of earning a university degree is reduced to to 0.743. The role of a large family concerning higher education showed a stronger relationship in the case of twins compared to nontwins. For twins, the sibling pattern has a decisive effect in educational attainment. Twins themselves have a 1.449 times higher chance of obtaining a higher education compared to nontwins (p = 0.101), and fraternal twins have half (0.517) the chance of obtaining a higher education compared to identical twins; but both results are not significant (p = 0.156).

Using vital statistics, yearly changes in the twinning and triplet rates by zygosity were investigated in the Czech Republic and the Slovak Republic during the period 1972–1995. Monozygotic (MZ) twinning rates in both countries had remained nearly constant (about 3 per 1000 total births) during that period. With a few exceptions, the dizygotic (DZ) twinning rates remained constant from 1972 to 1994, and increased in 1995 for both countries. MZ twinning rates for both countries were the lowest in Europe. As for triplet rates, overall rates increased significantly year by year in the Czech Republic, but not in the Slovak Republic. The triplet rate was significantly higher in 1995 than in the period 1972–1982 for both countries. The MZ triplet rate remained constant during that period in the Czech Republic. The trizygotic (TZ) triplet rates increased 3-fold for the Czech Republic and 4-fold for the Slovak Republic in 1972–1976 and 1992–1995. In the later period, the TZ rate was 1.5-fold higher in the Czech Republic than in the Slovak Republic. The quadruplet rate increased 2.3-fold from 2.9 per million births in 1982–1986 to 6.7 in 1992–1995 in the Czech Republic. The corresponding values were 2.7, 2.20 and 5.9-fold in the Slovak Republic. Both the Czech and the Slovak Republics were not affected by fertility drugs and assisted reproductive techniques until recently.