The aim of this study was to evaluate the differences in the metabolic profile and body composition of monozygotic (MZ) twins concordant and discordant for the practice of physical exercise. The sample consisted of 92 MZ twins (72.5% female and 27.5% male, mean age 25.4 ± 5.69 years), registered with the Brazilian Registry of Twins, residing in Natal, Brazil. Data collection was carried out between the years 2016 and 2018. On day 1, subjects underwent a whole-body fitness evaluation, including measures of weight, height, body composition by Dual-Energy X-ray Absorptiometry and the Cardiorespiratory Exercise Test. On day 2, 10 ml blood samples were collected (overnight fasting) to determine the lipid profile and fasting glucose. The sample was separated into three groups: Active Concordant twins (Concordant A, n = 44 subjects), Inactive Concordant twins (Concordant I, n = 22 subjects) and Discordant pairs for Physical Exercise (Discordant PE, n = 26 subjects). The results demonstrated a difference between the discordant twins for exercise and also between the active versus sedentary groups, indicating a causal effect of exercise on the fat percentage, maximum oxygen consumption (VO2max) and second ventilatory threshold variables. Between groups, a difference was also observed between the groups in ventilatory threshold, very low-density lipoprotein and triglycerides. We concluded that, regardless of genetics, the practice of physical exercise was sufficient to generate alterations in body composition and VO2max in MZ twins, but not in the lipid profile or fasting glucose.

Scholarly literature claims that health declines in populations when optimism about investing in the future wanes. This claim leads us to describe collective optimism as a predictor of selection in utero. Based on the literature, we argue that the incidence of suicide gauges collective optimism in a population and therefore willingness to invest in the future. Using monthly data from Sweden for the years 1973–2016, we test the hypothesis that the incidence of suicide among women of child-bearing age correlates inversely with male twin births, an indicator of biological investment in high-risk gestations. We find that, as predicted by our theory, the incidence of suicide at month t varies inversely with the ratio of twin to singleton male births at month t + 3. Our results illustrate the likely sensitivity of selection in utero to change in the social environment and so the potential for viewing collective optimism as a component of public health infrastructure.

Dichorionic diamniotic (DCDA) twin pregnancies after single blastocyst embryo transfer have been reported recently, although a blastocyst ovum is generally believed to divide into monochorionic twin pregnancy. We investigated the incidence of DCDA twin pregnancy after single blastocyst embryo transfer and their zygosity. This prospective cohort study included 655 consecutive twin pregnancies that were managed from 2006 to 2014 at our institution. Chorionicity and amnionicity were determined using first-trimester ultrasonography and/or placental pathology. Zygosity was analyzed if the cases were DCDA twins after single blastocyst embryo transfer. Among 655 twin pregnancies, there were 348 DCDA cases, 295 monochorionic diamniotic (MCDA) cases and 12 monochorionic monoamniotic cases. Single blastocyst embryo transfer was performed in 43 cases. Six out of the 43 (14%) cases involved DCDA twin pregnancies and the other 37 cases involved MCDA twin pregnancies. Three DCDA twins born after single blastocyst embryo transfer, wherein frozen embryo transfer (FET) was performed in the natural cycle, were dizygotic, and the other three cases, wherein FET with hormone replacement therapy was performed, were monozygotic. DCDA twin pregnancy occurred in 14% (7% for monozygotic and 7% for dizygotic) of twin pregnancies after single blastocyst embryo transfer cases.

John C. Loehlin, professor of psychology at the University of Texas, at Austin, passed away on August 9, 2020. John was 94 years of age. He will be remembered for his seminal contributions that have furthered the field of twin research in so many ways. An overview of his career is presented, with fond memories from his son and several colleagues. This tribute is followed by reviews of twin research on cognitive skills and social mobility, risk of subarachnoid hemorrhage, heritability of selected dental traits and twin-to-twin transfusion syndrome. Finally, twin-related topics of human interest include identical twins with different health histories, twins born 97 days apart, retiring twin tennis players, TikTok twins and twin reality stars.

We examined the item properties of the Two Peas Questionnaire (TPQ) among a sample of same-sex twin pairs from the Washington State Twin Registry. With the exception of the ‘two peas’ item, three of the mistakenness items showed differential item functioning. Results showed that the monozygotic (MZ) and dizygotic (DZ) pairs may differ in their responses on these items, even among those with similar latent traits of similarity and confusability. Upon comparing three classification methods to determine the zygosity of same-sex twins, the overall classification accuracy rate was over 90% using the unit-weighted pair zygosity sum score, providing an efficient and sufficiently accurate zygosity classification. Given the inherent nature of twin-pair similarity, the TPQ is more accurate in the identification of MZ than DZ pairs. We conclude that the TPQ is a generally accurate, but by no means infallible, method of determining zygosity in twins who have not been genotyped.

The extended twin model is a unique design in the genetic epidemiology toolbox that allows to simultaneously estimate multiple causes of variation such as genetic and cultural transmission, genotype–environment covariance and assortative mating, among others. Nick Martin has played a key role in the conception of the model, the collection of substantially large data sets to test the model, the application of the model to a range of phenotypes, the publication of the results including cross-cultural comparisons, the evaluation of bias and power of the design and the further elaborations of the model, such as the children-of-twins design.

Increasingly, consumers have been able to seek DNA testing online to explore their personal genetic information. This increased access to a range of genomic tests has raised concerns among health professionals tasked with providing guidance and support to patients requiring genetic/genomic testing. Individuals will seek genomic testing for a range of purposes; equally, the medical marketplace offers a range of different test types. The Human Genetics Society of Australasia (HGSA) published their first statement on Direct to Consumer Genetic Testing (2012 PS02). This is a revised statement, which considers developments in the field of online DNA testing, including rapid technological changes, diversity of applications and decreasing costs of testing. It draws from the first empirical nationwide study (Genioz – Genomics: National Insights of Australians) and insights from consumers with experience of this technology. The rapid adoption of these tests and the broad range of potential consequences have informed perspectives within this statement. It is the position of the HGSA that both individuals/consumers and health care professionals/providers should be supported to make informed choices about online DNA testing. This means adequate and ongoing education and resources should be available for individuals/consumers and health care professionals/providers before, during and after testing. Health care professionals/providers should be appropriately trained, have relevant experience and should be able to demonstrate (or provide evidence of) a current certification in their field of practice. This statement was ratified at the 2018 HGSA Council Meeting and was recently reviewed in 2019 for consistency with other HGSA position statements.

Dr Nick Martin has made enormous contributions to the field of behavior genetics over the past 50 years. Of his many seminal papers that have had a profound impact, we focus on his early work on the power of twin studies. He was among the first to recognize the importance of sample size calculation before conducting a study to ensure sufficient power to detect the effects of interest. The elegant approach he developed, based on the noncentral chi-squared distribution, has been adopted by subsequent researchers for other genetic study designs, and today remains a standard tool for power calculations in structural equation modeling and other areas of statistical analysis. The present brief article discusses the main aspects of his seminal paper, and how it led to subsequent developments, by him and others, as the field of behavior genetics evolved into the present era.

Using vital statistics in Japan (1995–2008), 154,578 live-born twin pairs (128,236 monozygotic [MZ] and 180,920 dizygotic [DZ]) were identified. The proportion of severe discordance among live-born twin births was twice as high in Japanese than Caucasian infants. There were 1858 MZ and 1620 DZ infant deaths. Computation of the relationship between infant mortality rate and birth weight discordance among the twins was performed. Discordance levels were classified into seven groups: <5%, five groups from 5–9% to 25–29%, and ≥30%.The mortality rate was significantly higher in MZ than DZ twins for discordances except at 5–9% and 10–14%. The lowest rate for MZ twins was at 5–9% (7.5 per 1000 live twins) and significantly increased from 10–14% (9.4) to ≥30% (83.4), while the lowest rate for DZ twins was at <5% (6.7), which significantly increased at 10–14% (8.0) and from 25–29% (12.1) to ≥30% (35.5). The relationship was also computed in two gestational age groups (<28 and ≥28 weeks). For births at <28 weeks, three discordances (after 20–24%) in MZ twins were associated with adverse mortality rate. For births at ≥28 weeks, the same relationship was obtained after 10–14% in MZ and after 20–24% in DZ twins. The relationship from 2002 to 2008 showed that the mortality rates significantly increased after 10–14% for both types of twins. In conclusion, five discordance levels in MZ and three levels in DZ twins were associated with adverse mortality rates.

Twin data and the results generated by research are driven by the behavioral and physical attributes of twin participants. However, many investigators working with twin data have limited personal contact with actual twin-pairs. This situation may be limiting with respect to interpretation of results and formulation of new hypotheses. In an attempt to rectify this issue, key aspects of the interconnected lives of co-twins in three monozygotic male twin-pairs are presented. The section that follows includes a review of twin research on Kleine–Levin syndrome, political discussion, twin relationship quality and urinary cortisol level, and guidelines for determining sample sizes. The final part of this report presents twin-related news items relating to twins’ same day deliveries, death of a twin Holocaust survivor, the Vindman twins, a twin festival in France and the tiniest twins on record.

It is likely that genetic factors play a role in the susceptibility to, and recovery from, COVID-19. A survey of ongoing twin research could produce findings likely to help in the prevention and management of this pandemic. This survey is followed by a review of research comparing selected features of asthmatic and nonasthmatic twins, links between twin mammals and COVID-19, and relationships between twin delivery and the three ‘Rs’. The final section of this article presents newsworthy twin-related items, some associated with COVID-19. They include a summary of the Rainman Twins film, a study of anal prints, the ‘Twins’ Irish pub, newborn twins ‘Covid and Corona’, the death of a surgeon who separated conjoined twins, and Twinco, a twin-based supply company.

One of Nick’s key early achievements at QIMR was to establish a twin study on melanoma risk factors. The Brisbane Twin Nevus Study (BTNS) had an initial focus on nevus (mole) count in adolescents but, reflecting Nick’s broad interests, expanded in scope enormously over the decades. In the skin cancer arena, BTNS was essential to genetic discoveries in melanoma, eye color and pigmentation. Later studies amassed data on thousands of phenotypes, ranging from molecular phenotypes such as gene expression to studies where gene mapping findings in adolescents turned out to have translational potential in late-onset diseases. Nick’s twin data have formed the basis for an enormous range of discoveries, with Nick and his colleagues continuing to capitalize on these data.

This article describes Dr Nathan Gillespie’s PhD training and supervision under Professor Nick Martin and their ongoing collaborations. Drs Gillespie and Martin have collaborated on numerous biometrical genetic analyses applied to cross-sectional and longitudinal twin data, combined molecular and phenotypic modeling, as well as genomewide meta-analyses of psychoactive substance use and misuse. Dr Gillespie remains an active collaborator with Professor Martin, including ongoing data collection, analysis and publications related to the Brisbane Longitudinal Twin Study.