This short essay recounts the author’s interactions with Nick Martin in the years they both worked with Lindon Eaves at Virginia Commonwealth University. Although coming from very different academic traditions, they became close colleagues building their young careers together. Nick generously shared his statistical genetics expertise and the author taught Nick a thing or two about psychiatric illness.

Chimeras, individuals whose genetically distinct cell lines originated from different zygotes, are attracting attention among twin researchers. This is because chimerism may occur more frequently than previously suspected, having been observed among 8% of dizygotic (DZ) twins and 21% of DZ triplets. The presence of separate cell lines in these twins may hold clues to disease susceptibility, developmental complications and histocompatibility features. Next, new twin research on physical development is offering fresh insights on height and hormones, height and education, and pelvic development. Finally, twins having twins, twins deserting the military and twins composing music raise new thoughts about reproduction, behavior and talent.

Prior research based on Swedish data suggests that collective optimism, as measured by monthly incidence of suicides, correlates inversely with selection in utero against male twins in a population. We test this finding in the US, which reports the highest suicide rate of all high-income countries, and examine whether monthly changes in overall suicides precede changes in the ratio of male twin to male singleton live births. Consistent with prior work, we also examine as a key independent variable, suicides among women aged 15−49 years. We retrieved monthly data on suicides and the ratio of male twin to singleton live births from CDC WONDER, 2003 to 2019, and applied Box-Jenkins iterative time-series routines to detect and remove autocorrelation from both series. Results indicate that a 1% increase in monthly change in overall suicides precedes a 0.005 unit decline in male twin live births ratio 6 months later (coefficient = −.005, p value = .004). Results remain robust to use of suicides among reproductive-aged women as the independent variable (coefficient = −.0012, p value = .014). Our study lends external validity to prior research and supports the notion that a decline in collective optimism corresponds with greater selection in utero.

This report presents the uncommon case of a 154-day delayed delivery in a spontaneous twin pregnancy associated with uterine atony. After abortion of the first fetus at 16 weeks, a healthy male was born at 38 weeks. Postpartum hemorrhage due to uterine atony, which was successfully treated with prostaglandins, occurred.

Adoption agencies are expending greater efforts toward placing adoptive twins and siblings together in the same home. However, new sources of reared apart twins continue to be identified. Some of these sources are described, with suggestions for how individual cases can be used more fruitfully by interested researchers. Next, recent findings on the fitness of female twins from male–female pairs and the heritability of olfactory thresholds are summarized. Finally, new thoughts about some artificially conceived infants and artificial reproductive methods are considered. A generous donation to the Twins Days Festival, in Twinsburg, Ohio is also described.

Lam et al. (2018) respond to a commentary of their paper entitled ‘Large-Scale Cognitive GWAS Meta-Analysis Reveals Tissue-Specific Neural Expression and Potential Nootropic Drug Targets’ Lam et al. (2017). While Lam et al. (2018) have now provided the recommended quality control metrics for their paper, problems remain. Specifically, Lam et al. (2018) do not dispute that the results of their multi-trait analysis of genome-wide association study (MTAG) analysis has produced a phenotype with a genetic correlation of one with three measures of education, but do claim the associations found are specific to the trait of cognitive ability. In this brief paper, it is empirically demonstrated that the phenotype derived by Lam et al. (2017) is more genetically similar to education than cognitive ability. In addition, it is shown that of the genome-wide significant loci identified by Lam et al. (2017) are loci that are associated with education rather than with cognitive ability.

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.

The purpose of this research was to determine the frequency of mutation of the cytochrome CYP3A5 genes and transport proteins SLCO1B1 and MDR1 in patients with coronary heart disease in the Kazakh nation. A prospective cohort clinical and genetic study was conducted. The study was conducted in 2017−2019. Medical records containing information about drug prescription conducted in hospitals and outpatient departments were carefully analyzed. In the examined group of 178 patients treated with statins, a significant frequency of genetic variants that determine the increased risk of complications of statin use was revealed. There was a tendency toward an increase in the activity of creatine phosphokinase (CPK) in the blood upon detection of the A6986G mutation of the cytochrome gene and SLCO1B1 (c.521T>C) gene of the transport protein OATP1B1. In the studied Kazakh population, the presence of a homozygous mutant SLCO1B1 gene of the transport protein can be recommended as a genetic marker for the undesirability of using antihypercholesterolemic therapy with statins, which simultaneously leads to a decrease in the effectiveness of treatment and an increase in the risk of side effects.

A look at scientific studies of sports performance is informative in view of the 2012 Olympic games in London. The athletic life histories of selected twin competitors supplement the scientific data. Reviews of recent twin studies concerning cardiometabolic risk factors, postnatal brain maturation, parental responses to rearing triplets, and the birth of an unusual chimaeric twin pair are presented. General interest items include the separate careers of twin soccer players, an unexpected identical triplet birth, the interesting life of twin Geoffrey D. Langlands, and the Lone Twin Boat Project.

This population-based cohort study investigated the prevalence, potential risk factors, and consequences of birth weight discordance (BWD) among same-sex twins. We retrieved data from the automated system of healthcare utilization databases of Lombardy Region, Northern Italy (2007–2021). BWD was defined as 30% or more disparity in birth weights between the larger and the smaller twin. Multivariate logistic regression was used to analyze the risk factors of BWD in deliveries with same-sex twins. In addition, the distribution of several neonatal outcomes was assessed overall and according to BWD level (i.e., ≤20%, 21–29, and ≥30%). Finally, a stratified analysis by BWD was performed to assess the relationship between assisted reproductive technologies (ART) and neonatal outcomes. We identified 11,096 same-sex twin deliveries; 556 (5.0%) pairs of twins were affected by BWD. Multivariate logistic regression analysis showed that maternal age ≥35 years (OR 1.26, 95% CI [1.05,5.51]), low level of education (OR 1.34, 95% CI [1.05, 1.70]), and ART (OR 1.16, 95% CI [0.94, 1.44], almost significant due to the low power) were independent risk factors for BWD in same-sex twins. Conversely, parity (OR 0.73, 95% CI [0.60, 0.89]) was inversely related. All the adverse outcomes observed were more common among BWD pairs than non-BWD ones. Instead, a protective effect of ART was observed for most neonatal outcomes considered among BWD twins. Our results suggest that conception after ART increases the risk of developing a high disparity between the weights of the two twins. However, the presence of BWD may complicate twin pregnancies, compromising neonatal outcomes, regardless of the modality of conception.

This article begins with the story of a 51-year-old Los Angeles, California man, Justin Goldberg, whose daughter caught a glimpse of his striking look-alike at a popular market. Many people have so-called doppelgängers, but this occurrence is especially intriguing — the individual in question, born in New York City in the mid-1960s to an unwed mother, was an adoptee placed by the Louise Wise Adoption Agency. This agency, under the guidance of a prominent psychiatrist, decided to place twins in separate homes. Some of these twin children were part of a controversial child development study that was hidden from them and their parents. Next, recent and current twin research on heart transplantation, distinguishing monozygotic co-twins, twin conceptions via oocyte donation and factors affecting craniofacial traits are summarized. The article concludes with highlights on twins in the media, specifically, a triplet delivery in the United Kingdom, self-concept and consciousness in conjoined twins, Colombian twin trainers, skin grafting to save an identical co-twin, lack of physical flaws in Dolly the cloned sheep, possible opposite-sex conjoined twins, and the passing of the remaining twin from the world's longest separated pair.

The first classical twin studies, recognizing the potential of comparing findings in identical twins, have previously been reported to be those by Siemens and by Merriman, both published in 1924. However, we would like to bring to attention a study performed by Walter Jablonski, 2 years earlier (1922), investigating the contribution of heredity to refraction in human eyes. Jablonski examined the eyes of 52 twin pairs and by comparing the size of within-pair differences between identical and non- identical twins was able to infer the heritability of a trait. Therefore, this is likely to be the first reported classical twin study.

Dizygotic twinning rates have changed over time, which has been seen as a sign of a decline in fecundity. Since a woman's birthweight has been shown to be a marker of her fecundity, maternal birthweight may correlate with subsequent twinning rates. In the Danish National Birth Cohort (1996–2002), we examined if maternal birthweight, and whether she was born at term or preterm, correlated with her probability of multiple birth. For 20,719 live born infants, we had self-reported information about maternal birthweight, collected during the first wave of the 7-year follow-up, and information on multiple births from record linkage. The association between maternal birthweight and multiple births was investigated by use of logistic regression and presented as odds ratios (ORs) with 95% confidence intervals (CIs). Compared to women born at term with a birthweight of 3001–4000 g, women with a birthweight > 4500 g appeared to have higher chance of multiple birth while women with a birthweight of 4001–4500 had a lower chance, especially if the analysis was restricted to women with a BMI < 25 (Adjusted ORs 2.3 [1.0–5.4] and 0.4 [0.2–0.9] respectively). Odds ratios for dizygotic twinning were of the same magnitude. In women with a BMI ≥ 25, no obvious pattern was present. Our findings do not indicate that twinning is a fecundity indicator. Women with a birthweight that may indicate a pregnancy complicated with gestational diabetes had the highest rate of multiple birth. These findings are new and should be put to a critical test in other data sources.

Avoiding overeating and being physically active is associated with healthy aging, but methodological issues challenge the quantification of the association. Intrapair comparison of twins is a study design that attempts to minimize social norm-driven biased self-reporting of lifestyle factors. We aimed to investigate the association between self-reported lifestyle factors and subsequent survival in 347 Danish twin pairs aged 70 years and older and, additionally, to investigate the reliability of these self-reports. The twins were interviewed in 2003 and followed for mortality until 2015. They were asked to compare their appetite and physical activity to that of their co-twins in different stages of life. On an individual level, we found a positive association between current self-reported physical activity and late-life survival for elderly twins. This was supported by the intrapair analyses, which revealed a positive association between midlife and current physical activity and late-life survival. A positive association between lower appetite and late-life survival was found generally over the life course in the individual level analyses but not in the intrapair analyses. Kappa values for the inter-twin agreement on who ate the most were 0.16 to 0.34 in different life stages, and for physical activity 0.19 to 0.26, corresponding to a slight-to-fair agreement. Approximately, 50% of the twin pairs were not in agreement regarding physical activity, and of these twins 75% (95% CI: 67–82%) considered themselves the most active twin. These findings indicate a still-existing tendency of answering according to social norms, even in a twin study designed to minimize this.

The expansion of genetic and genomic testing in clinical practice and research, and the growing market for direct-to-consumer genomic testing has led to increased awareness about the impact of this form of testing on insurance. Genetic or genomic information can be requested by providers of mutually rated insurance products, who may then use it when setting premiums or determining eligibility for cover under a particular product. Australian insurers are subject to relevant legislation and an industry led standard that was updated in 2019 to introduce a moratorium on the use of genetic test results in life insurance underwriting for policies <AU$500K. The Human Genetics Society of Australasia has updated its position statement on genetic testing and life insurance to account for these changes and to increase the scope of the statement to include a wider range of personally-rated insurance products, such as life, critical care, and income protection products. Recommendations include that: providers of professional education involving genetics should include ethical, legal, and social aspects of insurance discrimination in their curricula; the Australian Government take a more active role in regulating use of genetic information in personal insurance; that information obtained in the course of a research project be excluded; insurers seek expert advice when making underwriting decisions regarding genetic testing; and engagement between the insurance industry, regulators, and the genetics profession be improved.