This study reports on the inter- and intrarater reliability of physical features observation. Study subjects were 176 Chinese adult persons, consisting of 89 males and 87 females. Three trained research assistants responded simultaneously and respectively to 12 items regarding the subject's physical features including ‘hair’, ‘Mongoloid folds’, left and right ‘ear lobes’, ‘earwax’, ‘nostril shape’, ‘tongue rolling’, left and right ‘hitchhiker's thumb’, ‘mid-digital hair’ and left and right ‘simian crease’ at the moment of interview. And 14 days later, these subjects received the same observation once again. The results showed that the inter- and intra-observer agreements of ‘hair’, ‘earwax’, ‘tongue rolling’, ‘mid-digital hair’ and ‘simian crease’ were almost perfect with most kappa (κ) coefficients ≥ .80, while ‘Mongoloid fold’ and ‘nostril shape’ showed poor inter-observer agreement and ‘nostril shape’ showed poor intra-observer agreement (κ < .40). Two other physical features, ‘hitchhiker's thumb’ and ‘ear lobes’ showed moderate inter-observer agreement and three features, ‘hitchhiker's thumb’, ‘ear lobes’ and ‘Mongoloid fold’, showed moderate intra-observer agreement (.40≤ κ <.80). In conclusion, this study suggests that as far as reliability is concerned, the five features which were ‘hair’, ‘earwax’, ‘tongue rolling’, ‘mid-digital hair’ and ‘simian crease’ could be considered in zygosity determination of Chinese adult twins, while the two features, ‘Mongoloid fold’ and ‘nostril shape’, should be abandoned.

Only recently has the special bereavement associated with the loss of a twin been generally recognised. With the notable exception of Joan Woodward, few have written on the subject. We, therefore, offer a shortened version of a paper written nearly 30 years ago. George Engel was himself a lone twin as well as being an eminent psychiatrist and psychoanalyst in the United States. The length of the paper precludes reprinting the totality but we hope readers will be encouraged to seek out the original published in The International Journal of Psycho-analysis in 1975, Vol 56 part 1, 23–40 .

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.

We aimed to analyze the characteristics of development of relative weight in Japanese triplets from birth until 12 years of age. Data were collected through a mailed questionnaire sent to mothers of triplets asking for information recorded in medical records. Altogether we had information on 1,061 triplet children of 354 mothers born between 1978 and 2006. For these births, data on triplets’ height and weight growth, gestational age, sex, parity, and maternal age at delivery were obtained from records in the maternal and child health handbooks and records from the schools where children receive health check-ups. In addition, information on maternal height and weight was obtained. Triplets have a lower ponderal index at birth and lower body mass index (BMI) compared with the general population until 12 years of age, except for the period during 1 and 3 years of age. Moreover, birth weight had the strongest contribution to BMI of triplets until 6 years of age. After 9 years of age, maternal BMI was a significant factor affecting BMI of triplets.

Mapping genetic risk factors for endometriosis continues from early studies on women’s health initiated by Nick Martin and Susan Treloar. Their initial recruitment of endometriosis cases and family members received a major boost and became a flagship project within the Cooperative Research Centre (CRC) for the Discovery of Common Human Disease. We extended the study through a formal collaboration with Professor Stephen Kennedy and his group in Oxford. Our first joint scientific meeting was held in Brisbane and was sadly memorable as the day the planes were flown into the Twin Towers in New York. Our initial collaboration expanded into the International Endometriosis Genetics Consortium (IEGC). The IEGC now has 15 groups around the world, and the most recent meta-analysis will be published this year.

Life history theory studies the evolution of traits related to reproductive fitness. Fertility and parental investment are key life history traits which, from an evolutionary standpoint, appear strongly interrelated. The aim of this work was to analyze the genetic and environmental structure and relationship of two behaviors associated with reproductive fitness: total number of offspring and mean duration of breastfeeding. A total of 1,347 women distributed in 239 monozygotic pairs, 236 dizygotic pairs, and 393 individual twins from opposite sex pairs provided information about their reproductive history. We conducted separate univariate analyses to study the sources of variance of both variables; and a bivariate analysis, with threshold liability models. The sources of variance for number of children and breastfeeding were best explained by a model including familial and unique environmental factors, being E = 0.54 (CI 95%: 0.44, 0.66) and E = 0.46 (CI 95%: 0.34, 0.61), respectively. The phenotypic correlation between number of children and breastfeeding was low but significant (r = 0.16, CI 95%: 0.07, 0.25). Familial correlation between these variables did not reach significance, but unique environmental correlation did (re = 0.20, CI 95%: 0.02, 0.37). In conclusion, results do not support the existence of a clear common structure for the number of children a woman has and the time she spends breastfeeding them, at least in modern societies. The relationship found was mainly due to unique environmental factors. More research on these and related phenotypes is needed to better understand women's reproductive decisions and how natural selection acts on the life history traits.

This article explores some rare sides of twin research. The focus of this article is the sad plight of the Dionne quintuplets, born in Canada in 1934. However, several other studies belong in this category, such as Dr Josef Mengele’s horrifying twin research conducted at the Auschwitz concentration camp, Dr John Money’s misguided attempt to turn an accidentally castrated male twin into a female, Russian scientists’ cruel medical study of conjoined female twins and Dr Peter Neubauer’s secret project that tracked the development of separated twins. Reviews of current twin research span twins’ representation of self-image, twins with Kleine–Levin Syndrome, heteropaternal twinning in lemurs and factors affecting risk of dental caries. Media coverage includes a pair of high-society models, a book about the ‘Winkelevii’ twins, Super Bowl twin teammates, a family with three sets of fraternal twins, a twin sister surrogate and a near presidential twin.

Two families with twins — one in which adult DZ twins continue to face the risk of frontotemporal dementia, and the other in which a mother copes with brittle bones in a newborn infant MZ pair — are featured. The nature and progress of the disorders are reviewed and personal perspectives from the twins and other family members are variously provided. Next, four current twin studies are briefly summarized. Topics include sex differences in social support systems, new reproductive technologies, comparative twin-singleton frequencies of placenta previa, and the nature and complications of heterotopic pregnancies. This article concludes with a brief sampling of human interest stories involving twins: a different looking DZ pair, newborn sextuplets, conjoined twins and opposite-sex twin athletes.

TwinsMX registry is a national research initiative in Mexico that aims to understand the complex interplay between genetics and environment in shaping physical and mental health traits among the country’s population. With a multidisciplinary approach, TwinsMX aims to advance our knowledge of the genetic and environmental mechanisms underlying ethnic variations in complex traits and diseases, including behavioral, psychometric, anthropometric, metabolic, cardiovascular and mental disorders. With information gathered from over 2800 twins, this article updates the prevalence of several complex traits; and describes the advances and novel ideas we have implemented such as magnetic resonance imaging. The future expansion of the TwinsMX registry will enhance our comprehension of the intricate interplay between genetics and environment in shaping health and disease in the Mexican population. Overall, this report describes the progress in the building of a solid database that will allow the study of complex traits in the Mexican population, valuable not only for our consortium, but also for the worldwide scientific community, by providing new insights of understudied genetically admixed populations.

This article describes the Australian Twins Economic Preferences Survey (ATEPS). The data set comprises a wide variety of preference and behavioral measures (risk aversion, impatience, ambiguity aversion, trust, confidence) elicited using incentivized decision tasks. One-thousand one-hundred twenty Australian adult twins (560 pairs) completed the survey, making it one of the largest data sets containing incentivized preference measures of twins. As the survey was conducted during the COVID-19 pandemic, we also collected information on experiences related to the pandemic, along with a variety of questions on political attitudes and mental wellbeing. We hope that ATEPS can make a valuable contribution to social science and genetics research.

A January 2017 reunion of 10-year-old reared-apart Chinese twin girls was captured live on ABC's morning talk show Good Morning America, and rebroadcast on their evening news program Nightline. The twins’ similarities and differences, and their participation in ongoing research will be described. This story is followed by reviews of twin research concerning genetic and environmental influences on media use, twin relations across the lifespan and the breast-feeding of opposite-sex twins. Popular interest items include twins in fashion, the second twin pair born to an internationally renowned tennis star, twin primes and twin pandas.

Twin reversed arterial perfusion (TRAP) sequence is a rare anomaly in 1% of monochorionic twin pregnancies. Few TRAP sequence cases have a rudimentary heart with cardiac motion in the acardiac twins. Herein, we investigated the venous Doppler waveform in two cases of TRAP sequence with a rudimentary heart with cardiac motion in the acardiac twin. Although both cases had veno-venous anastomoses, the venous Doppler waveforms of the umbilical vein differed. A comparison of the characteristics of the cases indicated that the existence of a ductus venosus, or greater heart pulse power, in the acardiac twin might impact the venous Doppler waveform in the pump twin.

Familial twinning and fertility traits were investigated in Nigerian mothers of dizygotic (DZ) twins (MoDZT; N = 972) and controls (N = 525) who responded to our person-to-person interview, which included questions on pregnancy history and family history of DZ twinning. Controls were defined as women who are not twins themselves and do not have twins in their first-degree relatives. Over 95% of the participants were Yoruba. We found that Nigerian MoDZT had an average of 4.0 (±2.6) pairs of twins among their relatives, and of these, the prevalence of DZ twins was significantly higher than that of monozygotic (MZ) twins (45.9% vs. 25.8%). Controls had an average of 0.5 (±0.4) pairs, and over 95% of the controls had no twins in their relatives. These results suggest genetic influences on DZ twinning in Nigerians. MoDZT were significantly younger in their mean age at first child, and had higher parity than controls, suggesting increased fertility in MoDZT. As compared to mothers with a single set of twins, mothers (N = 130) with multiple sets had significantly more twins among their relatives (5.4 pairs vs. 3.7 pairs) and had their first twins at a younger age (28.4 vs. 30.7 years), indicating that mothers with multiple sets of twins might have higher genetic propensity for twinning associated with earlier age at twin pregnancy. Our findings argue for genomewide association studies for DZ twinning in Nigerians, and may help to develop intervention strategies to overcome infertility/subfertility problems.

The dynamics of rare X-linked recessive traits is explored by simulation. The model follows the prevalence of affected males and carrier females as separate but correlated variables. Different mutation rates and selection coefficients are introduced for males and females. A virtual population based on a published study of hemophilia B in the west of Scotland is followed at weekly intervals over many years. Speculative values of critical parameters to mimic the real population are proposed.

Between 2006 and 2021, the Hungarian Twin Registry (HTR) operated a volunteer twin registry of all age groups (50% monozygotic [MZ], 50% dizygotic [DZ], 70% female, average age 34 ± 22 years), including 1044 twin pairs, 24 triplets and one quadruplet set. In 2021, the HTR transformed from a volunteer registry into a population-based one, and it was established in the Medical Imaging Centre of Semmelweis University in Budapest. Semmelweis University’s innovation fund supported the development of information technology, a phone bank and voicemail infrastructure, administrative materials, and a new website was established where twins and their relatives (parent, foster parent or caregiver) can register. The HTR’s biobank was also established: 157,751 individuals with a likely twin-sibling living in Hungary (77,042 twins, 1194 triplets, 20 quadruplets, and one quintuplet) were contacted between February and March of 2021 via sealed letters. Until November 20, 2022, 12,001 twin individuals and their parents or guardians (6724 adult twins, 3009 parents/guardians and 5277 minor twins) registered, mostly online. Based on simple self-reports, 37.6% of the registered adults were MZ twins and 56.8% were DZ; 1.12% were triplets and 4.5% were unidentified. Of the registered children, 22.3% were MZ, 72.7% were DZ, 1.93% were triplets, and 3.05% were unidentified. Of the registered twins, 59.9% were female (including both the adult and minor twins). The registration questionnaire consists of eight parts, including socio-demographic and anthropometric data, smoking habits and medical questions (diseases, operations, therapies). Hungary’s twin registry has become the sole and largest population-based twin registry in Central Eastern Europe. This new resource will facilitate performing world-class modern genetic research.