Ten classic works in twin research are described. These volumes are rich in data, interpretation, and impact, and contain life history material that adds depth and dimension to the quantitative findings. Next, research on social relationships in older twins, superfecundated twinning in chimpanzees; effects of embryo transfer on conjoined twinning and the reduced frequency of in vitro multiples is reviewed. Finally, there has been considerable public interest surrounding the first identical twin renal transplant, an identical triplet wedding; identical twin boxers, a twin living in space, and a politically active twin pair.

Highlights from a conference, ‘Healthier Kids: Insights From Twin Research’, held in Melbourne, Australia on December 5, 2014, are summarized. In addition to informative and exciting presentations and discussions of recent findings, three key themes emerged: (1) results from twin research have important implications for non-twins, (2) researchers from diverse disciplines should be encouraged to participate actively in twin research, and (3) investigators, twins and families need to work more collaboratively. Next, papers and studies concerning the biological bases of monozygotic (MZ) twinning, gestational diabetes and post-partum depression are reviewed. Finally, general interest reports of rare quadruplets, mistaken identity, prenatal laser surgery and a royal twin birth are provided.

Background: Single fetal death (sFD) in monochorionic twin pregnancies is associated with substantial morbidity and mortality in the survivor. The aim of our study was to evaluate the rate of cerebral lesions detected at fetal Magnetic Resonance Imaging (MRI) and to correlate the results with the neurologic outcome of the survivors of monochorionic twin pregnancies after sFD. Methods: Between 2005 and 2012, 11 monochorionic twin pregnancies with sFD and subsequent fetal MRI of the survivor were included. All neonates underwent neurologic assessment after birth and 56% of surviving infants underwent long-term neurologic assessment. MRI findings and neurologic outcome of the survivors were evaluated. Results: Gestational age at sFD was 20.9 (±2.9) weeks; 55% (6/11) of survivors of monochorionic twin pregnancies after sFD showed cerebral lesions at fetal MRI; 72% (8/11) of all survivors had normal neonatal neurologic outcome: all survivors with normal fetal MRI and 50% of survivors with cerebral lesions at fetal MRI. Long-term neurologic assessment was normal in all tested patients with normal fetal MRI and in one of three tested patients with cerebral lesions at fetal MRI. Conclusion: Survivors of monochorionic twin pregnancies after sFD show a high rate of cerebral lesions at fetal MRI. The importance of cerebral lesions at fetal MRI in survivors after sFD in monochorionic twin pregnancies is uncertain. All tested survivors with normal fetal MRI showed normal neurologic outcome but only one of three survivors with cerebral lesions at fetal MRI showed normal long-term neurologic outcome.

Dr John MacMillan, Senior Staff Specialist at Genetic Health Queensland and Associate Professor of Medicine at The University of Queensland, passed away on December 21, 2014, aged 55 years. John was founding director at Genetic Health Queensland and was well known for his research contribution into the genetic basis of neurological disease.

Physicians and other medical professionals do not always provide new parents with an accurate diagnosis of their twins’ zygosity. An overview of this problem is presented, supplemented by an interview with a mother who recently learned that her 2-year-old ‘dizygotic (DZ)’ twin girls are actually ‘monozygotic (MZ)’. Reviews of two case studies, one of twins with sex-discordance and chimerism and the other of twins with congenital amegakaryotic thrombocytopenia, follow. Two additional studies, one a twin analysis of attractiveness to mosquitoes and the other a study of twins coping with crisis, are also described. Several articles and letters from the popular media, concerning less favored twins, paternity issues surrounding superfecundation, twins with late-onset Tay-Sachs disease, and triplets admitted to MIT are informative and insightful.

Approximately 60% of children with reading difficulties (RD) meet criteria for at least one co-occurring disorder. The most common of these, attention deficit-hyperactivity disorder (ADHD), occurs in 20–40% of individuals with RD. Recent studies have suggested that genetic influences are responsible. To assess the genetic etiologies of RD and the comorbidity of RD and two ADHD symptom dimensions –– inattention (IN) and hyperactivity/impulsivity (H/I) –– we are conducting the first longitudinal twin study of RD and ADHD. Data from twin pairs in which at least one member of the pair met criteria for proband status for RD at initial assessment, and were reassessed 5 years later, were subjected to DeFries-Fulker (DF) analysis. Analyses of reading composite data indicated that over 60% of the proband deficit at initial assessment was due to genetic influences, and that reading deficits at follow-up were due substantially to the same genetic influences. When a bivariate DF model was fitted to reading performance and IN data, genetic influences accounted for 60% of contemporaneous comorbidity and over 60% of the longitudinal relationship. In contrast, analysis of the comorbidity between reading performance and H/I indicated that common genetic influences accounted for only about 20% of the contemporaneous and about 10% of the longitudinal relationships. Results indicate that (1) genetic influences on RD are substantial and highly stable; (2) the comorbidity between RD and IN is due largely to genetic influences, both contemporaneously and longitudinally; and (3) genetic influences contribute significantly less to the comorbidity between RD and H/I.

Twin pairs discordant for disease may help elucidate the epigenetic mechanisms and causal environmental factors in disease development and progression. To obtain the numbers of pairs, especially monozygotic (MZ) twin pairs, necessary for in-depth studies while also allowing for replication, twin studies worldwide need to pool their resources. The Discordant Twin (DISCOTWIN) consortium was established for this goal. Here, we describe the DISCOTWIN Consortium and present an analysis of type 2 diabetes (T2D) data in nearly 35,000 twin pairs. Seven twin cohorts from Europe (Denmark, Finland, Norway, the Netherlands, Spain, Sweden, and the United Kingdom) and one from Australia investigated the rate of discordance for T2D in same-sex twin pairs aged 45 years and older. Data were available for 34,166 same-sex twin pairs, of which 13,970 were MZ, with T2D diagnosis based on self-reported diagnosis and medication use, fasting glucose and insulin measures, or medical records. The prevalence of T2D ranged from 2.6% to 12.3% across the cohorts depending on age, body mass index (BMI), and national diabetes prevalence. T2D discordance rate was lower for MZ (5.1%, range 2.9–11.2%) than for same-sex dizygotic (DZ) (8.0%, range 4.9–13.5%) pairs. Across DISCOTWIN, 720 discordant MZ pairs were identified. Except for the oldest of the Danish cohorts (mean age 79), heritability estimates based on contingency tables were moderate to high (0.47–0.77). From a meta-analysis of all data, the heritability was estimated at 72% (95% confidence interval 61–78%). This study demonstrated high T2D prevalence and high heritability for T2D liability across twin cohorts. Therefore, the number of discordant MZ pairs for T2D is limited. By combining national resources, the DISCOTWIN Consortium maximizes the number of discordant MZ pairs needed for in-depth genotyping, multi-omics, and phenotyping studies, which may provide unique insights into the pathways linking genes to the development of many diseases.

Aldur W. Eriksson, the noted twin scholar and human geneticist, passed away on Friday, April 3, 2015, at the age of 88. He had a full, highly productive and accomplished life. He is survived by his wife Maj-Britt and their son Staffan and his family. His death leaves a void, not only among relatives and friends, but among the many who had known him and in the scientific community, particularly that of twin researchers worldwide.

An impressive literature has revealed that variation in virtually every measurable phenotype is the result of a combination of genetic and environmental influences. Based on these findings, studies that fail to use genetically informed modeling strategies risk model misspecification and biased parameter estimates. Twin- and adoption-based research designs have frequently been used to overcome this limitation. Despite the many advantages of such approaches, many available datasets do not contain samples of twins, siblings or adoptees, making it impossible to utilize these modeling strategies. The current study proposes a measurement strategy for estimating the intergenerational transmission of antisocial behavior (ASB) within a nationally representative sample of singletons using an extended pedigree risk approach that relies on information from first- and second-degree relatives. An evaluation of this approach revealed a pattern of findings that directly aligned with studies examining ASB using more traditional twin- and adoption-based research designs. While the proposed pedigree risk approach is not capable of effectively isolating genetic and environmental influences, this overall alignment in results provides tentative evidence suggesting that the proposed pedigree risk measure effectively captures genetic influences. Future replication studies are necessary as this observation remains preliminary. Whenever possible, more traditional quantitative genetic methodologies should be favored, but the presented strategy remains a viable alternative for more limited samples.

Age difference in the etiology of the relationship between childhood negative emotionality (NE) and conduct problems (CP) has not been previously investigated. Mothers of 662 pairs of twins completed questions on the emotionality (NE) scale of the EAS temperament survey and the CP scale of the Strengths and Difficulties Questionnaires (SDQ) via a telephone interview. Twin data were analyzed separately in younger (ages 3 to 7 years; mostly pre-schoolers) and older children (ages 8 to 13 years; mostly elementary school children). The phenotypic correlation between NE and CP increased from 0.33 among younger twins to 0.43 among older twins. Bivariate model-fitting analysis was performed to determine age difference in the etiology of the relationship between NE and CP. Among younger twins, the correlation between NE and CP was entirely explained by additive genetic factors common to NE and CP. Among older children, however, a small but significant amount of unique environmental correlation emerged to account for about 47% of the phenotypic correlation between NE and CP. The remaining 53% of the phenotypic correlation was due to shared additive genetic factors. We speculate that environmental factors associated with school adjustment may exert influences on the relationship between NE and CP among elementary school children.