The June 2016 death of our esteemed colleague, Dr Irving I. Gottesman, was felt as an extreme loss at so many levels by colleagues, students, friends, and family across the globe. Irv's stellar contributions to the field of twin research will continue to be remembered and cited for many years to come. In commemoration of his life and work, I organized a symposium at the 16th meeting of the International Society for Twin Studies, held in Madrid, Spain, November 16–18, 2017. The panelists included mostly former students, as well as colleagues, who presented their scientific research and personal remarks reflecting Irv's profound influence in shaping their lives and careers. A chronology of Irv's academic positions and honors is included in the introduction to this special issue of Twin Research and Human Genetics, followed by brief sketches of the panel participants; their scholarly papers and personal reflections follow.

Play among peers is an important developmental context for child socialization. We have earlier shown that children at genetic risk for aggression were more likely to be treated aggressively by unfamiliar peers during peer play, reflecting genotype-evoked behaviors manifested during play. In this study, 118 5-year-old twin pairs were paired randomly with an unfamiliar, same-age, same-sex child, thus controlling for parent- and child-chosen environments (passive and active rGE). Twins played separately from each other with unmatched children. Play behaviors were coded for aggressive and assertive behaviors. Children were also independently rated by parents for rule-breaking problem behaviors at age 5, and 97 children were rated again on these behaviors 2–10 years later. Analyses showed that children at genetic risk for early rule-breaking were more likely to have partners who behaved more aggressively, suggesting that this was evoked behavior during play. Some evidence of an ‘early bloomer’ phenomenon emerged via early difficult temperament and parent-rated delinquency significantly predicting later delinquency. Children's play, which is one of the most important influences on early development, requires further study from an rGE perspective.

Irving Gottesman is known primarily for his work in psychopathology, but he also had a long-standing interest in understanding psychological development generally (typical and atypical). Through his mentorship, he also influenced work in gender development. Characteristics related to sex and gender are ideally suited to study the interplay of genes and environment across development. We discuss how gender development is influenced by gonadal hormones present during early life, but not in a simple way. We describe some of the challenges and opportunities to extend our understanding of the complexity of gender development. Throughout, we consider the kinds of questions Gottesman would likely have asked and emphasize his influence on our work.

Dr. Irving I. Gottesman was very creative in applying twin research designs to test hypotheses and predictions about human behavior. Two designs stand out among others: twins reared apart from birth and the twin-family study. I (NS) was inspired to incorporate both of these approaches into my own research programs at California State University, Fullerton. Gottesman's involvement and contributions to reared-apart twin research are described, followed by selected findings from the Minnesota Study of Twins Reared Apart (MISTRA), the Fullerton Study of Chinese Twins Reared Apart (CTA), a case report on separated monozygotic (MZ) female twins from South Korea, and doubly exchanged MZ male twin pairs from Colombia, South America. Recent findings from an ongoing twin-family study of social closeness, and future directions, are also presented.

In 1968, long before the publication of Stephen J. Gould's The Mismeasure of Man, Herrnstein and Murray's The Bell Curve, or Arthur Jensen's Bias in Mental Testing, Irving Gottesman published a book chapter that addressed head-on the issues that would define the relationship between the genetics of social behavior and large-scale social theory for the next 50 years. That he could do so with his characteristic scholarly thoroughness and scientific tough-mindedness without once lapsing into regressive hereditarianism is a testimony to the scope of his scientific knowledge and the generosity of his intellectual spirit.

One of Irving I. Gottesman's many contributions to behavior genetics, and part of his enduring legacy, was his introduction of the term ‘endophenotype’ to the field of psychiatry. Gottesman argued that focusing on endophenotypes, rather than complex heterogeneous clinical diagnoses, could help elucidate disease etiology. Although a different strategy for gene identification ultimately proved successful (that of amassing extremely large sample sizes in order to overcome the ‘noise’ of heterogeneity and have sufficient power to find genes of very small effect), the endophenotype concept continues to make a meaningful contribution to the field. The endophenotype concept forced the field to move beyond a simple disease model of finding genes ‘for’ psychiatric outcomes, and reminded us that genes are quite distal from complex behavioral outcomes and disorders. Endophenotypes called our attention to the steps along that pathway. In that process, the concept of endophenotypes evolved and expanded to include discussion of the role that other intermediary traits and psychological processes play in the development and genetic etiology of psychiatric and substance use disorders. As large-scale consortia continues to identify genes and generate genome-wide polygenic scores that are associated with behavioral outcomes, the next important step will be to characterize the pathways and mechanisms by which genetic risk unfolds. This essential step of mapping risk from genes to behavior is an evolution that follows naturally from the endophenotype concept, and could ultimately translate into improved prevention and intervention for individuals who are pre-disposed to mental health challenges.

Genetic predispositions play an important role in the development of internalizing and externalizing behaviors. Understanding the mechanisms through which genetic risk unfolds to influence these developmental outcomes is critical for developing prevention and intervention efforts, capturing key elements of Irv's research agenda and scientific legacy. In this study, we examined the role of parenting and personality in mediating the effect of genetic risk on adolescents’ major depressive disorder and conduct disorder symptoms. Longitudinal data were drawn from a sample of 709 European American adolescents and their mothers from the Collaborative Studies on Genetics of Alcoholism. Results from multivariate path analysis indicated that adolescents’ depressive symptoms genome-wide polygenic scores (DS_GPS) predicted lower parental knowledge, which in turn was associated with more subsequent major depressive disorder and conduct disorder symptoms. Adolescents’ DS_GPS also had indirect effects on these outcomes via personality, with a mediating effect via agreeableness but not via other dimensions of personality. Findings revealed that the pattern of associations was similar across adolescent gender. Our findings emphasize the important role of evocative gene–environment correlation processes and intermediate phenotypes in the pathways of risk from genetic predispositions to complex adolescent outcomes.

Irving I. Gottesman played an important role for psychiatric genetic research in Denmark through more than 40 years of collaboration with Danish scientists, resulting in important twin and family studies based upon the unique national registers available in Denmark.

Like families, professors transmit bits of wisdom to their students, not just by what they say but what they do. Colleagues share methods and data, but more importantly, ideas and visions. Irving I. Gottesman was unique in this respect, both brilliant in his field and wise when it came to teaching, guiding, and supporting. His academic pedigree extends across many generations and will for generations to come.

Human twinning poses a conundrum for researchers, given that the female uterus is optimally designed to carry a single fetus. Evolutionary explanations of dizygotic (DZ or fraternal) twinning provide insight and understanding of this reproductive event. This review is followed by summaries of recent twin research and reports concerning twins in a triplet pregnancy, double embryo transfer; a twin study of health anxiety, and delayed delivery of the second twin. Twin events reported in the media include a pair of digital marketing twins, a speech by a bereaved twin at the March for Our Lives in Washington, D.C., twins who exchanged places, and a same-team reunion of twin football players.