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.