Building on Ellis et al.’s theorization for potent dimensions of environmental adversity, the present work sought to evaluate how environmental harshness and unpredictability might function directly and in interaction with child sensory processing sensitivity (SPS) to shape the development of child socioemotional functioning. Participants were 235 young children (Mage = 2.97 at the first measurement occasion) and their parents, who were followed for two consecutive annual measurement occasions. Child SPS was measured through behavioral observation across multiple tasks within the laboratory setting. Greater environmental unpredictability was significantly associated with the development of children’s externalizing problems over a year only for children with high SPS. Follow-up analyses indicated that the unpredictability-x-SPS interaction was consistent with differential susceptibility, such that high SPS children showed greater increases in externalizing problems under high unpredictability, but also lower increases/greater decreases in externalizing problems under low unpredictability. Such association did not apply to children with low SPS.

Virtual twins (VTs) are defined as same-age unrelated siblings raised together from early infancy. This special class of adoptive siblings replays the rearing situation of twins, absent genetic relatedness. The first such pair was identified and studied in 1990 at the University of Minnesota, leading to the creation of the Fullerton Virtual Twin Study (FVTS) at California State University, Fullerton (CSUF) the following year. The registry currently includes 169 VT pairs, mostly children, with new pairs identified on a regular basis. These sibling sets provide a direct estimate of environmental influences on developmental traits and, as such, offer informative comparisons with ordinary monozygotic and dizygotic twins, full siblings and adoptive brothers and sisters. The sample characteristics, assessment battery and findings to date are summarized in this 2019 update.