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Fullerton Virtual Twin Project: Overview and 2019 Update

  • Nancy L. Segal (a1) and Francisca J. Niculae (a1)


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.


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Author for correspondence: Nancy L. Segal, Email:


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McGue, M., Bouchard, T. J., , Jr., Iacono, W. G., & Lykken, D. T. (1993). Behavioral genetics of cognitive ability: A lifespan perspective. In Plomin, R. & McClearn, G. E. (Eds.), Nature, nurture and psychology (pp. 5976). Washington, DC: APA Press.
McGuire, S. A., & Segal, N. L. (2013). Peers networks: Overlap in twin, sibling and friend dyads. Child Development, 84, 500511.
McGuire, S. A., Segal, N. L., & Hershberger, S. L. (2012). Parenting as phenotype: A behavior genetic approach to understanding parenting. Parenting: Science and Practice, 12, 192201.
McGuire, S., Segal, N. L., Whitlow, B., Gill, P., & Clausen, J. (2010). Sibling trust and trustworthiness. In Rotenberg, K. J. (Ed.), Interpersonal trust during childhood and adolescence (pp. 133154). Cambridge, England: Cambridge University Press.
O’Neal, I. C., Paek, S. H., & Runco, M. A. (2015). Comparison of competing theories about ideation and creativity. Creativity: Theories – Research – Applications, 2, 145164.
Rotenberg, K.J., Fox, C., Green, S., Ruderman, L., Slater, K., Stevens, K., … Carlo, G. (2005). Construction and validation of a children’s interpersonal trust belief scale. British Journal of Developmental Psychology, 32, 271292.
Saucier, G., & Goldberg, L.R. (1998). What is beyond the Big Five? Journal of Personality, 66, 497524.
Scarr, S., Weinberg, R. A., & Waldman, D. (1993). IQ correlations in transracial adoptive families. Intelligence, 17, 541555.
Schelling, T. C. (1960). The strategy of conflict. Cambridge, MA: Harvard University Press.
Segal, N. L. (1997). Same-age unrelated siblings: A unique test of within-family environmental influences on IQ similarity. Journal of Educational Psychology, 89, 381390.
Segal, N. L. (2000). Entwined lives: Twins and what they tell us about human behavior. New York: Plume.
Segal, N. L. (2005). Indivisible by two: Lives of extraordinary twins. Cambridge, MA: Harvard University Press.
Segal, N. L. (2010). Twins: The finest natural experiment. Personality and Individual Differences, 49, 317323.
Segal, N. L. (2012). Born together-reared apart: The landmark Minnesota twin study. Cambridge, MA: Harvard University Press.
Segal, N. L. (2017). Twin mythconceptions: False beliefs, fables, and facts about twins. San Diego, CA: Elsevier.
Segal, N. L., & Allison, D. B. (2002). Twins and virtual twins: Bases of relative body weight revisited. International Journal of Obesity, 26, 437441.
Segal, N. L., Feng, R., McGuire, A. S., Allison, D. B., & Miller, S. (2008). Genetic and environmental contributions to body mass index: Comparative analysis of monozygotic twins, dizygotic twins and same-age unrelated siblings. International Journal of Obesity, 33, 3741
Segal, N. L., & Hershberger, S. L. (2005). Virtual twins and intelligence: Updated and new analyses of within-family environmental influences. Personality and Individual Differences, 39, 10611073.
Segal, N. L., Li, N., Graham, J. L., Miller, S. A., & McGuire, S. A. (2015). Virtual twins: Individual differences and evolutionary perspectives. Paper presented at the International Society for the Study of Individual Differences, London, Ontario, Canada.
Segal, N. L., Li, N. P., Graham, J. L., & Miller, S. A. (2015). Do parents favor their adoptive or biological children? Predictions from kin selection and compensatory models. Evolution and Human Behavior, 36, 379388.
Segal, N. L., McGuire, S. A., Graham, J. L., & Stohs, J. H. (2013). Fullerton Virtual Twin Study: An update. Twin Research and Human Genetics, 16, 451454.
Segal, N. L., McGuire, S. A., Havlena, J., Gill, P., & Hershberger, S. L. (2007). Intellectual similarity of virtual twin pairs: Developmental trends. Personality and Individual Differences, 42, 12091219.
Segal, N. L., McGuire, S. A., Miller, S., & Havlena, J. (2008). Tacit coordination in monozygotic twins, dizygotic twins and virtual twins: Effects and implications of genetic relatedness. Personality and Individual Differences, 45, 607612.
Segal, N. L., McGuire, S. A., & Stohs, J. (2012). What virtual twins reveal about general intelligence and other behaviors. Personality and Individual Differences, 53, 405410.
Segal, N. L., & Montoya, Y. S. (2018). Accidental brothers: The story of twins exchanged at birth and the power of nature and nurture. New York: St Martin’s Press.
Segal, N. L., Munson, J. E., Marelich, W. D., Goetz, A. T., & McGuire, S. A. (2013). Meeting of minds: Tacit coordination in monozygotic and dizygotic adolescent and adult twins. Personality and Individual Differences, 58, 3136.
Segal, N. L., Orozco, E. N., Preston, K. J., & Gerkens, D. R. (2019). Nonverbal cognition across genetic and environmentally informative kinships: The case for opposite-sex twin pairs and virtual twins. Journal of Experimental Child Psychology, 182, 144150.
Segal, N. L., Stohs, J. H., & Evans, K. (2011). Chinese twin children reared apart and reunited: First prospective study of co-twin reunions. Adoption Quarterly, 14, 6178.
Segal, N. L., Tan, T. X., & Graham, J. L. (2015). Twins and virtual twins: Do genetic (as well as experiential) factors affect developmental risks? Journal of Experimental Child Psychology, 136, 5569.
Tan, T. X., Marfo, K., & Dedrick, R. F. (2010). Early developmental and psychosocial risks and longitudinal behavioral adjustment outcomes for preschool-age girls adopted from China. Journal of Applied Developmental Psychology, 31, 306314.
Tapp, A. L., Maybery, M. T., & Whitehouse, A. J. O. (2011). Evaluating the twin testosterone transfer hypothesis: A review of the empirical evidence. Hormones and Behavior, 60, 713722.



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