Monozygotic (MZ) twins reared apart from birth are a rare natural experiment, allowing unique insights into genetic and environmental factors underlying behavior. Accidentally switched MZ twins are an even rarer subgroup of separated twins with only nine documented cases world-wide. This presentation will review the complete chronology (1941 – 1988), life histories and research findings from these cases, as well as the psychological and legal implications posed by these unusual situations. The material is based on information gathered personally from the twins and/or their families in eight of the nine cases. Particular attention will be given to the most recent switched twins, which involved a double exchange: the switching of one newborn MZ male twin from two different pairs, born in Colombia, South America in 1988. This event generated a series of genetically and environmentally informative kinships--some familiar, but some unique. Familiar sibling sets are the two MZA twin pairs raised in dramatically different environments. The unique sibling pairs consist of (a) two virtual twin pairs (same-age unrelated siblings) who believed they were dizygotic (DZ) twins and who were raised as such; and (b) two pairs of “replicas” (unrelated siblings who were not reared together, but who genetically replicate the unrelated reared-together pairs; comparing the replicas with the virtual twins yields a measure of shared environmental influence). Selected findings on physical measures, health characteristics, mental abilities and epigenetic profiles are published (Segal & Montoya, 2018), but new findings in these areas will be presented, supplemented by photographic material from Colombia.
The nine switched-at-birth MZ twin pairs variously come from Switzerland (n = 1), Puerto Rico (n = 2), Canada (n = 1), Poland, (n = 1) the Canary Islands (Gran Canaria; n = 2) and now Colombia (n = 2). Five pairs are male and four pairs are female. The ages at which the truth about their birth was discovered range from 18 months to twenty-eight years. Despite the extremely different environments in which some of the co-twins were raised, some striking similarities in their behavioral and medical characteristics were observed. At the same time, differences in mental abilities and other behavioral traits were expressed that were possibly tied to events in their respective life histories. An epigenetic analysis of the Colombian twins and a biological sister (who was raised with one brother, but apart from the other), found that the pairs of genetic relatives were most alike with one exception, possibly associated with prenatal factors (Segal, Montoya, Loke & Craig, 2017). The various cases can be viewed against the backdrop of extant findings from MZ and DZ twins raised apart and together, but each individual switched-at-birth pair should be evaluated with caution.
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., Montoya, Y.M., Loke, Y.J., & Craig, J.M. (2017). Identical twins doubly exchanged at birth—Genetic and environmental influences on the adult epigenome. Epigenomics, 9(1), 5-12.
Cognition: Education, Intelligence, Memory, Attention , Personality, Temperament, Attitudes, Politics and Religion , other
