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Reared-apart (separated) identical twins have great appeal. Though exceedingly rare, many people see them as providing the ultimate Rosetta Stone-type method of teasing apart the potential roles of nature and nurture as causes of human behavioral variation. They share a genetic identity while—in theory—growing up and living under entirely different environmental conditions. The two main ways they have come to our attention have been anecdotal stories of individual twin pairs said to share “spooky” and “eerie” behavioral similarities, and a small handful of scientific studies based on a sample of twins. Here, I focus on the latter.
The “Minnesota Study of Twins Reared Apart” (MISTRA) is one of the most famous and widely cited studies in the behavioral sciences. The study began in 1979 and ended in 2000. Many academic publications based on the MISTRA data have appeared since 2000. The MISTRA IQ study was key, due to cognitive ability's long-standing central role in the “nature-nurture debate” and the fields of psychology and behavioral genetics. The study was published in a 1990 edition of Science, one of the world's top scientific journals. Although the researchers assessed other psychological characteristics in that article, I refer to it here as the “IQ study” because this was the main MISTRA IQ publication.
The 1990 MISTRA sample consisted of 56 reared-apart MZ twin pairs (monozygotic, identical; 100% genetic similarity) and 30 reared-apart DZ twin pairs (dizygotic, fraternal; average 50% genetic similarity). The three MISTRA cognitive ability (IQ) measures were the “Wechsler Adult Intelligence Scale” (WAIS), the “Raven's Progressive Matrices/Mill-Hill Vocabulary Scale composite,” and the “First Principal Component of Special Mental Abilities” (FPC). Reared-apart MZ twins are known as “MZA” pairs; reared-apart DZ twins are known as “DZA” pairs.
Study initiator psychologist Thomas J. Bouchard, Jr., along with David Lykken, Matthew McGue, Nancy Segal, and Auke Tellegen