Introduction

This chapter describes the claim that the classical studies were truly studies of reared-apart twins. Newman and colleagues performed the first systematic twins reared-apart (TRA) study, but the claim that the study produced valid evidence in favour of genetics is highly questionable. Juel-Nielsen both assumed and concluded that MZA behavioral similarity is caused by twins' identical genotypes, a circular argument was repeated by the Minnesota TRA researchers. By far the most well-known and influential TRA study is the Minnesota Study of Twins Reared Apart (MISTRA), which was carried out by Thomas Bouchard, Jr. and colleagues between 1979 and 2000, with many additional publications based on the MISTRA data appearing since 2000. Researchers and journalists claiming that MZA pairs have many similarities usually highlight the reported similarities, and downplay or ignore the differences. The Finnish TRA researchers were unsure about the generalizability of their findings to the rest of the population.