In a study of twins reared apart in the 1980s and 1990s, a sample of identical twins who were raised in separated environments produced scores on three measures of intelligence that were as similar to each other as a sample of twins who were raised together. though the sample sizes were relatively small, this research has been repeated with approximately the same findings. we can have some confidence that these findings are valid because of the critical thinking principle of falsifiability replicability extraordinary claims occam’s razor