The lost study: a 1998 adoption study of personality that found no genetic relationship between birthparents and their 240 adopted-away biological offspring

Abstract

In 1998, Robert Plomin and his Colorado Adoption Project (CAP) colleagues published the results of a longitudinal adoption study of personality. They found an average personality test score correlation of only 0.01 between birthparents and their 240 adopted-away 16-year-old biological offspring, suggesting no genetic influences on personality. However, the researchers interpreted their results in the context of previous twin studies, produced an average 14% heritability estimate, and concluded that nonadditive genetic factors underlie personality traits. The author challenges these conclusions and notes that the near-zero correlation stands in contrast to other types of behavioral genetic methods, such as twin studies, that are more vulnerable to environmental confounds and other biases. The author shows that authoritative psychology texts frequently fail to mention this 1998 CAP study. When it is mentioned, the original researchers' conclusions are usually accepted without critical analysis. The author also assesses the results in the context of the 20-year failure to discover the genes that behavioral geneticists believe underlie personality traits. He concludes that this 1998 investigation is a "lost study" in the sense that, although it is one of the most methodologically sound behavioral genetic studies ever performed, its results are largely unknown.