Although the three naturalistic experimental designs described in this article represent novel approaches to building on the rigor of quantitative genetic methods from the life sciences, they are not without limitations. For example, if selective placement or matching of adoptive and birth parents is present, or if adoptees have ongoing contact with their birth parents, the design assumptions in the parent-offspring adoption design are violated and associations between birth parents and adoptees might not only include genetic and prenatal influences, but may also include postnatal environmental influences. In addition, it can be difficult to determine the best proxy of an adult trait in a young child. For example, when examining children’s heritable risk for antisocial behavior by studying criminal behavior in the birth parents, what is the best measure of inherited risk for antisocial behavior in a 4-year old child? The science in the selection of these variables is far from precise at this time. Similarly, in the siblings reared apart cross-fostering design, it is important to consider potential “third” variables. The many environmental differences between adoptees and their biological
