The full parent-offspring adoption design in human populations shares a unique design feature with the “common garden” approach used in life sciences research: the environment is hypothesized to have an influence on individual units reared together in that environment, and its effects can be separated from genetic influences by restricting the variance in either the environment or in the genetic background of the individuals in that environment (see Figure 1). Whereas in plant research this refers to, for example, different seed variants grown in specific lighting, temperature, and/or water conditions that are hypothesized to affect plant health, in the parent-offspring adoption design, this refers to adopted children reared in a family environment in which they are genetically-unrelated to the rearing parents. Adopted children who are placed with nonrelatives at birth are genetically unrelated to their rearing parents, yet share the family environment and the larger social context with their rearing parents. In the absence of systematic biases in the selection of adoptive families by birth parents or vice versa, similarities between an adopted child and his or her adoptive parents
