Although they provide useful information, they do not address the process of development. It is difficult to generalize the twin pair study results to the general population. Some believe that heredity and environment are intricately entwined and cannot be divided into separate influences. A range of reaction is a person's unique, genetically determined response to a range of environmental conditions. This accounts for how people respond in different ways to the same environment.
Canalization is the genetic restriction of development to one or a few potential outcomes.
Highly canalized traits require extreme environmental conditions to deter their genetically set outcomes. The concept of genetic-environmental correlation states that our genes influence the environments to which we are exposed. In a passive genetic-environmental correlation, a child has no control over the environment available to him or her. Parents create an environment compatible with their own heredity. In an evocative genetic-environmental correlation, a child behaves in ways consistent with his or her own heredity.