The next section describes the methods in use for studying behavior genetics. There are numerous approaches used in identification of the loci that contribute to the overall combined genetic and environmental influences for behavioral disorders. Quantitative methods, such as twin and adoption methods for humans and inbred strain and selection methods in animals, estimate genetic and environmental contributions to phenotypic variance in a population. The most common research methodologies used in human behavior research are twin studies, family studies, and adoption studies. Environmental influences can be separated into two types, shared and non-shared environments.
Shared environment is the environment shared by sibling reared together in the same family.
This includes such variables as socioeconomic status and education level. Non-shared environment is that environment unique to the individual, including variables such as peer group characteristics. Resemblance among family members is a function of both genes and a shared environment. The utilization of twin and adoption studies can indicate the extent to which family resemblance is due to shared genes and the extent to which it is due to shared environments. Therefore, to the extent that genes are influential, identical twins should be more alike than DZ twins.