Editing Human behaviour genetics (section)

===Twin and adoption studies===
The basic understanding of behavioural genetics requires the separate study of effects of genes and environment influence on human behaviour. Such as, the genetic effects in a trait are discernible if pair of genetically identical (monozygotic twins) are much similar to one another than pair of genetically non-identical (dizygotic twin).

Twin and adoption studies describe the extent to which family resemblance is due to shared genes and the extent to which it is due to shared environments. Behavioral Scientist uses twin studies to examine hereditary and environmental influences on behavioural development.

For instance, some researchers also study adopted twins: the [[adoption study]]. The adoption design produces estimates of various genetic and environmental components of variance, similar to the twin design. Furthermore, the adoption design facilitates

(1) the identification of specific environmental influences that are unaffected by heredity (e.g., the effects of life stressors),

(2) the analysis of heredity's role in ostensibly environmental relationships, and

(3) the evaluation of genotype-environment interactions and correlations.<ref name=":0" />

In this case the adoption disentangles the genetic relatedness of the twins (either 50% or 100%) from their family environments. Likewise the classic [[twin study]] contrasts the differences between [[Twin|identical]] and [[fraternal twins]] within a family compared to differences observed between families. This core design can be extended: the so-called "extended twin study" which adds additional family members, increasing power and allowing new genetic and environmental relationships to be studied. Excellent examples of this model are the Virginia 20,000 and the QIMR twin studies.

Generally, if the observed behaviour and cognitive traits have a genetic component, then genetically similar relatives resemble to each other as comparative to individuals who share lesser component of genome. In case of environmental influence, researchers study the two broad classes of effects in behavioural genetics such as shared environmental factors causing them to behave similarly and the other one is nonshared environmental factors causing them to behave different from one another. For example, siblings raised together in same environment will have more evident shared environment influences whereas in relative siblings raised apart from each other will have non-shared environmental influence. The understanding of the effects of genes and the influence of shared and nonshared environment on human behaviour provides a comprehensive data for genetic and environmental relatedness.<ref name=":0" />

Also possible are the "children of twins" design (holding maternal genetic contributions equal across children with paternal genetics and family environments) and the "virtual twins" design - unrelated children adopted into a family who are very close or identical in age to biological children or other adopted children in the family. While the classical twin study has been [[Twin study#Criticism|criticized]] they continue to be of high utility. There are several dozen major studies ongoing in countries as diverse as the US, UK, Germany, France, the [[Netherlands]], and Australia, and the method is used widely on phenotypes as diverse as [[dental caries]], [[body mass index]], [[ageing]], [[substance abuse]], [[human sexuality|sexuality]], [[intelligence|cognitive abilities]], [[Personality type|personality]], [[values]], and a wide range of [[psychiatric disorder]]s. This is broad utility is reflected in several thousands of peer-review papers, and several dedicated societies and journals.