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Human behaviour genetics is a subfield of the field of behaviour genetics that studies the role of genetics in human behaviour. It looks at genes that code for behavioural traits and their physiological processes as well as how those genes interact with the environment. The field is an overlap of genetics and psychology. Classically, human behavioural geneticists have studied the inheritance of behavioural traits.



History
In 1869, Francis Galton published the first empirical work in human behavioural genetics, Hereditary Genius. Here, Galton intended to demonstrate that "a man's natural abilities are derived by inheritance, under exactly the same limitations as are the form and physical features of the whole organic world." Like most seminal work, he overstated his conclusions. His was a family study on the inheritance of giftedness and talent. Galton was aware that resemblance among familial relatives can be a function of both shared inheritance and shared environments. Contemporary human behavioural quantitative genetics studies special populations such as twins and adoptees.

The initial impetus behind this research was to demonstrate that there were indeed genetic influences on human behaviour. In psychology, this phase lasted for the first half of the 20th century largely because of the overwhelming influence of behaviourism in the field. Later behavioural genetic research focused on quantitative methods.

Contemporary behavioural quantitative genetics
Currently, the largest branch of human behavioural genetics is psychiatric genetics which studies phenotypes such as schizophrenia, bipolar disorder, and alcoholism.

Recent trends in behaviour genetics have indicated an additional focus toward researching the inheritance of human characteristics typically studied in developmental psychology. For instance, a major focus in developmental psychology has been to characterize the influence of parenting styles on children. However, in most studies, genes are a confounding variable. Because children share half of their alleles with each parent, any observed effects of parenting styles could be effects of having many of the same alleles as a parent (e.g. harsh aggressive parenting styles have been found to correlate with similar aggressive child characteristics: is it the parenting or the genes?). Thus, behaviour genetics research is currently undertaking to distinguish the effects of the family environment from the effects of genes. This branch of behaviour genetics research is becoming more closely associated with mainstream developmental psychology and the sub-field of developmental psychopathology as it shifts its focus to the heritability of such factors as emotional self-control, attachment, social functioning, aggressiveness, etc.

Several academic bodies exist to support behaviour genetic research, including the International Behavioural and Neural Genetics Society, Behavior Genetics Association, the International Society for Psychiatric Genetics, and the International Society for Twin Studies. Behaviour genetic work features prominently in several more general societies, for instance the International Behavioral Neuroscience Society.

Methods of human behavioural genetics
Human behavioural geneticists use several designs to answer questions about the nature and mechanisms of genetic influences on behaviour. All of these designs are unified by being based around human relationships which disentangle genetic and environmental relatedness.

So, for instance, some researchers study adopted twins: the adoption study. 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 identical twins 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.

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 criticized they continue to be of high utility. There are several dozen major studies ongoing, in countries as diverse as the USA, UK, Germany, France, the Netherlands, and Australia, and the method is used widely in fields as diverse as dental caries, BMI, ageing, substance abuse, sexuality, cognitive abilities, personality, values, and a wide range of psychiatric disorders. This is broad utility is reflected in several thousands of peer-review papers, and several dedicated societies and journals (See Twin study).