Studying animal domestication offers an opportunity to understand the mechanisms of evolution. Domestication is associated with a change in selection pressures; selection for production traits is introduced, and animals are faced with larger and denser social groups. It is not unexpected then that domestication produces a simultaneous change in a number of traits, both physiological and behavioural. This correlated change in traits, e.g. egg production and social behaviour has been termed the “domestic phenotype”. However, it has been shown that selection for one trait alone among the many associated with the domestic phenotype can lead to simultaneous changes in others. This may be a result of such traits being inherited together because of pleiotropy or close linkage of several genes affecting different traits. A chicken growth QTL has previously been found in an intercross between White Leghorn layers (WL) and their main wild ancestor, the red junglefowl (RJF). This QTL has also been found to influence explorative and social behaviours. This thesis aims to characterize this QTL further with respect to social and emotional behaviours, and tries to clarify whether pleiotropy or linkage is responsible for the many observed effects. This is done using behavioural phenotyping, genetic marker genotyping, QTL- and gene expression analysis of an intercross line between RJF and WL, and to some extent of the parental RJF and WL lines themselves. The results show that domestication in these chickens has led to increased social tolerance to unfamiliar conspecifics and a tendency to a decrease in the propensity of chickens to explore the environment, and that these effects are partly explained by the previously described growth QTL. The results also indicate that close linkage of genes, rather than pleiotropy, may be responsible for the multiple effect of the QTL, as different traits to some extent seem to be influenced by different areas within the larger QTL region. This information, in combination with that of other studies and with existing and upcoming genetic research techniques, may be used in the design of future breeding programs that take animal behaviour and welfare as well as production traits into account. Findings like these may also be of use in directing research in human psychiatric genetics.