The objective of this thesis was, by using two model strains of chicken, red junglefowl and White Leghorn layer, to study differences in social behaviour of the species that could be derived from domestication and production related selection processes. The social phenotype characterisation was done to be able to further study the association of the social differences to specific production related traits as well as to certain genome areas in intercross and backcross lines generated from these two parental populations.The first part of the thesis covers phenotypic differences between the two parental stocks in social motivation (sociality), social discrimination ability and preferences as well as the dynamics of aggression and hierarchy establishment under unstable social circumstances. These features are crucial for the coping of fowl in its social environment. The underlying social motivation and related characteristics were measured first in chicks, while the fourth study was designed to investigate if the social motivation differences across the breeds would persist into adulthood and if so, would correlate with various production related traits, as growth and egg production, in the F₃ progeny. The last paper analysed the effect of genotype along a domestication linked Growth1 QTL on some of the social behaviour differences. The behaviour tests revealed a consistent breed specific behavioural difference, which was linked to the novelty and stressfulness of each test situation. The junglefowl chicks behaved in a manner that is considered to be adaptive in the wild; they for instance explored and foraged more in the novel environment. Conversely, the Leghoms appeared more fearful and showed a higher level of attachment to familiar companions in the tests. These breed specific behaviour patterns persisted over age and correlated also with several production related traits in the F₃ progeny. Even though selection for egg production has affected the social behaviour only on a quantitative level (frequencies and intensities) the dynamics of aggression and hierarchy establishment differed remarkably between the breeds when studied in unstable groups; Leghoms responded to regrouping with greater aggression and had more difficulties to adapt to strangers. The genetic study in the last paper revealed a link between social motivation and exploratory behaviour and two adjacent marker areas within the Growth1 QTL, indicating at least two involved genes in this area. The conclusion of this thesis is that social motivation, which is considered to underlie most social behaviour traits in fowl, has been influenced by selection for high production in the studied White Leghom stock in comparison with junglefowl. Furthermore, the found breed differences seem to segregate together with some production related traits in an advanced intercross generation, and to be associated with a QTL-region, which has been suggested to play an important role in the domestication of chickens.