Domestication has led to profound changes in animal behaviour, physiology, and brain morphology. This phenomenon is known as the domestication syndrome. One of the main early drivers for these phenotypic changes is hypothesised to be selection for tameness, i.e. reduced fear of humans and that this trait is heritable. Previous studies on domestication in animals have shown that selection for tameness not only changes inter-specific behaviour, as shown towards humans, but also intra-specific behaviour. This thesis examines how selection for tameness in Red Junglefowl (Gallus gallus) influences social interactions and cerebellar traits, shedding light on the mechanism underlying domestication-related changes. Paper I, II, and IV, focus on how social behaviour in chickens has been shaped by domestication, with domesticated breeds often displaying different social strategies compared to their wild ancestor. To investigate whether selection for tameness alone can drive these changes (paper I), we studied Red Junglefowl lines selected for either high (HF) or low (LF) fear of humans over multiple generations. In a semi-natural setting, we separated HF from LF and made groups consisting of two males and two females. LF birds exhibited more social conflicts and increased male crowing, while HF birds spent more time in proximity and engaged in more non-aggressive social pecking. These differences reflects behavioural distinctions found between wild Red junglefowl and domesticated laying hens, supporting the hypothesis that domestication-related social changes can arise as a side effect of tameness. In addition to changes in conspecific interactions, domesticated chickens exhibit reduced fearfulness and stress responses compared to their wild counterparts (paper II). Behavioural testing in a risk-taking scenario of domesticated White Leghorns (Gallus gallus domesticus) and parental Red junglefowl chicks confirmed that junglefowl were more fearful but also more exploratory. Importantly, social partners helped buffer fear responses in both breeds, with a stronger effect in Red Junglefowl. This suggests that domestication has not only reduced overall fear but also modified the role of social support in managing stress. To investigate the effect of a social buffer further, we gave adult chickens the choice of a known vs unknown social buffer (paper III). Four gradient groups of domestication were tested: White leghorn (WL), the parentals of unselected Red Junglefowl (RJF), low-fear Red Junglefowl (LF), high-fear Red Junglefowl (HF). The birds were tested one by one, in an arena, where they could freely investigate the area, but on each side, there were a couple of same sex birds, one with known birds, and one with unknown birds. We found that LF birds engaged in more non-aggressive social exploration and emitted more food calls; behaviours more commonly associated with domesticated chickens. While unselected RJF males showed increased interest in unfamiliar birds and displayed more agonistic behaviours, whereas this pattern was not mirrored in the HF and LF lines, suggesting that selection for tameness and fearfulness alters social coping. Notably, LF males showed similarities to domesticated White Leghorns, further supporting the idea that tameness selection contributes to shifts in social behaviour. Surprisingly, the WL females showed more agonistic behaviour, mostly towards strange birds, which could be explained by increased motivation to feed and consequently resource defence. The cerebellum, a key brain region in motor coordination and complex cognitive functions, has by different degrees, been shown to be affected by domestication in several species. While domesticated chickens have smaller relative brain size than their wild ancestors, their cerebellum has been relatively enlarged. To explore whether this pattern could be attributed to tameness selection, we examined brain morphology in HF and LF Red Junglefowl (paper III). LF birds exhibited reduced relative brain mass, particularly in the non-cerebellar brain regions, aligning with patterns observed in dully domesticated breeds. HF birds had higher neuronal density in the cerebellum, suggesting that while overall cerebellar size is increased in domesticated-like birds, it does not affect the number of neurons in the cerebellum. Hence, the reason for conserving the cerebellum size, is not more neurons. Thus, to map out the reason for the increased relative cerebellum size, different cerebellar behavioural tests assessing motor coordination, balance and social navigation, were conducted (paper V). There was no significant difference in coordination or balance between HF and LF chicks, but HF birds completed an obstacle course more quickly, possible due to increased social motivation. LF birds exhibited more reorientation pauses, which may reflect higher exploratory tendencies rather than deficits in movement control. Although the HF birds had shorter latency in an obstacle course, there was no significant difference in a rotarod test, which indicates that HF does not necessarily have superior motor skills.  In a Y-maze test assessing social preferences, HF birds showed preference for a non-aggressive rooster, indicating a possible link between fearfulness and risk-taking behaviour and social orientation, whereas this was not observed in the LF birds. The findings suggest that selection for tameness alone can drive significant changes in social behaviour, mirroring domestication effects seen in the modern domesticated chickens. These changes include increased social conflict, altered social exploration patterns, and a shift in social attraction. While LF birds have relatively larger cerebellum, this does not necessarily enhance coordination but may influence social processing, highlighting the complex relationship between tameness, behaviour, and brain morphology.