Domestication is an evolutionary process in which animals become adapted to a life in close proximity to humans. There are typically specific selection pressures associated with this, including living in larger social groups than their wild ancestors. We hypothesized that the genotype at a major growth QTL could affect aspects of social behavior in chickens as well. We performed social behavior tests in red junglefowl (RJF) and White Leghorn (WL) chickens and in chickens from a selected advanced intercross line (SAIL) between RJF and WL, selected for different genotypes at a microsatellite marker locus within the QTL region. Four-week-old pure WL inspected strangers significantly more than pure RJF. Male 4-wk-old SAIL birds, homozygous for the WL allele at the marker locus, differed from those with RJF alleles in a similar way as the pure WL differed from RJF. Furthermore, 155- to 170-d-old male SAIL birds homozygous for the WL allele at the marker locus were less aggressive to unfamiliar conspecifics in a dominance test. The results suggest that domestication has caused changes in social behavior, which, in males, may partly depend on variations in the genotype at the growth QTL where the avian homolog of the arginine vasopressin receptor 1a (AVPR1a) is located. This gene is therefore one of several putative candidate genes for future research.