Three themes have dominated the sociology of education: the sub-cultural thesis, the socio-cultural advantage thesis, and the biogenetic thesis. With regards to the last, one should try to account for differences in the behavior of identifiable social groups by looking for social causes. The relationship between the strength of the bonds attaching youths to social groups and the development of academic competence was investigated using data selected from a comprehensive study of a sample of 4077 drawn from the 17,000 secondary school students in western Contra Costa County in the spring of 1965. The social groups taken as salient in the environment of youths are the family, the peer group, the school, and the student's perception of the society as a whole. The Hemnon-Nelson group IQ test was the criterion measure. The large disparities of some 15 IQ points between the off-spring of professional and of lower-class parents, and between blacks and whites are entirely interpreted by the set of intervening environmental variables. (JM)