  Homophily, the tendency of individuals to associate with others who share
similar traits, has been identified as a major driving force in the formation
and evolution of social ties. In many cases, it is not clear if homophily is
the result of a socialization process, where individuals change their traits
according to the dominance of that trait in their local social networks, or if
it results from a selection process, in which individuals reshape their social
networks so that their traits match those in the new environment. Here we
demonstrate the detailed temporal formation of strong homophily in academic
achievements of high school and university students. We analyze a unique
dataset that contains information about the detailed time evolution of a
friendship network of 6,000 students across 42 months. Combining the evolving
social network data with the time series of the academic performance (GPA) of
individual students, we show that academic homophily is a result of selection:
students prefer to gradually reorganize their social networks according to
their performance levels, rather than adapting their performance to the level
of their local group. We find no signs for a pull effect, where a social
environment of good performers motivates bad students to improve their
performance. We are able to understand the underlying dynamics of grades and
networks with a simple model. The lack of a social pull effect in classical
educational settings could have important implications for the understanding of
the observed persistence of segregation, inequality and social immobility in
societies.
