To be eligible for the current study, children needed to be aged three years or over at T1 and still in day care (i.e., not have started school) one year later, at T2. Of the 290 children who met these criteria, 209 children met the additional inclusion criterion of having complete teacher data at both T1 and T2. Six children were later excluded from the analyses on the basis of their friendship nominations (see section on measurement of best friends’ behavior below), bringing the final sample size to N = 203 (50% female).