define social development.
The UC Berkeley-Stanford study found that all children who attended preschool at least 15 hours a week displayed more negative social behaviors such as trouble cooperating or acting up, when compared with their peers. The discrepancies were most pronounced among children from higher-income families.
The kids are getting more educationally, but they are also under more pressure - so they compensate by acting out in that typical toddler way. (my 2 cents)