What does the research say about the development of bullying in early childhood?

"Bullying can pose a serious threat to children’s immediate and long-term health and well-being, and can have profound impacts on all children involved in bullying behaviors, whether as the one bullying others, the one being bullied, or the one witnessing bullying. At least some of the roots of bullying behaviors, and conversely the roots of positive pro-social skills, can likely be found in adverse and positive experiences from early childhood, yet the research literature on these connections is limited. The early childhood field lacks a coherent, theoretical model that identifies the factors contributing to aggressive or “mean” behavior in young children, and establishes the developmental link between this early behavior and later bullying behavior."