[2] From 2000 to 2004, he worked as a postdoctoral researcher (with Siegwart Lindenberg and Hans Ormel) and as a data manager at TRAILS.

He is a member of the steering committee of the 22 million GUTS[4] (Growing Up Together in Society) research program, funded by the Ministry of Education, Culture, and Science.

[10] He edited a special issue of the Journal of Research on Adolescence on selection and influence processes in adolescent research[11] and a special issue of the International Journal of Behavioral Development on social norms and behavioral development.

[13][14] His group also collected high quality datasets, such as TRAILS, SNARE,[15] KiVa NL,[16] and PRIMS.

[23] Longitudinal social network studies using SNARE focused on how friends select and influence each other in terms of academic achievement,[24] aggression,[25][26] and helping.

Longitudinal social network studies using KiVa NL have focused on the co-development of bullying and victimization on the one hand, and antipathies,[32] defending,[33] friendships,[34] and popularity on the other.

He delivered keynote addresses at the 2nd World Anti-Bullying Forum and the 27th meeting of the International Society for the Study of Behavioral Development.