Baltes was active in various national and international organizations including the US Social Science Research Council (where he served as chair of the Board of Directors from 1996 until 2000), the German-American Academy Council, the Berlin-Brandenburg Academy of Sciences, and the European Academy of Science. Regarding interdisciplinarity, Baltes was engaged primarily in two projects: he chaired (together with Karl Ulrich Mayer) the Berlin Aging Study and, together with the Sociologist Neil Smelser, he was co-editor-in-chief of the 26-volume International Encyclopedia of the Social and Behavioral Sciences (Elsevier) which appeared in 2001.