After one of my first lectures at an international conference, Robert Goy, director of the Wisconsin Primate Center, invited me to come work with him. Initially, I went on my own to Wisconsin, since it was supposedly for one year only. This was in 1981. Within weeks, however, Bob talked me into staying. Later Catherine, by then my wife, joined me.

Before moving to the US, I had thought long and hard about which research direction to take, and decided on reconciliation and social reciprocity as promising topics. The first, I had discovered myself in Arnhem. At the time no one worked on animal conflict resolution - nowadays a major research area. It led to my second book, and the first one written directly in English: Peacemaking Among Primates (1989). The book earned the 1989 Los Angeles Times Book Award as well as an unflattering cartoon in the newspaper. Later, Filippo Aureli and I co-edited a comprehensive volume on the same topic, Natural Conflict Resolution (2000).

With my assistant, Lesleigh Luttrell, I studied macaques at the Vilas Park Zoo, in Madison, Wisconsin. We did so for ten years, and though I greatly enjoyed it, something was missing. I set up two ape studies, for which I had to go south. One was a study of bonobos at the San Diego Zoo. Obviously, Our Inner Ape could never have been written without this total immersion into bonobo behavior, which produced so much data and so many photographs that I still regularly draw on this material.

More than a decade after my San Diego stay, celebrated photographer, Frans Lanting, and I decided to produce a book on bonobos using Frans' unique pictures from the wild. But we first had to deal with the prudishness of American editors - rather baffling to two Dutchmen. Bonobos were seen as too hot! So, we first published in the German GEO Magazin, which put a copulating pair on its cover with the title Frieden durch Sex ("Peace through Sex"). Then we approached Scientific American, which took our story as well. Only after both trial balloons failed to cause public outcry, did we bring out Bonobo: The Forgotten Ape (1997).

My second "expedition" from Wisconsin was to test reciprocity in the food sharing among chimpanzees at the Yerkes Primate Center in Atlanta. Upon my return to Wisconsin, I noticed how much I had missed working with apes. In 1991, I joined the faculty of Emory University so that I could permanently work with the apes at the Yerkes' Field Station.