I worked at the Arnhem Zoo from 1975 through 1981. This was facilitated by the zoo director, Anton van Hooff, being my advisor's brother. Together they had established a colony of around 25 chimpanzees on a one hectare (two-acre) island - still the world's largest such colony today.

Every morning, rain or shine, I would cycle to the zoo to spend hours watching chimps. I did little else. Working at my desk, I would keep an ear to the chimps. At the slightest noise, I had my binoculars in hand to follow the spectacle. Over this six-year period, I supervised more than twenty students, who helped out with observations and the collection of thousands of data points.

Most of the ideas central to Our Inner Ape were formed during this time, such as those relating to power, sex, conflict resolution, empathy, cooperation, and reciprocity. I was directly inspired by the chimps. Perhaps because of my closeness to the apes, I developed an aversion to the simplifications of theoretical biologists who liked to depict us, and by extension all other primates, as nasty and selfish. I saw a much more complex picture, including a variety of genuinely positive tendencies.

In 1977, I received my Ph. D. in biology at the University of Utrecht.