In earlier work, the author has presented an economic case for complementarity as the dominant form of social relations. This paper considers the psychological impact of social systems based on substitution (conflicts of interest) vs. complementarity (concerts of interest). Here, reference is made to a psychological literature on negative vs. positive emotions and their health effects, along with neurological research showing humans hard-wired for empathy. The paper extends a horizonal case for complementarity into its psychological links on cognitive function and mental health. Substitution and complementarity yield opposite findings for competition and cooperation. The psychological and neurological literature reinforces a case for complementarity and cooperation, showing competition serves us ill and should be rejected for cooperation as a surer means to a mentally healthy society.