Nowadays, we increasing value the broad physical, ethnic, racial, and cultural diversity of human beings. “How wonderful that humans come in all sorts of sizes, shapes, colors, ethnic groups and cultures.” So long as we conduct our behaviour within sanctioned norms. This presentation will focus upon the above paradox: In stark contrast to our delight in the physical, ethnic and cultural expressions of human diversity, there is, at the same time, a perhaps increasingly narrow tolerance for a variety of behavioural and experiential human differences. In such human realms, present-day cosmopolitan societies increasingly call for behavioural and experiential conformity rather than diversity. And if we cannot conform? We propose that the phenomenon of mental illness arises as a consequence of the phenomenon of human diversity coming up against constraints and limitations in expressed and experienced mental and behavioural realms. This presentation will focus upon the primary role that human diversity plays in mental illness. We will discuss adaptive strengths associated with the extraordinary diversity of humans (and our pets and domestic animals) as well as vulnerabilities accompanying this diversity. For example, diversity associated with skin pigmentation has enabled humans to extend across the globe. A consequence, however, is an enhanced vulnerability to skin cancer for some with fair skin and to Vitamin D deficiency for others with dark skin. Psychological diversities can be viewed in an analogous, pervasively more problematic manner. And furthermore, unlike physical diversities, often increasingly celebrated, mental and psychological diversity are – with notable exceptions, increasingly problematic.