Scientific disciplines are uplifted and expanded by unifying themes and theories that can link disparate constructs, research results, scientists, and practitioners. A good example is self-determination theory, a unifying theory and sub-theories for human motivation and personality. Another example is the Big Five personality assessment, which invigorated personality psychology. On the other hand, observers have criticized the field of positive psychology for lacking a unifying meta-theory that brings together the numerous constructs, study results, and scientists.