Developmental psychologists have a long history of using triangle metaphors to
conceptualise the social constitution of psychological development. In this paper, we
present a genealogy of triadic theories, to clarify their origins, distinctions between
them, and to identify key themes for theoretical development. The analysis identifies
three core triangle models in the developmental literature. Each theory relies on some
combination of the terms subject, object, other and sign, and they can be distinguished
by the core psychological dynamic which they entail. We distinguish an emotional triangle
rooted in psycho-analysis, a mediational triangle rooted in the work of Vygotsky,
and a sociocognitive triangle originating with Piaget. Despite their differences, the analysis
reveals a common theme of the transformation from external mediation to internal
mediation. Contemporary research and possible future directions are discussed in the
light of the theoretical distinctions that our genealogy has revealed