The nine type Enneagram of personality that Oscar Ichazo taught and Claudio Naranjo brought to the United States arrived without the layer that most defines how the system is taught today: a scale of psychological health running within each type from an integrated, healthy expression down through an average, more defended one to a genuinely unhealthy, pathological extreme. Don Richard Riso, working first alone and then with Russ Hudson, built that scale, the levels of development, into the core of the framework in Personality Types: Using the Enneagram for Self Discovery, first published in 1987 and substantially revised with Hudson as full co-author in 1996. Where the earlier oral teaching described each type by a fixed set of traits, Riso and Hudson's levels let the same basic type describe a wide range of actual behavior, from admirable to destructive, depending on how defended or open a person is at a given time.
They added a second classificatory axis alongside the traditional body, heart and head triads: the Hornevian groups, drawn from psychoanalyst Karen Horney's three neurotic trends toward, against and away from other people, and set out fully in their 1999 book The Wisdom of the Enneagram. Compliant types orient toward an internalized standard; assertive types orient toward self expression and control; withdrawn types orient toward inner retreat. The two groupings, triad and Hornevian group, describe different things and do not coincide, giving a reader two independent lenses on the same nine types rather than one.