Alongside type and wing, Riso and Hudson's framework, developed jointly and set out fully in their 1999 book The Wisdom of the Enneagram, adds a third independent layer: the instinctual variant, one of three basic survival drives, self preservation, social or sexual, that each person favors as their dominant instinct regardless of which of the nine Enneagram types they are. A self preservation dominant person of any type orients toward physical security, comfort and resources; a social dominant person orients toward group belonging, status and community; a sexual, or one to one, dominant person orients toward intense individual bonding and chemistry with particular others. Combined with the nine types and the levels of development, the instinctual variants produce twenty seven distinct subtype combinations, each type expressed through each of the three instinctual stackings.
The addition answers a gap the type and wing system alone left open: two people of the identical type and wing can look strikingly different in daily life, and Hudson and Riso's account attributes much of that difference to which instinct is dominant rather than to the type itself varying. A self preservation Eight, for instance, channels the type's characteristic assertiveness toward securing material and physical resources, where a social Eight channels the same underlying drive toward leadership and group loyalty, two visibly different personalities built on the same core type structure.