I've been working my way through Pat Wyman's Three Keys to Self Understanding which I've found fairly thought provoking. Wyman works as a therapist doing affective therapy at a practice where they MBTI type and eneatype people as part of her practice's (of eight affective therapists) initial intake process. Over the years, she noted that during intake, people mostly were functioning out of their enneagram type, but after successful therapy they seemed to function more out of their MBTI type.
Wyman covers three main areas: MBTI, enneagram and "inner child healing." For the sake of this post, I'm going to ignore inner child healing (the name alone induces major eye-rolling... still, I can't comment on how effective approach it is therapeutically).
My main complaint about the book is that it doesn't go deep enough spelling out enneagram and MBTI interactions. The focus of the book is really breaking down forced use of enneagram defenses by doing inner child work.
Enneagram Effects on the Dichotomies
Wyman holds that the dichotomies are just that: dichotomous. She claims the reason people test "in the middle" on an MBTI dichotomy is most often a result of a clash between their MBTI type ("Core Self") and their enneatype ("Defense System"). That means that in her view one's enneagram doesn't change your MBTI type, but instead how you test and behave—especially when under stress. Note that every type doesn't affect every dichotomy (some are fairly neutral). She gives some examples:
Extraversion vs Introversion: A Three's need to perform and seek attention pushes one in the E direction. A Four's or Five's internal nature pushes one in the I direction.
Sensing vs Intuition: A One's need for detail pushes one in the S direction. Conversely, a Four's future orientation and quest for uniqueness pushes one towards intuition.
Thinking vs Feeling: A Three's result-oriented nature or an Eight's harsheness pushes one in the Thinking direction, A Four's emotional focus pushes one in the Feeling direction.
Judging vs Perceiving: A One's "do it perfectly" or a Three's "take no risks, focus on results" pushes one in the J direction, while a Seven's need to experience every possibility pushes one in the P direction.
My Guesses on how Enneagram Type Affects Functions (and I'd love additional feedback/correction here):
Type 1: Te (judging everything according to The Rules™) & Si enhancing (noticing every detail short of perfection). Fe to a lesser extent. (very J in general)
Type 2: Very unhealthy Fe enhancing. SJ in general (especially ISxJ, serving as power behind the throne)
Type 3: Somewhat Te enhancing (efficiency, results) somewhat Fe (very socially appearance conscious), somewhat Ti, pushes towards Extraversion
Type 4: Fi (plumbing one's own emotions) and Ne (& Ni somewhat) enhancing (seeking the unattainable and loving symbols).
Type 5: Ti enhancing, perhaps Si, pushes strongly towards introversion
Type 6: Enhances Ni (search for hidden agendas, motivations), enhances Fe (loyalty to group) & a Te a bit (respect for existing systems), Si (respect for traditions, obsessively scanning for signs of danger)
Type 7: Enhances Se and/or Ne (love of novelty, hatred of restrictions). Enhances Fi (charming, narcissistic). Very E in general.
Type 8: Enhances Ti (iconoclastic, dislike or authority), More E in general (audacious, attention seeking). T in general (conflict-y)
Type 9: Feeling enhancing (maybe more Fe, but definitely both Fe and Fi), very I in general (not making waves, escaping internally), very P in general (avoiding expressing choices)
Wyman also talks about how an extraverted Three (for example) will test as extremely extraverted on the MBTI. Or an judging One would test as extremely judging. Personally, this explanation helps explains why my ESTJ-1 father seems like such an ESTJ caricature (even at his age).
Wyman farther claims that people who are pulled in opposite directions by MBTI and Enneagram suffer increased distress; those pulled in opposite directions tend to be exhausted by being forced to use their non-preferred functions. If sufficiently unhealthy, they may entirely lose touch with who they really are. However, the distressed incurred by those with opposed MBTI/enneagram types may force them to seek help sooner than those pulled in the same direction by their MBTI and enneagram types.
I personally find her break-down of MBTI and enneagram type fairly convincing. It fits reasonably well with INFP variations I see here and on PersC. I'd already found the most useful way for me to get value out of the enneagram was thinking of it like a long-term version of Freudian defense mechanisms. Wyman makes a similar claim and uses the enneagram as a starting point for outlining the unhealthy defense systems of her clients. Her model is fairly fluid, though, and includes the possibility of building defenses out of qualities of one's MBTI type. Those looking for empirical evidence beyond her claiming "this is what I've seen" will be disappointed.