Evolution of typologies and thought systems in general
It seems that brain chemistry is the current typological fashion, somewhat like the concepts of "information" and "information processing" in the mid-20th century. The idea of personality types is ancient, but with each new period of philosophical or scientific development it seems that the types are reformulated in contemporary terms, leading to an evolution and inevitable drift of the types themselves through cumulative subtle changes in descriptions and emphasis.
Today a dominant philosophical/cultural trend is the increasing scientific understanding of how the brain functions. More and more, you hear words like "serotonin" over the church pulpit and in everyday conversation. Typologists realize that in order to remain culturally relevant, they need to somehow respond to the dominant trends in psychology and philosophy, much like the Pope feels pressure to formulate some sort of official stance on issues such as AIDS and climate change.
An important effect of the need to remain culturally relevant is that thought systems evolve to encompass ever more phenomena. For instance, sects founded on a few narrow doctrinal principles over time expound views on virtually all culturally relevant matters. Or, limited typologies such as the forerunner to the Enneagram gradually evolve into general typologies of personality. If there's a greater market for a system of general personality types than for a system of, say, vices and how Christian hermits can overcome them, then why not try to break into that market? If other typologies offer relationship descriptions, then why don't we add some on, too? These are classic examples of memetic competition among overlapping thought systems.
Personally, I am most sympathetic to science and less so to thought systems based on imagination and mental constructs. I feel that science memes are more lasting and tend to build on each other and generate permanent, tangible progress, whereas other kinds of thought systems basically run around in circles with a periodicity of 10 years, 100 years, 1000 years, or whatever with almost no innovation. When innovations in thought systems do occur, it seems to often be the result of an "injection" of scientific, or factual knowledge. But I digress...
Typological applications of brain chemistry
If any readers are aware of other typologies that have attempted to link brain chemistry to personality types, please leave me a note.
1. Helen Fisher's types (discussed in depth in previous post) -- based on neuroscience from the outset, but with the choice of four types possibly influenced by millenia-long typological traditions. Uses 2 neurotransmitters (dopamine and serotonin) and 2 hormones (testosterone and estrogen).
2. Enneatypes. See article "The Enneagram and Brain Chemistry" which links the 9 enneatypes to 3 different neurotransmitters (dopamine, serotonin, and norepinepherine) and 3 different levels thereof, which technically produces 27 types, but only 9 are given.
3. Keirsey Temperaments. According to a poster at the enneagraminstitute.com forum, the book The Edge Effect "ties in neurotransmitters in the brain with the four Keirseyan temperaments (NF, NT, SP, SJ). Naturally, that means he focuses on four neurotransmitters: acetylcholine, dopamine, seratonin, and GABA."
To further quote this poster ("Patrick"), "I did a little Googling, and it seems there are actually ten or more known neurotransmitters. But of course fans of four-type systems will pick four, and enneagram fans will go for three or nine." Exactly my thoughts. While we like the idea of there being 2 x 2 (4) or 3 x 3 (9) or 4 x 4 (16) combinations because it makes for much easier subcategorization, it is unclear whether nature actually needs us to have such a tidy number of important neurotransmitters rather than, say, 6, 11, 17, or 26.
The attempts to link the Enneagram and Keirsey temperaments strike me as amateurish, and the article linking Enneagram to neurotransmitters seems to suffer from trying to immediately relate new entities (neurotransmitters) to existing thought structures (Enneatypes) without learning about the new entities in sufficient detail first. Lofty language such as "We hereby propose a theory of personality whereby high, medium, and low activity of each of these three neurotransmitters systems are distributed in an enneagrammitically logical way" only serves to cement that impression. But this is, of course, just a hypothesis, and hypotheses are often like that.
In any event, attempts to link type and brain chemistry will probably grow more sophisticated as more information from research becomes available.
Right now, many people are testing the waters by suggesting hypotheses about the correlation between brain chemistry and personality types. Based on my experience studying religions and the history of their interaction with science, I would predict that either the neuroscience ends up radically changing the typology, or the typology abandons attempts to correlate types with brain chemistry altogether. Ultimately, typology and neuroscience are driven by different and often incompatible approaches; one has a system in mind and needs "meat" to put on the bones of that system, whereas the other has no system in mind, just a collection of observed facts, and begs some systematic explanation. In practice, it seems, typologies are rarely willing to give up their systems, nor science -- its facts.
For a perfect illustration of this quandary, read the article "Enneagram and Science" at wagele.com.