I'm interested in the study of Personality Types.
That's like phrenology, right?
How is this different from, say, horoscopes...positing peoples' personalities based on the sign of the zodiac under which they were born?
This is very different. With a horoscope (or phrenology :-), I would take one fact about you and try to extrapolate knowledge of your personality, ideal career, likes and dislikes, or your future, from that one fact.
With MBTI or Keirsey, you describe your own personality, based on many factors. You answer a series of questions, identifying your likes and dislikes, your interests, what sorts of activities energize you and what things leave you cold. You identify your personality type.
What about the Forer effect?
The "Forer effect" refers to the observation that (some? many?) individuals "will give high accuracy ratings to descriptions of their personality that supposedly are tailored specifically for them, but are in fact vague and general enough to apply to a wide range of people." Is that a fault of the experiment? Or a fault of the person?
Personally, I believe that the Forer effect results when someone with a weak sense of their own personality is confronted with an admittedly "vague and general" description. "Hey, that could be me, I suppose." Well, duh.
I've read the 16 MBTI descriptions and their Keirsey counterparts. They're not similar. I see myself in the description of the INTJ type (mine). I see some of me in some of the other types, while other types are so far off from me that I have no trouble believing these profiles are very different, geared for very different personalities.
For what it's worth, I don't consider the original Forer description to be particularly accurate, at least, not for me. It's so obviously wishy-washy. But, maybe that's just how my personality type sees things. :-)
For further reading: