Keirsey breaks the 16 types into four groups. NF, NT, SP, and SJ. At first, I thought this approach was a bit incorrect, assuming that STs and SFs would be better grouped together, I've seen rather interesting proof to the contrary. SJs and SPs are in fact a better grouping. Here's why. NF types tend to develop Ni, Ne, Fi, and Fe as their primary functions, regardless of which are their dominant and secondary functions. For example, INFJs begin with Ni and Fe, but are likely to develop strong Ne and Fi usage. NT types tend to develop Ni, Ne, Ti, and Te as their primary functions, regardless of which are their dominant and secondary functions. For example, INTJs begin with Ni and Te, but are likely to develop strong Ne and Ti usage. This pattern is what lead me to believe that the S types followed the same pattern, but in fact, they do not. SJs tend to develop Si, Fe, Te, and either Fi or Ti as their primary functions. SJs seem to avoid developing Se as much as they avoid developing their N functions. SPs tend to develop Se, Fi, Ti, and either Fe or Te as their primary functions. SPs seem to avoid developing Si as much as they avoid developing their N functions. In other words, there seem to be four models for how the human mind develops, very Se, very Si, intuitive thinking, or intuitive feeling. Very interesting indeed, because the implication is that the mind develops based on how it processes information. Here is a breakdown of the populations of the various groups... 48% SJs (ISTJ 12, ESTJ 9, ISFJ 14, ESFJ 13) 32% SPs (ISTP 7, ESTP 6, ISFP 9, ESFP 10) 11% NTs (INTJ 2, ENTJ 2, INTP 3, ENTP 3) 09% NFs (INFJ 1, ENFJ 2, INFP 3, ENFP 3) Clearly, we live in a world dominated by the SJ types, and the N types (NFs and NTs combined) are the minority.