There is biological variability in virtually all genetic traits... because of "other genes" and environment. Although by Occam's Razor one should not propose complexity for which there is no evidence, from what one knows about other genetic traits, one should have a prepared mind for such complexity. It's possible that the A− trait and temperament (however you define it, e.g., a reaction time) are neither polygenic nor genetically heterogeneous, but I wouldn't bet on it. Could genes that affect hormones, physical stature or hair growth interact with the NPA genes? Sure. In the past we have noted short and tall stature, premature baldness, etcetera, in some of the NPA types.
The Five Factor Model and others of this ilk are empirical and arbitrary. In one hundred years they will be mentioned in passing... along with the phlogiston theory and Ptolemy's model of the earth as the centre of the universe. By "empirical" we mean that they do produce some numbers, but they have no basis in scientific theory and can be neither proved nor disproved. By "arbitrary" we mean that there is no basis for choosing five factors other than the fact that we happen to have five fingers on each hand. It's like insisting on a theory of light based on some high number of arbitrary colors, when in fact only three primary colors are required.
« Back to index