I've observed this: Identical (monozygotic) twins are always: a) Identicals, b) conflictors. I don't have a too large sample from which I've drawn this conclusion, but it makes sense because VI-wise conflictors are very similar (in external traits/appeareance , not actual VI of facial expressions). This suggests that conflictors share similar genetic configuration, and that at an undetermined point, a "switch" is activated that sets the final type. What this "switch" could be, I do not know.