These are the basic rules that can be used to settle these questions:
- Whether it is tihetuka or dvihetuka is determined at the moment of grasping a new bhava at a cuit-patisandhi moment.
- Then that (tihetuka or dvihetuka) is not changed during that whole bhava. For example, if one is born a human with tihetuka patisandhi, it will not change to dvihetuka (and vice versa), when that person is born a human many times during that bhava.
- There is a third category called “ahetuka”, which is the lowest. Beings in the four realms (and even a few humans) are born with ahetuka (which does not mean without hetu, but just means with “no good hetu”). Those humans born with ahetuka patisandhi are mentally retarded, and thus can be recognized.
- What kind of a birth (ahetuka, dvihetuka, tvihetuka) is determined by the kamma beeja (or the kamma vipaka) responsible for the new bhava.
- Therefore, even a human with tihetuka patisandhi can be born with an ahetuka patisandhi in the next bhava. Conversely, an animal could be born with a tihetuka human in the next bhava.
- Those are changed permanently only with magga phala. Of course anyone with a magga phala will always be reborn with a tihetuka patisandhi.
Please ask questions if anything is not clear.