As Carl Jung rightly pointed out, many a mother with multiple children can tell you that each one had its own particular “character” from very early on.
This is because all humans – and probably all mammals as well, for that matter – are born with certain innate propensities. Carl Jung, Katherine Briggs, Isabel Myers, Aušra Augustinavičiūtė and others noticed this and tried to classify those propensities. Later on less careful observers mixed this all with the concept of personality, which leads to all sorts of confusion, because personality is a related concept but not all the same thing. This is another long topic which I had better not attempt to tackle here.
The point is that it seems clear that these propensities we observe are basically digital – i.e. on/off lights. There are no obviously observable “grey zones”. Sure, plenty of pundits don’t agree, but that’s mostly because they insist on mixing up the concepts of personality and character. With such muddled concepts they are not likely to learn anything useful at all, so best not to waste much time on them.
The challenge I want to touch on here is to identify what actually causes these on/off settings. Though genes are certainly not the only possible explanations, they are the most common cause for such phenomena. And – now we actually have at least one candidate! The candidate gene that is the so-called Rs4680 gene, also known as Val158Met. With one exception, each chromosome comes in pairs, so normally humans will have two each, of which one may or may not be dominant. The Met-Met combo is classified as “tendentially more exploratory”, while the Val-Val combo is the opposite. Could it be a “P-J” gene? Given the fact that testing does not seem to indicate that either the Met or Val type is dominant, it doesn’t seem like a good match for an on/off scenario. Nonetheless, this is just the kind of link we need to identify, so it’s definitely a topic worth following. There are two useful discussions of it here:
There is also a highly technical article in Nature, but it’s not too easy to understand: