Hypothesis: Link Between Hormonal Levels and MBTI Type?
We know that hormones play a large role in brain function, particularly in relation to mood and personality disorders. Not enough norepinephrine? ADHD, depression, schizophrenia. Oxytocin deficiency? Sociopathy, narcissism, anxiety. Imbalanced melatonin? S.A.D., bipolar.
Furthermore, intentional increases of certain hormones have shown marked personality shifts. Nasally administered oxytocin showed an increase is empathy, trust, and generosity (also, envy and schadenfreude,) and a decrease in fear and difficulty remembering negative social experiences. Lower levels of oxytocin showed reduced trust of strangers and increased cultural and racial biases.
Dopamine, seratonin, oxytocin, vasopressin, and norepinephrine are all linked with social bonding and attachment. Cortisol and epinephrine work together in the formation short-term memories of emotional events and flashbulb memories.
Obviously it's not quite so linear as that, but it seems clear that there's a link between hormones and personality.
I wonder, then, if the 16 MBTI types are (at least in part) a result of slight variations in baseline hormonal levels. For instances, do Feeling Perceiving types regularly operate with higher levels of oxytocin than Thinking Judging types? Are Extroverted types secreting less norepinephrine than Introverted types? Do Intuitive types have more vasopressin than Sensing types? Not enough to be an unhealthy imbalance, but enough to show a marked difference in cognitive patterns.
It's just a hypothesis. There has been a fair amount of behavioral study in regards to MBTI type, but essentially none when it comes to the biological side of things.