There is no consensus that there are '16 personality types'. The idea of 16 personality types appears to be most prominent in the lucrative commercial world of Myers-Briggs testing, which is commonly used in industry, but which is not favoured by the modern psychologists who research personality testing.

Generally, instead, modern psychologists who research personality testing prefer 'Big Five'/Five-Factor/NEO personality tests, which have five personality traits, and which instead of dividing people into two categories within a factor (e.g., introvert vs extravert), divide them into five (e.g., 'extremely high', 'relatively high', 'about average', 'relatively low', and 'extremely low'). This leads to, in effect, 3125 personality types, so five factor advocates don't really talk much about 'personality types' in their research, but focus much more on the individual factors. The reasons for preferring the Five-Factor model include it having more test-retest reliability than the Myers Briggs (e.g., people's personalities are more stable on the Five-Factor model than they are on the Myers-Briggs), the five factors are better separated statistically in a way that's more reliable than the four in the Myers-Briggs (the questions are better chosen to just get at those factors and not other things, in other words), and, well, saying someone is 'about average' on a trait is often more useful and thus more predictive than putting them in one of two boxes.

Additionally, research also seems to show that scores on the Five-Factor model (even if they're more stable than those on the Myers Briggs overall) aren't fixed - peoples personalities do change over their lifespans, and that variation is different in different cultures. Which is to say that, even if there's 3125 different personality combinations possible on the Five Factor test, people aren't stuck forever in 'types' - and that research suggests that some of why we change or don't change isn't due to biological changes as we get older but instead because of expectations in a culture. Additionally, someone who is introverted in one situation where they feel attacked, might be much more extroverted in another situation where they feel more comfortable in being who they are.

That said, these personality type inventories - whether Myers-Briggs or Five Factor, are products of the 20th century (with the Five Factor approach being developed and coming to prominence in the 1980s and 1990s, and Myers Briggs being something originally developed across the 1910s to 1940s). It's important to remember that the questions are designed to elicit reliable responses in people who've probably done multiple choice tests before and understand the (relatively new) concept of psychological testing culturally (i.e., they've probably done a Buzzfeed test in their time). Robert McCrae, one of the people behind the Five-Factor test, has done a bunch of cross-cultural research with the test, and clearly seems to think it's universal across cultures, but other researchers have found poorer validity of these tests in a couple of settings they describe as being 'largely illiterate indigenous society'.

Which is to say that, historically, you cannot go back and ask Socrates or his wife to do a personality test without the ability to go on a Bill & Ted style bogus journey through the past. For psychologists to be satisfied that the personality factors in something like the Five Factor model aren't just culturally specific but are more universal over millennia of time full of cultural change, they'd typically want to have a whole bunch of data from the past, from people who have specifically done these tests, which they can statistically analyse to see whether the statistical factor analysis that differentiates between trends in how people respond to factors also applies in ancient populations. That data simply doesn't exist.

So, in summary, modern psychologists don't think there are 16 personality types in the first place. In regards to what psychologists do favour - the categories in the five factor model, based on statistical analysis of the personality test results of a modern, literate population - there's still a big question over whether that would apply to an ancient population, especially as at least some research shows limited applicability in other populations in the modern day world. There's no data, one way or the other, that would provide definitive proof.

And that's without getting into discussing psychologists that think there are serious problems with the atheoretical Five-Factor approach. In truth, the Five-Factor approach does has the advantage of - unlike the Myers-Briggs approach - of not being originally based on old Jungian notions that nobody really takes seriously anymore. However, there are certainly those who would prefer that an approach to personality be based in separable physiological function (rather than the Five-Factor approach which is agnostic about such things), and those who believe that trying to turn something as complex as a personality into basic sets of numbers is a fraught exercise at best, both of whom would be more skeptical about the Five-Factor approach than the majority of psychologists.

So TLDR, if you told a personality psychology researcher that Socrates was an INTJ, they'd probably wince.

Thank you and your fulsome response is appreciated.

I’m not an academic, however I have a great interest in the past in how it influences the present. History repeats, and we fail to learn almost every time.

Accepted that personality labelling is of our time and can be subjective, this said, my theory is that core personality traits have been have been similar throughout history.

I realise this statement is naive, it explains to me why the same reactions continues to result in the same results. The really scary part is that issues are exactly the same.