I say no, although it is not like physics. We are talking about modest correlations, not strict laws.
What made the marshmallow test famous was the follow-up work which suggested that a child’s ability to defer gratification on the test helped predict future outcomes, such as SAT scores. These correlations with longer-term outcomes speak to the usefulness of the test in revealing some important trait.
A personality trait is a reflexive response, kind of by definition.
The reflexive response, or emotional response is another term, happens prior to the semantics of language, so our common everyday description of personality is inevitably flawed, deceptive. We are almost never in situations where the emotional response is not modified considerably with cultural externals of the moment, quite time where we can be observed.
I’m not sure that reflex is quite the appropriate way to characterize. Personality, in my view, is more about filtering our perceptions rather than triggering responses.
That is, the way we see the world (i.e. what things our perceptual systems highlight and ignore) determines our personality (i.e. how we choose to approach the world).
I say this only because I think the “perceptual filter” concept better explains the effect of personality not only on “System 1” thinking, but how it influences our “System 2” thinking.
I think the relative positions you assign to personality psychology and physics is roughly correct, but to focus only on that comparison misses the significance of personality psychology relative to the other social sciences.
It is obvious (in present times anyway) that an explanation of any social or psychological phenomenon is going to have a multitude of causal factors, and an indefinite number of other irrelevant factors that must be sifted through. The question on the mind of the working social scientist is “What models/tools/instruments should I deploy to quickly get some traction on this phenomenon that will give me the most bang for my buck?”
Now, a good social scientist may have a couple dozen such models they can employ to really dig into an issue. But when it comes to mapping a complex issue they need to know what they should reach for first. IQ is possibly the most effective tool of this sort, but it can also be politically inconvenient (and thus might be difficult to use as often as we would like). Aside from IQ, there are probably several other good candidates (e.g. SES) but I’d wager that personality (specifically the Big 5) comes out on top.
(BTW, this is what bothers me about so-called “Critical Studies”. They purportedly build off the notion that there are an infinite number of ways to interpret the universe, but for all of the different types of studies (marxist analysis, feminist analysis, post-colonialism, etc.) they are all variations on the same thing. It’s like someone who comes to a construction site with a toolbox full of nothing but hammers. They have sledge hammers, ball-peen, mallets, etc. So long as all they need smash things they are fine, but as soon they need to, say, cut something, they are out of luck)
I really like Myers Briggs, not to explain personality, but how I acquire and process information. MB is particularly helpful for anyone on the far end of a spectrum (raises hand) and less helpful for those in the middle. But S/N and P/J are extremely helpful, particularly if you’re very N and very P.
The Big 5 (OCEAN?) isn’t as useful for me, but it captures the willingness to be rude, and neuroticism.