[S]uppose you know for a fact exactly what someone's actual goals and desires are, and then you observe them doing a certain action. Then which is more likely to be the reason why they did the action:
A) their self-reported reason for doing the action, or
B) because it was the rational thing to do, given their goals and circumstances?
Even if you think that both A and B have low probabilities of being right, which one has a higher probability of being right?
The better explanation is B), for people are naturally terrible at introspection, but predictable if you can model them well. You have presumed that we know the modeling parameters—"you know for a fact exactly what someone's actual goals and desires are"—and we know that there exists ways to predict how people will operate. The secret comes from the term "Predictably Irrational", which is also a book title: we must be rational not according to a person's
(1) "self-reported reason",
but instead his/her
(2) "actual goals and desires".
In some sense, this seems like it will be a dissatisfying answer to your question, because it is not clear how well we can actually measure (2). It is not clear we can ever know a person's actual "goals and desires". If our options are instead,
A) their self-reported reason for doing the action, or
B') [the best obtainable model of] their goals and circumstances
, I would still choose B'), on the basis that people tend to be worse observers of themselves than others are of them. There remains a weakness in this model, though. It assumes that we can model people well, which may not hold for all personality types. For example, we were terrible at understanding Autism for a long time, and are only still just learning how to productively interact with severely autistic people. It could be the case that in interacting with severely autistic people, A) would be better.
Keeping my response to the original form of the question: I would rate two of the claims in your question as highly dubious.
1. "Introspection is a good guide to human motivations,
In his 2008 The Unreliability of Naive Introspection, Eric Schwitzgebel says:
We are prone to gross error, even in favorable circumstances of extended reflection, about our own ongoing conscious experience, our current phenomenology. Even in this apparently privileged domain, our self-knowledge is faulty and untrustworthy. We are not simply fallible at the margins but broadly inept. Examples highlighted in this essay include: emotional experience (for example, is it entirely bodily; does joy have a common, distinctive phenomenological core?), peripheral vision (how broad and stable is the region of visual clarity?), and the phenomenology of thought (does it have a distinctive phenomenology, beyond just imagery and feelings?). Cartesian skeptical scenarios undermine knowledge of ongoing conscious experience as well as knowledge of the outside world. Infallible judgments about ongoing mental states are simply banal cases of self-fulfillment. Philosophical foundationalism supposing that we infer an external world from secure knowledge of our own consciousness is almost exactly backward.
The second is like it:
2. "Rationality is a good way to explain human behavior.
See the book Predictably Irrational: The Hidden Forces That Shape Our Decisions:
My goal, by the end of this book, is to help you fundamentally rethink what makes you and the people around you tick. I hope to lead you there by presenting a wide range of scientific experiments, findings, and anecdotes that are in many cases quite amusing. Once you see how systematic certain mistakes are--how we repeat them again and again--I think you will begin to learn how to avoid some of them
Fortunately, neither of these claims need to be true in order for either answer to be plausible. This is because:
- A person's inability to accurately self-report beliefs does not mean he/she does not have beliefs, and nor does it mean that he/she cannot be well-modeled as having beliefs.
- Predictability does not require rationality.
This is good news to philosophers, who have a history of knowing that it takes a while to think in a properly philosophical manner. [citation needed] I'm sorry, I just had to say it that way. :-)