This question begs for a Cognitive Psychology tag (though I can't retag yet), but could also be Cognitive Ergonomics.
How should we handle tagging specific subject areas? Just the single most relevant subject? How do we determine which that is?
As I stated in my comment:
Why limit to just one relevant subject when it can be multiple? It are tags after all.
Worrying about this would only make sense when questions can cover a wide set of subject areas. I would imagine this isn't the case most of the time.
In the case of competing subjects and when the OP is only interested in one, it is up to the person who asks the question to tag the question with the desired subject.
I can think of different levels for tags. There are tags at the level of disciplines or sub-disciplines (cognitive psychology; developmental psychology; clinical psychology; social psychology), and then there are tags that pertain to particular features of the question (e.g., attention, perception, motivation, attribution, etc.).
Perhaps in the future, users may want to follow only questions on certain sub-disciplines. Thus, it may be useful to encourage people asking questions to add discipline/sub-discipline tags to questions. Or others could edit questions to add these.