Notwithstanding the dictionary definition of "dubious", I always think of it more along the lines of "skeptical". "Doubtful" simply means that you doubt what the person said. "Dubious" means that you probably didn't believe a word they said before they even opened their mouth!
I think in your example, that I would use "doubtful" although they could be interchangeable, depending on context.
When referring to a situation, I think I have the vaguest idea. If we are doubtful about a situation, it's more like trying to say we are not sure whether it will succeed - more of a YES or NO uncertainty. If we are dubious about a situation, it means there is more to the situation than we think, or we suspect other possibilities.
With regards to a "dubious person" and a "doubtful person", I've always thought that a dubious person is a person that is doubted (or doubtable) by others, while a doubtful person is a person that doubts others. Any thoughts?
Thank you for your response. I've read the post by kitenok. It was informative, but not informative enough in my opinion.
Judging by the usage of dubious and doubtful in situations (that is if I am correct), and after some more thought, I would say that a "dubious person" is a person who has questionable character, and people are unsure whether to trust him or not.
According to the example from the Macmillan Dictionary for Advanced Learners - "documents of doubtful authenticity" - doubtful here probably is used to indicate an uncertainty of whether the documents are authentic or not, so going by this rationale, when applied to a person, it should mean whether the "doubtful person" is a person or not, which doesn't make much sense. Or perhaps we could ask Kitenok if my first explanation was correct?
I've heard people saying "He is a doubtful person." used on someone who has a habit of doubting others, and it makes sense to logicize that "He is a dubious person.", like what kitenok gave of the example of himself, means the person has questionable character.
Or could it be that we should use "dubious" to describe a person and NOT "doubtful"? It would seem that these two words were created with very slightly different connotations in mind and formed during different periods (Middle English, Late, etc.).
I would agree with both you and Kitenok, both giving good description. Here is my two cents worth:
Doubtful leans toward adverbial because it describes an action (in thought or feeling) that a person may feel or hold as an idea. It can lean toward adjectival in describing a situation as is quite commonly used. For example, "I'm doubtful the Law School will accept me," and "My acceptance into Law School is doubtful," respectively. It is also adjectival regarding a person who doubts people or situations habitually rather than a single specific person or situation, for example "He's a doubting Thomas, doubtful of everything."
I had always thought that the "ous" ending indicated its adjectival nature, and if that is so then dubious would be used in describing an object or person their attributes, or a situation. For example:
1) "A person of dubious character" - a person we know but we know little of, or moreover with suspected negative attributes.
2) "The dubious provenance of the artifacts led me to doubt their authenticity." i.e. a questionable object.
Finally, with the foregoing in mind, if someone declared, "Oh, I'm dubious about whether you'll be accepted into Law school," suggests that their judgement of your acceptability is unreliable. More clearly, their judgement is doubtful not your acceptability.
Dubious carries negative connotation of whatever is being declared dubious, whether self, character, person, situation, or attributes. Doubtful is a simple clinical uncertainty.
In my estimation it has crept into common misuse initially as slang with a cachet of minor obscurity much like the word "bogus."