Q: Please help me. What’s the difference between “dubious” and “doubtful”?
A: These words are very similar and have overlapping meanings. Something that is doubtful is IN DOUBT; something that is dubious is a CAUSE OF DOUBT. Both have the same ancient Indo-European root, “dwo,” which gives us scores of words, including “two,” “double,” and many more. (You might say that someone who’s in doubt is wavering between two alternatives.)
Here are the acceptable meanings.
“Dubious” can mean: (1) questionable in character, or untrustworthy, as in “the company’s earnings report was dubious” or “her résumé was full of dubious job references”; (2) undecided or uncertain, as in “he’s dubious about switching jobs”; (3) open to question, as in the old cliché about the “dubious distinction.”
“Doubtful” can mean (1) subject to doubt, as in “his chances for recovery are doubtful”; (2) undecided or uncertain, as in “he’s doubtful about switching jobs”; (3) in doubt, as in “we were doubtful that it would work”; (4) suspicious, as in “she has a doubtful reputation.”
As you can see, there’s a lot of room for duplication. In some cases, you could choose either word; the difference would be one of nuance. But it seems that “dubious” has a more negative or ominous tone.