disbelief vs doubtfulness

disbelief and doubtfulness both are nouns.

Word	Noun	Adjective	Verb	Adverb
disbelief	Yes	No	No	No
doubtfulness	Yes	No	No	No

As nouns, doubtfulness is a hypernym of disbelief; that is, doubtfulness is a word with a broader meaning than disbelief:

Other hypernyms of disbelief include doubt, dubiety, dubiousness, incertitude, uncertainty.

disbelief (noun)	doubtfulness (noun)
doubt about the truth of something	uncertainty about the truth or factuality or existence of something
a rejection of belief	the state of being unsure of something