The psychology of ‘none of the above’

I am working on a design for a formal registration service that demands users categorise themselves according to a closed list of options. The list is not exhaustive, so some users don't see an option that works for them. However, user testing shows some reluctance to hit the 'none of the above' option. I'm wondering if the negative precept in the 'none of the above' wording has been explored anywhere? Is there any literature that explores the distinction between a negative 'None of the Above' and a more affirmative "A different option" (or equivalent). Can anyone point me in a useful direction? Thanks!