Saying that "some adults can also be a naif" is the same as saying that "some people can be an idiot." The Schoolteacher Shaw would far rather redraft the sentence into a sensible form of English, viz. "some people can be idiots." Plural with plural in harmony, Nige. "Some people are capable of being idiots" would be even better, but hey ho. And Nigel, in the mangled words of Mandy Rice-Davies, Iains would say that, wouldn't he? Don't be such a naif, Nige!