Actually, if belief is something that is learned, then this is not true. To remain agnostic you would have to be isolated from community members who do believe in Him. Alternatively, an individual would have to be given no reason from birth onward to believe that a God does exist, or, a person would have to have experienced nothing to lead them away from agnosticism. Someone would not have to be completely devoid of reason to show disbelief; instead, they would have to be devoid of reasons to offer their belief.
And personally, I don't think that rationality and belief are completely the same thing. If something can be one-hundred percent proven through reason, then it isn't really belief, is it? It's more of an undeniable fact. I personally believe in God because I have experienced certain things which have lead me to do so, but otherwise I would have remained agnostic.