I can offer some anecdotal evidence from the prob and stat class I taught yesterday. One of my students was in danger of flunking the course. I studied his papers, and discovered he didn't have the concept of the "number line". He was a non-traditional student, an ex-marine in his fifties, and evidently they didn't teach the number line when he was in school, or maybe he just didn't remember it from so long ago. He had taken the course before, and failed it, and at that time his teacher told him, "This is stuff you should have learned in high school." But that's no help.
I took fifteen minutes during my office hours to teach him the number line, and in no time at all he was putting numbers, both positive and negative decimals, in the correct place on the number line.
Now, in my opinion, that's what smart is. Smart isn't how much you know, it's how quick you learn.