Another way to look at Second-Order thinking is whether someone can understand C pointers intuitively. Joel Spolsky once wrote that understanding pointers is an aptitude, and I believe he was right. It is the same concept as second-order thinking - being able to follow several layers of indirection. Recursion and regular expressions fall into this group as well. I find otherwise very bright individuals that don't have something in their brain that allows them to grasp these concepts. I think it is how the brain is wired, like good hand-eye coordination or fast memory recall (aka being witty). Just because your brain is good at one of those traits doesn't mean you are good at all of them.