Why does it have to be causal? A variable can be a real predictor of another without any of them causing the other one (for instance, they may have a common cause). That doesn't mean the predictor is of any less value.
I would say that results on certain cognitive ability tests (i.e. "IQ tests") being at least a moderately good predictor for future performance in certain human activities is a rather well proven fact. For instance, the correlation between an IQ score and future job performance on a job with complex requirements is about 0.5.
Of course, that does not mean the relationship is 1-to-1 and that it is a perfect predictor (which it obviously isn't).
I would say that results on certain cognitive ability tests (i.e. "IQ tests") being at least a moderately good predictor for future performance in certain human activities is a rather well proven fact. For instance, the correlation between an IQ score and future job performance on a job with complex requirements is about 0.5.
Of course, that does not mean the relationship is 1-to-1 and that it is a perfect predictor (which it obviously isn't).