Psychometrician here.
The issue of whether “IQ” is a real phenomenon and whether is predicts or causes life outcomes are two separate issues.
We don’t understand intelligence as a neural level phenomenon, and therefore we don’t yet have ratio scalings of its magnitude. General intelligence, or “g” is defined as the first principal component extracted from an intellectual test. What this means put simply is that people who score high on one test of brain ability tend also to score high on others. We don’t know what it is in a physical sense that does this, but just label this regularity as “intelligence”.
Theories having much less support are those which argue for “multiple intelligences”, often labeling other human abilities as IQ. These models tend not to be supported across samples.
Now the question of predicting life outcomes. First of all, understand that psychology is not physics. You will not see “laws” in psychology (or very few), and thus most correlations or predictions are far lower than certainty.
On one hand, someone telling you that an R=.30 between intelligence and job performance isn’t that high, because it isn’t! That’s only 9% of the variance in job performance explained by intelligence. But, the important point is that intelligence is still our BEST predictor of general complex job performance. You see, we still aren’t very good at predicting job performance. Otherwise, hiring good people would be a hell of a lot easier!
So intellgence is a real statistical concept or regularity, and its still the best predictor relative to everything else we have.
Further reading for those interested:
Spearman’s g
factor analysis method
generalized validity
selection ratios and “g”
