What's missing: all the children whose parents raised them the same way, and who ended up committing suicide, or are in mental institutions, or fill the ranks of the homeless today.
This is a longitudinal study, not a cross-sectional one; attrition can be measured. You should also provide some evidence for the idea that hordes of high-IQ people are going homeless and committing suicide, as opposed to occasional ones with mental illness (especially schizophrenia), given that all the existing evidence tends to imply the opposite, if you don't want to come off as a Taleb-citing crank.
What's missing: all the children whose parents raised them the same way, and who ended up committing suicide, or are in mental institutions, or fill the ranks of the homeless today.