The general figure for the heritability of IQ, according to an authoritative American Psychological Association report, is 0.45 for children, and rises to around 0.75 for late teens and adults. src  Between 50%-75% of the variance in IQ is genetic. End of story. Note that this does not mean environmental factors cannot or have not driven IQ upwards. In fact, one of the reasons the genetic component of IQ may be so high today is that the environmental component of IQ is better controlled: IQ averages have gone up, while variance has gone down. That is to say, today the mean/sd are something like 100/15. In the past it would have been something like 70/30; which increases the variance (and decreases the heritability) by a factor of 4. As we become better and better at optimizing the environment to maximize cognitive performance, the only remaining source for variability is genetics.