It’s undeniable that intelligence has a hereditary component. Is the half of our DNA we inherit from each parent the higher or lower IQ part? It’s chance. Recall the story of George Bernard Shaw and the glamorous dancer Isadora Duncan on the topic of producing a child together. Duncan stated that Shaw had a magnificent brain and she had a glorious beauty; the combination would yield a remarkable child. Shaw replied with regret that he feared the result would embody his beauty and her brains.
IQ tests over the population at large have a normal distribution with a mean of 100 and a standard deviation (SD) of 15.
Combining two random draws over a large sample to produce a child will lead to a normal distribution with a mean of 100 and SD of 21.2 if the two random draws are uncorrelated. If you want the gory details check here. The standard deviation would be less if there is a correlation but always greater than the original standard deviation of 15.
As the population at large has a normal distribution with an SD of 15, there must be another influence that returns the child population’s average SD to 15. The environment acts to increase numbers of near average IQ and reduce IQ extremes.
The environment is important.