Indeed, some cognitive disabilities are based in genetics; but those are usually quite obvious. The rest of us humans are pretty much the same. Young child’s brain is, by nature, like a sponge, and impressions received at an early age are extremely critical.
Childrens' growth and development is also seriously affected by things that happened before they were born, like their mother's diet and exposure to local pollutants, that causes effects that could be mistaken for genetic differences. This is why twin studies and such things don't actually prove anything.
