I could start my explanations with the research that tracked the lives of twins given up for adoption at very young age and who were adopted by families of different social standing living in very different societies. Yet the twins showed just as high correlation in intelligence as those who lived together with their parents, proving that genes are the biggest determinant of intelligence. But I wouldn't use this line of argument as there are also equally compelling research that prove otherwise.
Rather I will back my claim with my observations. Observations of how incoherent all other possible determinants of intelligence are. The same environment that turns out a priest also turns out the criminal. Even our notorious universities have turned out many youths who have shattered academic records in high ranking foreign universities. Most of our popular musicians probably went to school that had no music class nor music teacher. You would expect Obasanjo to have come from Niger Delta and Jonathan to have come from South West. If something as external as the proper understanding of the value of rice is necessary for university admission then there is a Nigerian state which will not have as many professors as it has. No where have I come across an external factor having a logical consistent effect on the level of intelligence of everyone. Not even age.
What I have seen happen consistently is that bright parents give birth to bright children. Fashion crazy/intelligent parents give birth to fashion intelligent children. An Ibo man gives birth to an Ibo boy; Uche & sons limited. Even natural newscasters give birth to Radio Eleje. And there is often little the external factors can do about it. Even if you take the child of naturally bright parents and have him grow up in the jungle among monkeys and apes you will only be remaking Tarzan.You can lock an eaglet with chicks in a cage, it's never going to make it less an eagle.
Truthfully, there are some things the environment can influence. It can determine who gets a job or not (a graduate in US vs an equivalent one in Nigeria). It can determine how absolutely/narrowly smart you are (a professor vs the village judge). But the one thing it has almost no real effect on is how really intelligent you are. Because intelligence is more genes that every other thing combined.