In our books and lectures, we learned that the IQ of a person is strongly correlated with genetics from the many twin studies (where identical twins raised a part had more similar IQs versus non-related children raised together). This makes plenty of sense, but I was curious to find out if there was any evidence depicting the environmental effects on a person's intelligence.
In this article, researchers and graduate students Christopher Eppig, Corey Fincher and Randy Thornhill looked at the global diversity of intelligence, because there indeed is variations of average IQ across the world, even across nations. There are several explanations as to why this is, such as difference in education or perhaps that harsher, colder climates require more intelligent people to inhabit them (which must mean we Minnesotas are truly smart).
However, Eppig, Fincher and Thornhill provide a different explanation: infectious diseases. According to them, since humans spend an exhausting supply of energy building the brain as children, if they get really ill, a lot of that energy which should have gone to promoting the brain now goes to fighting the disease. Thus, the children have lower intelligence in the future.
There were many different trials conducted to support this hypothesis, such as finding lower IQ with people who were infected with intestinal worms as children, or areas in Mexcio had higher IQs if they underwent malaria eradication. This was also seen among different states in the US, with the "smarter" states having less infectious childhood disease.
Now this is good news indeed! This could be another explanation for the Flynn Effect (finding that average IQ scores have been rising at a rate of approximately three points per decade). This could also be the gateway for helping decrease the disparity between people's overall intelligence.