This article should make you rethink your example: http://www.theguardian.com/science/2013/sep/29/chimp-intelli...
It doesn't have to be. But it does vary from person to person (even if non linearly). What would you say affects the intelligence of people with mental retardation?
This article should make you rethink your example: http://www.theguardian.com/science/2013/sep/29/chimp-intelli...