Full Idea
Possibly a third of the variation in our intelligence is down to the environment in which we grew up - nutrition and education, for example.
Gist of Idea
About a third of variation in human intelligence is environmental
Source
New Scientist writers (New Scientist articles [2013], 2013.10.29)
Book Reference
-: 'New Scientist magazine' [ 2013], p.31
A Reaction
This presumably leaves the other two-thirds to derive from genetics. I am a big believer in environment. Swapping babies between extremes of cultural environment would hugely affect intelligence, say I.