I think the above issue also has to do with an increase in the number of harmful exposures, such as lead, mercury, and noise pollution, in an urban environment as well. Reference this study:
- Bridge Apartment Study, NYC, 1973
- Income restrictions
- All children attended same school
- Measures: reading test, auditory word discrimination
- Control for: parental education, # of siblings
- Floor of residence correlated with scores
http://htcexperiments.files.wordpres...en_exhaust.pdf