Those wishing to talk about correlation and causation may be interested in the 
following study:

Parasite prevalence and the worldwide distribution of cognitive ability
Christopher Eppig*, 
Corey L. Fincher and 
Randy Thornhill
+ Author Affiliations
Biology Department MSC03 2020, University of New Mexico, Albuquerque, NM 87131, 
USA 
*Author for correspondence ([email protected]). 
Abstract
In this study, we hypothesize that the worldwide distribution of cognitive 
ability is determined in part by variation in the intensity of infectious 
diseases. From an energetics standpoint, a developing human will have 
difficulty building a brain and fighting off infectious diseases at the same 
time, as both are very metabolically costly tasks. Using three measures of 
average national intelligence quotient (IQ), we found that the zero-order 
correlation between average IQ and parasite stress ranges from r = −0.76 to r = 
−0.82 (p < 0.0001). These correlations are robust worldwide, as well as within 
five of six world regions. Infectious disease remains the most powerful 
predictor of average national IQ when temperature, distance from Africa, gross 
domestic product per capita and several measures of education are controlled 
for. These findings suggest that the Flynn effect may be caused in part by the 
decrease in the intensity of infectious diseases as nations develop. 
If you are teaching students who are not quite ready to understand the analyses 
presented in the original article there is a very readable synopsis in the 
Economist:
http://www.economist.com/node/16510958?story_id=16510958
This might also be a useful article when addressing IQ issues.

-Don.


Don Allen 
Retired professor 
Langara College

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