Ice cream and intelligence

The Economist's daily chart on April Fools' Day demonstrated the correlation between per capita ice cream consumption and average score on the PISA reading scale (which measures reading for 15-year-old students in each country):

Clearly New Zealand students are underperforming in reading relative to expectations based on our ice cream consumption. But hold on before you start to wonder if Chick-fil-A is going to boost the intelligence of Americans. Of course, while the data is serious and the correlation is real, there is unlikely to be any causal relationship here (which was of course the point of The Economist's data team posting this on April 1).

Countries that are richer tend to consume greater quantities of animal protein. These countries also tend to spend more on education, which should in theory lead to better reading scores for richer countries (maybe that's the case in this graph?). Of course, there's a lot of other stuff going on, but those two contemporaneous relationships with national income are enough to ensure that there is a correlation between ice cream consumption and reading scores. That is, because ice cream consumption and national income are positively related, and reading scores and national income are positively related, then it would appear to a casual observer that ice cream consumption and reading scores are positively correlated.