What is the negative correlation between literacy and income?

One would think that literacy (ability to read) would improve ones chances to work at better paying jobs, so a positive correlation would result. That is the conclusion of the web link I provided. I do not claim it to be impartial, as it is from a website that promotes literacy. I don't have know of a study that concludes there is a negative correlation. If illiteracy percentages are calculated and then compared to per capita GDP for particular country, a negative correlation could result.