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Illiterates represent a nonneglectable percentage of the world population (about one fifth of the world population; www.uis.unesco.org/en/stats/statistics/literacy2000.htm). Average educational level of contemporary man is about 3-4 years of school. Just a few centuries ago, reading and writing abilities were simply uncommon among the general population. Writing, as a matter of fact, has only existed during the past 5 to 6,000 of years of human history. It may be conjectured that the acquisition of these skills may have somehow changed the brain organization of cognitive activity in general.
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Alfredo Ardila Mónica Rosselli (2007) studied this question.