Something did. In the mid 80s, psychologist James Flynn discovered that since 1900, everywhere intelligence tests have been given, scores have risen by three IQ points every ten years. IQ tests measure short term memory, spatial recognition, mathematical ability, and abstract reasoning.
The eponymous Flynn effect has created smarter people much faster than selective breeding ever could. Possible explanations for the Flynn effect include better nutrition, smaller families, heterosis, more stimulating environments, and the ability to use logic to work in a hypothetical situation. The last is the most significant change. In Flynn’s words: “We are the first of our species to live in a world dominated by categories, hypotheticals, non-verbal symbols, and visual images that paint alternative realities.
“There has been a transition from using the mind to manipulate the concrete world for advantage toward logical analysis of symbols increasingly abstracted from the appearance of the concrete world and even the literal appearance of the symbols themselves. This is what I call supplementing ‘utilitarian spectacles’ with ‘scientific spectacles’—which does not imply that the average person knows much science.” http://www.amazon.com/reader/1107609178?_encoding=UTF8&page=22
An example of thinking using “utilitarian spectacles” comes from Unitarian Universalist minister David Breeden in a description of a visit to his nearly illiterate parents. Fresh from his first year at college, he tried to explain Spinoza’s argument that we create our image of God depending on who we are. For instance, triangles would create a triangle god, ants would create an ant god. This idea made no sense to his pre-modern parents. “How could a triangle think; why would an ant think about God?” http://www.patheos.com/blogs/uucollective/2013/05/softballing-with-spinoza/
Flynn comments on the mind set of people like Breeden’s parents, “Note how the pre-modern mind refuses to abandon the concrete world and refuses to use logic to analyze a hypothetical situation. Today, we automatically classify things rather emphasize their differences, take the hypothetical seriously, and use logic to analyze both the hypothetical and abstract symbols.”
Breeden believes the ability to think in the abstract is a gift and a privilege. However, the Flynn effect shows that with education that focuses on abstract reasoning, such as that required to solve mathematical story problems, rather than memorizing, almost everyone can learn to employ logical analysis of symbols. Breeden’s education was a gift and a privilege, but his parents, raised in rural isolation, never had educational opportunities.