Trump speaks at a fourth grade level, he isn't curing any disease, that example is hyperbolic.
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It's not about Trump's ability to speak at a higher level (he obviously can -- he did a great job on Oprah and other public speaking engagements). It's about his target audience.
In all my public speaking classes and seminars, one of the rules was to know your audience. Your vocabulary and abstract ideas have to be communicated at a grade level the lowest common denominator is capable of comprehending without great effort. A common target grade level was 6th grade.
The average reading grade level for Americans is 7th to 8th grade. That's the average, meaning there are many who are lower -- maybe much lower -- than 7th grade.
So, if you're attempting to reach out to adults who may not use English as their first language, didn't finish high school, or simply do not relate to vocabulary they aren't exposed to regularly, where is the common denominator?
I'd say 4th to 5th grade is a pretty good guess, which is why Trump has been able to relate to so many American voters.
http://www.clearlanguagegroup.com/readability/