The folks at Vocativ used a Flesch-Kincaid readability test to assess the ease of comprehension of more than 600 presidential speeches, delivered by every Commander in Chief in American history. Notice a pattern?
Vocativ's analysis is noteworthy for several reasons. It's informationally dense, but not overwhelmingly so. Above, the length of a president's speech corresponds to the size of its representative circle (as presidential speeches have become simpler and more accessible, they've also gotten more long-winded). It's interactive, and not in a gimmicky way; it's easy, for example, to trace your way through America's presidential lineage by hovering your cursor over each of the 600+ data points. Said data is deep enough to lose yourself in exploration, if that's your thing, but not so deep as to become unnavigable.
But the best thing about this analysis is its refusal to draw pat conclusions from the data it presents (cf. this chart of rappers ranked by vocabulary size). Authors EJ Fox and Mike Spies asked historian Jeff Shesol, a former speechwriter for Bill Clinton, to examine the data, and the observations he brings to bear are subtle and instructive. Here's his reading of the steady decline in the speech grade level of presidential oratory:
"It's tempting to read this as a dumbing down of the bully pulpit," Shesol explains. "But it's actually a sign of democratization. In the early Republic, presidents could assume that they were speaking to audiences made up mostly of men like themselves: educated, civic-minded landowners. These, of course, were the only Americans with the right to vote. But over time, the franchise expanded and presidential appeals had to reach a broader audience."
And here are Shesol's thoughts on the important distinction between complexity of language and complexity of ideas:
...length and grade level aren't especially useful gauges for the intelligence (or impact) of a speech. The Gettysburg Address might be short, and the number of syllables in Lincoln's words few, but entire books have been written about that speech. John F. Kennedy and his speechwriter, looking to Lincoln for inspiration, concluded that you should never use a three-syllable word when a two- or one-syllable word will do. That economy of prose characterizes a lot of JFK's speeches.
And, he says, a lot of Obama and Bush's speeches, as well. "I think President Obama, no more or less than President Bush, tries to pack a lot of nuance and subtext into language that is as plain and straightforward as possible," he explains.