Part of the explanation may be specific to the most recent presidents. Trump and Biden are among the oldest presidents to take office and arguably both show signs of cognitive decline. In most respects, though, their use of language was consistent with a long-term trend that includes both Democrats and Republicans. Since the 1940s, presidential speeches have tended to hover around a middle-school level of comprehension.

You can argue that’s a good thing. The declining complexity of presidential rhetoric is correlated with an expanded electorate that can be addressed directly using broadcast media. The long, ornate speeches of the past weren’t necessarily superior, either. We remember Lincoln’s Gettysburg Address, which contains fewer than 275 words (there are discrepancies between various manuscripts and contemporary newspaper accounts), better than the two-hour stemwinder by Massachusetts politician Edward Everett, that was the headline event of the day.

Still, there’s a significant, if not unqualified, relation between complexity of language and complexity of thought. The example of Lincoln’s profound simplicity does not excuse the banality and vulgarity of more recent presidents.