“By he way, you may have seen that I have recently launched a Snapchat account,” Hillary Clinton told Iowans last month. “I love it—those messages disappear instantly, all by themselves.”

The joke was on herself, of course, referring to the much-criticized private email account she used as Secretary of State. Since the dawn of the television era, our most skilled politicians have deflected attacks by making wisecracks at their own expense.

But self-deprecating humor is in short supply on the 2016 campaign trail, where the candidates have mostly fired their one-liners at each other. A typical example: citing recent news reports about cyberattacks , Republican candidate Scott Walker said that Russia and China now know more about Clinton’s email server than do members of the U.S. Congress.

For both parties, of course, the easiest target is Donald Trump. “Finally, a candidate whose hair gets more attention than mine,” Clinton jibed. After Trump visited Iowa in his private helicopter, Bernie Sanders joked that he had left his own chopper at home.

These kinds of quips might draw a few chuckles from from a campaign crowd, but they won’t humanize candidates—or disarm their opposition. To do that, you need to poke fun at yourself. ...