Ms. Greenberg noted that the hand-to-heart motion was especially effective for female politicians. “Men are rewarded for showing their feelings, but women are still judged differently,” she said. “This is a way of showing emotion without crying.”

There is no way to pinpoint how or when the motion gained currency. When Angelina Jolie received a humanitarian award at the Sarajevo Film Festival in 2011, she put her hand on her heart several times to show how moved she was by the honor. But it’s rare at Hollywood awards ceremonies, possibly because movie stars holding statuettes don’t want to risk whacking themselves in the thorax.

The gesture is more common as a greeting or a sign of respect in parts of Asia and the Middle East, so it’s possible Mrs. Clinton picked it up while traveling as secretary of state. She did put her hand on her heart during a visit to Saudi Arabia in 2010 — but in that case, King Abdullah had inquired about former President Clinton’s health, and she was explaining that her husband had two stents put in one of his coronary arteries.

It could just be that the gesture is a nonverbal version of phrases that suddenly turn ubiquitous: “iconic,” instead of “special,” “granular” instead of “detailed,” “I’m good” instead of “no thank you.”

At the moment, the use of “unpack” in lieu of “explain” is popular: When Chris Wallace in a Fox News interview last Sunday suggested to Mrs. Clinton that her economic plan would create more costly government programs, she replied, “Well, but let’s unpack that.”

Sometimes, these expressions are infectious, but Mr. McGowan said he considered it unlikely that Mrs. Clinton adopted the habit unconsciously. (The Clinton campaign did not return calls asking for comment.) “There is probably no deeper analysis of the ways to communicate than in politics,” Mr. McGowan said. “I wouldn’t be surprised if the campaign tested it on focus groups.”