Quick question

This has been bothering me for awhile.
When did Americans start putting their hands over their heart during the National Anthem?
I mean, during the Pledge of Allegiance, I understand. But growing up, I do not remember doing anything more than removing my hat (if I was wearing one) during the National Anthem.
I’m not saying that I don’t like the custom as it has apparently evolved. I just want to know when it started.
And yes, I was impelled to ask this question because of all the uproar over a certain Dumbocratic presidential candidate who didn’t follow the custom recently.
Thoughts? (Of course, perhaps I am simply in early-onset Alzheimers.)