Did 9/11 change people's behavior during the playing of the national anthem?

Question

The Star-Spangled Banner was adopted as the official national anthem of the United States in 1931. Up until June 1942 there were no guidelines given as to how people were to behave during the playing of the anthem. Since then there have been a number of changes to the description of what people should do. For example, men were to remove their hats and women were to place their hand on their heart. Then it changed to instruct both men and women to place their hand on their heart when the flag was on display. Then it was changed again to instruct that your hand should be held over your heart whether or not the flag was displayed, essentially meaning for the entire duration of the anthem. To quote the source I've read (the Wikipedia article on the anthem):

Because of the changes in law over the years and confusion between instructions for the Pledge of Allegiance versus the National Anthem, throughout most of the 20th century many people simply stood at attention or with their hands folded in front of them during the playing of the Anthem, and when reciting the Pledge they would hold their hand (or hat) over their heart.
The Star-Spangled Banner: Customs and Federal Law

But it was the very next sentence that I found very interesting:

After 9/11, the custom of placing the hand over the heart during the playing of the national anthem became nearly universal.

It cites three sources directly after this sentence, however they are supporting references to the entire preceding paragraph. The links take you to legal documents, and none seem to support the change in custom after the 9/11 terrorist attack.

Is there any truth to this claim?

Edit: A commenter has linked a different Wikipedia article where another version of this claim is made:

According to the United States Code, those present should stand at attention with right hand over heart.[15][16][17] It's noteworthy, this tradition only acquired widespread practice after the year 2001, ...
US national anthem protests

Again, even here it just gives links to the United States Code as a citation. None of it supports the claim about 9/11 or the year 2001.

Answer 1

This is anecdotal, but I was taught in school in the 1970's to always stand facing the flag and put a hand over my heart for the Pledge of Allegiance or the National Anthem, unless you were in the military or scouting uniform, in which case you can salute. (This was actually a major selling point of scouting)

So this sentence from your WP link in my experience sounds about right:

On July 7, 1976, the law was simplified. Men and women were instructed to stand with their hands over their hearts, men removing their hats, irrespective of whether or not the flag was displayed and those in uniform saluting.

I can remember going to Tulsa Roughnecks games back in the late 70's and early 80's, and while it perhaps wasn't universal, pretty much everyone stopped and did this during the opening anthem. It was what you were expected to do. A lot of times, even people who couldn't see the flag (eg: in vending lines ouside the stadium bowl) would put a hand over heart.

This is all in Oklahoma though. Perhaps folks in more urbane parts of the country behaved differently.

I did interestingly find an article from 1999 about how lax folks had become in doing this though. This was of course 2 years before the attacks. So perhaps this particular public ritual was indeed at a bit of a low ebb at that point.

Blame it not so much on disrespect, said Veteran of Foreign Wars state commander Jay Bjorgo. Blame it on ignorance.

Bjorgo said a lot of people simply don't know the proper conduct when the anthem is played.