This graph. The idea being once discrimination is removed the true value is revealed and stable. People use it to compares to rates of being gay or trans.

But my problem with it is it suggests there's an upper limit on being gay or trans, which is probably true, but it would be also fine if there wasn't.
I agree we're too deep in it to have an idea. That said 3% is a number that keeps popping up in cultures around the world with more tolerant views to being transgender/trans adjacent. Like Samoa with fa'afafine.
