Sexing by upperlip slope, Frog Myth busters needed!

[Hypnotic]

I just rememberd I heard about this a long time ago, before I even owned my first frog.
That you could basicly sex most pacmans looking at the slope of the upper lip. If the upperlip is more rounded, it is a female. If more steep, it is a male.
I'm not sure if it's a myth or not, all I know is it seems like one, because I've never heard it mentioned before on here.
I'm also not sure where I picked it up, it was a long time ago that I've heard it, and I'd love to know a final conclusion.
Also, looking at the two frogs I own so far, the theory adds up, but it could be random luck.

First one is a confirmed male, with a steep upperlip, second one would be a guessed male.



First one confirmed female, note the rounded upper lip. Second one guessed female.


If you have pictures of your frogs that debunk this theory, please link them! I'm also really curious to see if anyone else has actually heard of this theory.