My understanding, perhaps incorrect, is that even after trying them on, most women still buy the wrong sizes (80-85%. Wikipedia has 4 citations to this stat, i haven't bothered to read them all, but here is one study: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2275741/).

That makes this even more fascinating to me, because while women apparently want to try them on so they can test "fit", whatever they are trusting to determine that fit is apparently causing them to buy the wrong bras most of the time.

Of course, it's also not clear if this is intentional (IE they uy the wrong bra size thinking it makes them look better or whatever), or accidental.

Interestingly, reading the results of that study, it could be they don't care. The study finds that various forms of pain are not correlated with improper bra size, so it could be women just wear what they think makes them look good, and since it doesn't cause any pain, they don't realize it's not the right size.

If it makes them look good, and doesn't hurt, by what measurement is it not the right size anyway?

Apparently, providing proper support, etc.

It turns out that just because something doesn't hurt, and looks good, doesn't mean it's good for you.

My naive, non-boob-having understanding is that using the wrong size will cause your boobs to sag faster, as well as causing other medical issues, even if it doesn't cause pain.

Wearing a bra will actually make your boobs sag faster, not the other way around. It's a very common misconception among women, though, who apparently can't be bothered to look these things up before shoving "advice" down your throat that "everyone knows" is correct.

http://en.wikipedia.org/wiki/Ptosis_%28breasts%29#Bras_don.2...