In general, it's seen as bragging if your salary is high, and mortifying if your salary is low, so it's usually just better to keep your mouth shut about it.

I've told a number of people my salary and regretted it every time. They seem to take a vicarious pleasure in telling other people how much money I make. "Oh, you should listen to delluminatus, he makes $xx thousand a year." Ugh.

Companies (especially large ones) don't like it as it limits their negotiation position it can also disclose indirect discrimination e.g. "oops all the females seem to be under paid by 10/20%" which can get expensive.

And it also limits employees when they want to move as the new employer knows how much they make now.

Yes, I understand why companies do it. They don't want people to know when they are under/overpaid.

What I don't understand is why people don't talk about it. If you tell and you learn you are overpaid then you feel good. If you learn you are underpaid, well, time to go negotiate/look elsewhere.

Do people think they ARE their salary? It's simply money that some people arbitrary decided to pay you...

Well employees are funny about money in general any HR professional will tell you that :-)

Having a potential employer know exactly what you where paid before puts you at a big disadvantage when it comes to negotiating a new salary /package.

Seems to be more of an American thing than a European thing. Not everyone will hide their salary here. Some will some won't.

I never saw someone disclose his/her salary in France. There seems to be an expectation that doing so would produce jealousy and bitterness. So it's not only an American thing.

If discussing salary in the workplace weren't taboo there'd be people unionizing left and right.

Asymmetrical information = leverage