Here's my guess: dawn of time.

I don't think it's a new thing. The new thing is the idea that people enjoy being in equal relationships. I don't buy it for a second. Relationships are never equal. The whole hurting thingy is simply feedback to affirm our various status in the relationship. Which is important for all parties.

I had a long deep discussion about just this with my slave yesterday. So she should get the credit for my theory. This is the first relationship she's had where she has no rights at all and is unconditionally obedient, and she tells me she's never been happier. Go figure.

We have loads of social pressures and perceived norms floating about. Who says the perceived norms is what is normal or healthy?