How to turn your wife or girlfriend into a sadist...

Well, you can't. And if you could - if you could press a mind modification button to make it happen - you shouldn't. 

However, if you give her the opportunity to explore Femdom in the right way, then experience shows she may well discover a sadistic streak. I've just read a book by Steven Pinker that in briefly talking about sadism explains how this works:

When we experience something we don't like, we feel aversion. When the experience stops, we feel a buzz of relief.

The neat thing is that the aversion wears out, leaving us with an increasingly intense buzz. This is why some things are an acquired taste. Chili, for example, or bungee jumping.

Why the - call it - rebound buzz? Perhaps the aversion module temporally burns out, so the buzz is there by contrast, like when you hit the shower after a run. Perhaps it's some Darwinian thing - the buzz is a reward for persisting in something you don't enjoy.

It doesn't really matter. The point is that the rebound buzz happens at the animal level, is visceral and automatic.

So if your partner whips, first beats to spanks you, then at first she may feel a very human aversion to causing pain, but when she stops doing it, she gets a rebound buzz. After a while, the rebound buzz becomes more intense and the aversion fades away.

Congratulations, your wife or girlfriend now enjoys hurting you. That makes her a sadist.

But how do you get her to hit you in the first place? Use the techniques in my Femdom how-to book, The Vanilla Dominatrix. Pick activities you think she'll like, suggest them, and, with consent, follow through (for godssake) by setting them up.

If you do this - only introduce activities that offer her something - then the whip or hairbrush usually first appear as a means to an end of her liking: "Here's a whip. Feel free to use it to direct me as I make love to you/do the chores."

Steven Pinker calls this "Instrumental Violence", violence done as a way of getting something. It's something humans cheerfully use whenever they feel morally OK about it, not just in the dim distant past, or in 3rd-world hellholes and 1st-world crime zones, but also on the civilised sports field. This makes it the easy one to get her to do, especially if it demonstrably produces results she enjoys ("Lick faster!" THWACK!!!)

(from "he stoops to worhsip")

Now the taboo's broken, the way opens for two more delicious kinds of violence: "Dominance Violence", in which she strikes you to show she's in charge; and  "Revenge Violence", which extends this into retaliation for slights - in other words, if she's cross, she hits you.

As described in my book, once you've got all three kinds of violence going, you've arrived at that dark point where the Femdom corner of your relationship feels real.

If Steven Pinker's theories are right, it should be round about now that she also starts to enjoy beating you for it's own sake. You'd better have a good firewall in place...