Your Cheatin' Heart
We humans love gossip. It probably plays a key role in what makes us the way we are. Why so?
For one thing, it's probably what keeps us from cheating even more than we do. Plato devotes one of his dialogs to the subject. It features the Ring of Gyges, evidently one of those created by Sauron, which confers invisibility. Gyges, a humble shepherd who found the ring, parlayed it into a kingdom. Socrates, though, thought that an internal moral compass would keep the honest man honest. Maybe so, but studies show that there aren't many such.
Shaq didn't participate in Plato's dialog, but he called it. People cheat because they can. Jonah Lehrer and Kristen Stewart probably thought that they could get away with cheating, so they did. To be sure, she probably had the better excuse - youth and hormones - but he had more to lose and less to gain. Not to minimize her current pain, but Kristen Stewart will still be young, beautiful, and rich whether Cedric/Edward (Robert Pattinson) takes her back or not. Jonah Lehrer's next gig is unlikely to be so easy to come by.