Why is it that mere ideas can activate the pleasure circuit when those ideas have nothing to do with evolutionary survival, or adaptive advantage at all? There are a couple of possible explanations.  I think the most likely one is that there are a lot of things that are created accidentally in evolution.  There are a lot of side effects.  So you can evolve a pleasure circuit for adaptive things so you will eat food and drink water and have sex.  And you can evolve social cognition and then you can connect up your social cognition center with your pleasure center.

And then you can take pleasure from positive social evaluation, which is something that you can imagine would promote group cohesion and in hunter/gatherer societies would be adaptive. But then you’ve built a machine that can be operated on in all kinds of ways in terms of culture and behavior.  

And we shouldn’t fall into the trap and think that all those ways are adaptive or useful just because they’ve evolved.  Evolution is a tinkerer and not an engineer as Francoise Jacob famously said.  And when you’re a tinkerer, you throw things together to solve the problem at hand.  You don’t build elegantly and you don’t build the way an engineer would build to try to consider all the possible contingencies.  You’re just solving the one problem that circumstances have dealt you at this moment.  

There’s a tendency, particularly when looking at brain function, to be over-awed by the brain and, this is understandable.  We say human consciousness is manifest in this two-and-a-half pounds of tissue in our skull and that’s amazing.  You say, "that’s amazing," but that doesn’t mean that when you lift the hood and look at how it's built, either anatomically or electrically or genetically, that what you see is well-engineered.  No.  It’s a freaking mess in there.  

It’s a freaking mess both at these biological levels and a lot of times it’s a freaking mess at the behavioral level.  The sort of thing that gets you a behavioral advantage in one situation can actually work to your detriment in another.  The pleasure circuit that makes it so that you are motivated to eat food and survive is great in one context, but when you figure out how to get heroin out of a poppy plant and you get pleasure from that and that can kill you, well that’s not so great.  But obviously we didn’t evolve to get pleasure from heroin.  This is just history.