I’m going around the world giving talks about Darwin, and usually what I’m talking about is Darwin’s strange inversion of reasoning. Now that title, that phrase, comes from a critic, an early critic, and this is a passage that I just love, and would like to read for you.

"In the theory with which we have to deal, Absolute Ignorance is the artificer; so that we may enunciate as the fundamental principle of the whole system, that, in order to make a perfect and beautiful machine, it is not requisite to know how to make it. This proposition will be found on careful examination to express, in condensed form, the essential purport of the Theory, and to express in a few words all Mr. Darwin’s meaning; who, by a strange inversion of reasoning, seems to think Absolute Ignorance fully qualified to take the place of Absolute Wisdom in the achievements of creative skill."

Exactly. Exactly. And it is a strange inversion. A creationist pamphlet has this wonderful page in it: "Test Two: Do you know of any building that didn’t have a builder? Yes/No. Do you know of any painting that didn’t have a painter? Yes/No. Do you know of any car that didn’t have a maker? Yes/No. If you answered 'Yes' for any of the above, give details."

A-ha! I mean, it really is a strange inversion of reasoning. You would have thought it stands to reason that design requires an intelligent designer. But Darwin shows that it’s just false.

Today, though, I’m going to talk about Darwin’s other strange inversion, which is equally puzzling at first, but in some ways just as important. It stands to reason that we love chocolate cake because it is sweet. Guys go for girls like this because they are sexy. We adore babies because they’re so cute. And, of course, we are amused by jokes because they are funny.

This is all backwards. It is. And Darwin shows us why. Let’s start with sweet. Our sweet tooth is basically an evolved sugar detector, because sugar is high energy, and it’s just been wired up to the preferer, to put it very crudely, and that’s why we like sugar. Honey is sweet because we like it, not "we like it because honey is sweet." There’s nothing intrinsically sweet about honey. If you looked at glucose molecules till you were blind, you wouldn’t see why they tasted sweet. You have to look in our brains to understand why they’re sweet. So if you think first there was sweetness, and then we evolved to like sweetness, you’ve got it backwards; that’s just wrong. It’s the other way round. Sweetness was born with the wiring which evolved.

And there’s nothing intrinsically sexy about these young ladies. And it’s a good thing that there isn’t, because if there were, then Mother Nature would have a problem: How on earth do you get chimps to mate? Now you might think, ah, there’s a solution: hallucinations. That would be one way of doing it, but there’s a quicker way. Just wire the chimps up to love that look, and apparently they do. That’s all there is to it. Over six million years, we and the chimps evolved our different ways. We became bald-bodied, oddly enough; for one reason or another, they didn’t. If we hadn’t, then probably this would be the height of sexiness.

Our sweet tooth is an evolved and instinctual preference for high-energy food. It wasn’t designed for chocolate cake. Chocolate cake is a supernormal stimulus. The term is owed to Niko Tinbergen, who did his famous experiments with gulls, where he found that that orange spot on the gull’s beak -- if he made a bigger, oranger spot the gull chicks would peck at it even harder. It was a hyperstimulus for them, and they loved it. What we see with, say, chocolate cake is it’s a supernormal stimulus to tweak our design wiring. And there are lots of supernormal stimuli; chocolate cake is one. There's lots of supernormal stimuli for sexiness.

And there's even supernormal stimuli for cuteness. Here’s a pretty good example. It’s important that we love babies, and that we not be put off by, say, messy diapers. So babies have to attract our affection and our nurturing, and they do. And, by the way, a recent study shows that mothers prefer the smell of the dirty diapers of their own baby. So nature works on many levels here. But now, if babies didn’t look the way they do -- if babies looked like this, that’s what we would find adorable, that’s what we would find -- we would think, oh my goodness, do I ever want to hug that. This is the strange inversion.

Well now, finally what about funny. My answer is, it’s the same story, the same story. This is the hard one, the one that isn’t obvious. That’s why I leave it to the end. And I won’t be able to say too much about it. But you have to think evolutionarily, you have to think, what hard job that has to be done -- it’s dirty work, somebody’s got to do it -- is so important to give us such a powerful, inbuilt reward for it when we succeed. Now, I think we've found the answer -- I and a few of my colleagues. It’s a neural system that’s wired up to reward the brain for doing a grubby clerical job. Our bumper sticker for this view is that this is the joy of debugging. Now I’m not going to have time to spell it all out, but I’ll just say that only some kinds of debugging get the reward. And what we’re doing is we’re using humor as a sort of neuroscientific probe by switching humor on and off, by turning the knob on a joke -- now it’s not funny ... oh, now it’s funnier ... now we’ll turn a little bit more ... now it’s not funny -- in this way, we can actually learn something about the architecture of the brain, the functional architecture of the brain.

Matthew Hurley is the first author of this. We call it the Hurley Model. He’s a computer scientist, Reginald Adams a psychologist, and there I am, and we’re putting this together into a book. Thank you very much.