How does the mathematics of sex differ from the chemistry of sex? Or the sociology of sex?

We use sociology, chemistry and math in the same way we use poetry and writing -- to try to connect to the world. And each method highlights something different. I like the way that math helps me make sense of things.

Can you explain one of the juiciest equations in the book, "The Rule of 12 Bonks"?

This math helps us answer the question, "How many partners should I have before I settle down?" The background is quite interesting: Despite high divorce rates, when it comes to falling in love, we refuse to take advice. If you were buying a DVD player and you were told there was a 50 percent chance it would break down, you would really think hard before buying it, wouldn't you? You certainly wouldn't buy the first one you came across, and you would probably ask advice from friends. It seems like when it comes to marriage, we're acting all crazy. This mathematician [Peter Todd from the Max Planck Institute for Psychological Research in Germany] said, hold on, maybe there are some mathematical equations that make us act this way. He thought, in this borderless world where we have an almost unlimited number of potential partners, how many partners should we test before we settle down? The math showed a very revealing pattern. If you use this simple rule, it will give you very good results: Test a sample of 12 partners. Then, after you get to 12, continue testing but take the next best partner that comes along (it could be partner number 13 or partner number 40). Todd found that doing this will give you a 75 percent chance of picking someone with the qualities that you want.

Of course, we can't rely upon mathematics to tell us what those qualities are, right? We have to make up our own list of criteria.

That's right. And mind-blowing sex doesn't even have to be one of them. In fact, you don't really need to sleep with 12 partners -- you could be dating platonically, I suppose. But you need to have 12 relationships with 12 partners. I called it the "Rule of 12 Bonks" rule because it's a reflection of my cheekiness. I'm modern woman in a modern world and sex needs to be one of my criteria!

What about all those people in relationships who aren't even close to their 12th partner? Should they dump their current partner and keep looking, just to be sure?

No. This strategy promises a 75 percent chance of success. It's not foolproof. These people could very likely be in that lucky 25 percent. There are some people that would find true love with Number 3, and other who would need to go up to Number 105.

But it's still interesting that this is such a simple strategy. Back in the '80s, a popular idea in mathematics was that our brains worked with strict mathematical rules, and if we could only create a computer to replicate these rules as fast as possible, we could feasibly replicate human thinking. Now they're thinking that our brains actually work with a bunch of very simple strategies that don't always get perfect results. This "12 Bonks Rule" is one of them.

When I tried to understand the equations, I couldn't even figure out what the symbols meant. Why didn't you include a glossary of symbols?

You know what's interesting? A number of American publishers asked for more math, they wanted me to break things down as much as possible. Honestly, that made me laugh. First of all, there are thousands of math books in the library. And no one is reading them! As an author, I thought that no one would want to hear about the hardcore math. I thought that my job was to create the link between math and sex, to make math more interesting in that way. Who would know that readers would want more math?

It's important to realize that to show the background and explanations for those equations would take another whole book. That work is very complicated. Some of the math that I talk about -- you would need a Ph.D. to understand it. I didn't want to bog the reader down. As for a glossary, if I had included one, the book would be twice the size it is now.

So, I have to ask: Have any of your mathematical strategies helped you in the bar or the bedroom?

Yes, the part about the ups and downs of love in the first chapter. [In this section of the book, Cresswell discusses the work of Steven Strogatz, now a professor at Cornell University, who reworded a common undergraduate mathematics problem to explain the evolution of Romeo and Juliet's love affair. Strogatz shows that Romeo's love depends on Juliet's responses, and vice versa.] The whole idea that when you fall in love you have this horrible experience of an emotional roller coaster -- that helped me understand what I've felt in the past. It helped me deal objectively with those yucky parts -- the anxiety, the nervousness, the self-doubt -- and to remind myself that I'm just going to have to get through it.

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