The calculus of coitus

A new book explores "The Rule of 12 Bonks" and other mathematical equations that help explain sex and relationships.

By Corrie Pikul

Published September 13, 2004 4:42PM (EDT)

Clio Cresswell certainly isn't the first single girl to agonize over the unpredictability of a new relationship, or wonder how many frogs she'll have to kiss before she finally meets her prince. But she's probably one of the few who believe that the answers to such romantic conundrums can be found in numbers -- and not the soft stuff like numerology, either. To make sense of love and life, the 30-year-old Australian mathematician instead turns to hardcore mathematics and mind-bending equations.

In her breezy new book, "Mathematics and Sex," Dr. Cresswell attempts to show that this dreaded discipline is more scintillating -- and more relevant -- than most of us ever dreamed. The book presents current mathematical research that can be used to answer questions like: How will we know when we've found "the one"? How much should individual partners compromise in a relationship? Who has better orgasms, men or women? Cresswell also uses mathematical equations to show how dating services work and why most people will end up happier if they actively proposition as many desirable partners as possible.

While at times Cresswell goes too far in dumbing down the subject -- for people who actually know something about math, the equations she uses will seem sparse and incomplete -- the connections she creates between abstract equations and human behavior are thought-provoking, unexpected and, well, fun.

Defying stereotypes of the unkempt academic, Cresswell (who was voted one of Australia's most beautiful people by a celebrity magazine in 2001) divides her time between lecturing about math at the University of New South Wales, writing an advice column for a women's magazine, juggling public speaking engagements, and talking about relationships and other issues on Australian TV. Salon spoke to her by phone while she was packing for a trip to the U.S.

How did you ever think to connect mathematics and sex?

Mathematics is the relationship of patterns. Obviously, when there are two people interacting, you can witness how they're responding to each other, and you can discern patterns in their behavior. We're very comfortable using math to predict where the stock market is going. But we rarely take the leap that math can be used in other situations.

While doing research, I came across these interesting equations that used math to figure out how much one should compromise in a marriage. These mathematicians took couples, observed their emotional responses to each other, and used similar prediction techniques to chart their relationships and to calculate whether the couples would stay together or break up.

I found these equations fascinating. When I brought them up them in my public speaking engagements, people would invariably laugh, but they'd also ask me more about the equations after the lecture. When I told a friend about this, he suggested that I see if there were more equations like them out there. I said no way -- I was sure it was just a fluke. But when I started researching the area of math and relationships, I found quite a bit of research.

It sounds like you and other mathematicians have figured out the secret to happiness with another person. So, fill us in!

Psychologists John Gottman and Catherine Swanson, and the mathematician James Murray [all from the University of Washington] looked at newlywed couples interacting for 15 minutes, and charted their body language and positive/negative reinforcement. They used techniques that are often used to analyze the stock market to help them analyze happiness and grumpiness. They found these negativity thresholds: Some people express their negativity as soon as they feel it, others hold it in and try to "empathize" for as long as possible with their mate. Gottman and the rest found that the couples that had a low negativity threshold, that expressed their negative feelings straightaway, had a better chance of success. They were still married six years later. The other couples, those who made more of an effort to empathize and stay quiet longer, were less successful. Most ended up divorced.

That's totally counterintuitive.

Yes. These days, there's a lot of talk about empathy: If your partner is doing something wrong, you should try to understand where they're coming from, you should look at their background, their childhood. The math I saw showed that this might actually be a bad approach. What works, according to the math, is when couples are quick to say, "You know, when you do that, that really gets on my nerves!" This coincided with what some psychologists have found: When couples have high standards and stick to them by complaining when things aren't going right, that might actually be a positive thing for the couple. It makes them strive towards these high standards. When members of the couple empathized and kept giving in, they lowered their standards.

Did you write this book to make mathematics more sexy, or to make sex more mathematical, more logical?

It saddens me that society has painted math in such a negative way. Many people miss out on the wonders of mathematics. This book just seemed like the sort of "Sex and the City" style way to try to bridge that gap, to make people realize that math can be fun. And I thought that the whole concept of math and sex, the pairing of the two, was just so funny!

I'm not much of a math person. Help me understand the mathematician's approach to understanding love and sex.

OK, ready? A mathematician would choose a subject -- like love -- and would start thinking, "I think there may be patterns that arise from this subject of love." We would then ask ourselves, "What are the key factors that go into love?" That's where we start by making an abstract move: We have to write the problem into abstract mathematical notations. For love, we might have two people. We might call these people X and Y. Then we would ask, "How are these two people going to interact?" We'd create sample equations with X and Y. For example, we might create one equation predicting that X and Y would fall in love, and then suddenly hate each other the next day. There are obvious patterns to human interaction, so we'd test equations to see what looks right what doesn't look right, what matches what we've observed in the real world and what doesn't. We might prepare an equation, plug in variables, and then say, "Hmmm, that equation may be mathematically correct, but the chance of that happening in the real world is highly unlikely." So we'd pick another equation.

We'd play with different equations and different mathematical analyses to tell us what people are doing in real relationships. In picking equations we'd come across patterns. We may start to see patterns that we may not have noticed otherwise. These patterns may show us things about relationships that we may not have seen or expected.

Like what?

In our love example, our mathematician might say, "People's emotions for each other will oscillate, then calm down in to a strong bond." If we want to take this even further, we may show our equations to psychologists for another opinion, and they may respond, "Well, that's exactly what we've observed!" Then we mathematicians will think to ourselves, "Clearly, we're on to something."

It sounds like, contrary to popular opinion, mathematics is not done in a vacuum. There's always an interplay between math and real life, between what mathematicians observe happening in the tangible world around them, and the patterns they create in the lab.

That's exactly right. People think that math is completely separate, but in my opinion, math is a form of expression. Math is just another way of making sense of the world, like dancing or writing.

So, in order to create order out of chaos, mathematicians assign people and their behaviors symbols, and try to create equations where behavior tends to be predictable?

Yes, and because we're using the abstract variables to try and understand these patterns, it helps us look at the patterns in a different way. It gives us a fresh understanding of how individual elements work together.

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.

Corrie Pikul

Corrie Pikul writes about women's issues and pop culture. She lives in Brooklyn.

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