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[August, 2013]

Foreword to 20th Anniversary edition of Robin Baker’s Sperm Wars

Todd K. Shackelford

Oakland University 2

As an undergraduate at The University of New Mexico in the early 1990s, I took a course on the of human sexual behavior with evolutionary biologist Randy Thornhill. This course would change my life, setting me on a career path in academia focused on investigating the evolutionary psychology of . I loved the course, all of the material. But it was Randy’s presentation and discussion of a particular area of research that most profoundly affected me, an area of research that I have pursued as an evolutionary psychologist for more than two decades. About midway through the term, Randy introduced the research on human conducted by evolutionary biologists Robin Baker and Mark Bellis. I could not believe what I was hearing—it was amazing! Baker and Bellis had provided the first empirical evidence that humans have evolved adaptations to sperm competition. Their work indicated, for example, that men adjusted the number of sperm in their ejaculates as a function of the proportion of time they spent apart from their regular partner (a proxy for the risk of sperm competition) since the couple last had sex. And women contrived to place the ejaculates from different men into competition. What’s more, women deployed their orgasms to bias the retention of the sperm from favored men. The evidence was there, published in the most prestigious journals in evolutionary science. Having spent most of my undergraduate career in psychology, learning mostly about humans, this was my first exposure not just to sperm competition in humans, but to sperm competition, more generally. What an amazing phenomenon, producing such beautiful adaptations in species as diverse as nematodes, birds, snakes, and humans.

I have been obsessed with sperm competition theory and its application to humans since first learning about the research conducted by Baker and Bellis. My own research (see www.ToddKShackelford.com) has focused on identifying the evolved psychology in men and 3 women “designed” to solve the adaptive problems of sperm competition recurrently confronted by our ancestors. One of our first studies (published in 2002 in the journal Evolution and Human

Behavior) followed directly from Baker and Bellis’s research documenting ejaculate adjustments in men as a function of sperm competition risk. Baker and Bellis had shown that human males, like the males of many other socially monogamous, paternally-investing species, inseminate greater numbers of sperm into their regular partner following a period of separation—time during which men cannot account for their partner’s sexual behavior. But how do men do this?

What is going on psychologically? There must be evolved psychological mechanisms that register the greater risk of sperm competition, then send this information along to other evolved mechanisms, finally to cause an increase in the number of sperm delivered in the next ejaculate.

Our research therefore attempted to fill in the psychological gap between sperm competition risk and ejaculate adjustment. In series of studies, we have done just that. We have documented, for example, that with a greater proportion of time spent apart from their regular partner since the couple’s last copulation, men report that their regular partner is more sexually attractive and more physically attractive, both to them and to other men. They also report greater copulatory urgency—they want to have sex as soon as possible, and they even admit to being more likely to coerce sex from their regular partner following periods of greater sperm competition risk. And we have only just scratched the surface.

My research career was and continues to be inspired by the pioneering work of Robin

Baker and Mark Bellis. Their 1995 book, Human sperm competition, summarized their own brilliant research and still today provides a superb scholarly introduction to sperm competition theory, and to the application of this theory to humans. Human sperm competition was an academic book, designed to share their theoretical and empirical harvest with scientists and 4 scholars. The book documents in meticulous detail the samples, methods, and statistical analyses they used to conduct their research, and is capped by a thorough listing of the many hundreds of scholarly references upon which their research was built. The book you are about to read, Sperm wars, is designed instead to bring this work to a broad audience of non-academics, people who are curious about human sexual behavior (who isn’t?!). Sperm wars is a masterpiece of science writing, and for good reason was an international bestseller when it was first published in 1996.

Robin Baker is the rare academic who also writes beautifully for popular audiences. Indeed, he has since gone on to publish several fiction books—each inspired by some of his own groundbreaking work on human sperm competition, including Primal, Caballito, and The hitchhiker’s child. Few popular science books are so important and impactful that they are published again in an anniversary edition. Sperm wars is one of those few books, republished in

2006, the 10-year publication anniversary, and with a new introduction by Robin Baker. And here we are now, at the 20th anniversary of Sperm wars, another republication in hand. As well it should be. Sperm wars, like no other book, brought to a stunningly wide audience the magnificent, even stupefying, phenomena of sperm competition in humans. Through a series of engaging short stories, followed by Baker’s insightful commentary, the reader is led through fictional, but real-to-life events of men and women navigating the vagaries of sexual conflict and sperm competition. It is time once again to celebrate Robin Baker’s intellectual genius and especially his classic book, Sperm wars. I have now lost count of the number of times I have read this book, gaining something new each time over the past two decades. If this is your first read, I envy you.