The Physics of Crowds Once Social Scientists Inspired Physicists

books and arts The physics of crowds Once social scientists inspired physicists. Now the reverse is true.

Critical Mass: How One Thing AP Leads to Another by Philip Ball William Heinemann: 2004. 644 pp. £25 To be published by Farrar, Straus and Giroux in the US Steven Strogatz

Psychohistory is a remarkably accurate theory of humans’ collective behaviour. It predicts the rise and fall of political move- ments and empires, and even the sweep of civilization itself. Like the calculations at the root of the insurance industry, it is a fundamentally statistical science, valid only for groups large enough for the vagaries of individuals to average out. Of course, this theory is the stuff of science fiction, as readers of Isaac Asimov’s Foundation series will recognize. But in the past decade or two, the dream of psychohistory — although admittedly in a much more modest form — has seduced a motley crew of physicists, complex-systems theor- ists and social scientists. Borrowing the tools of statistical mechanics and nonlinear dynamics, and armed with unprecedented computer power and huge data sets, they Human interest: physicists have tried to explain processes from the action of crowds to the economy. have re-examined some classic puzzles about human group behaviour and its social, statistical point of view, Maxwell wrote: political upheavals in seventeenth-century economic and political ramifications. These “Those uniformities which we observe in England that gave rise to Thomas Hobbes’ include the causes of boom–bust cycles in our experiments with quantities of matter vision of utopia, to van der Waals’ theory the economy; the long-tailed distribution of containing millions and millions of mole- of phase transitions between liquids and wealth; the stampedes of panicked crowds; cules are uniformities of the same kind as gases. Furthermore, unlike so many recent how altruism can emerge in a group of self- those explained by Laplace and wondered at books about complexity theory, this one sets ish individuals; and the infuriating waves by Buckle arising from the slumping togeth- everything in proper historical context, and of congestion that can clog traffic on an er of multitudes of causes each of which is by does not oversell its ideas. Ball even draws otherwise open highway. The goal, in short, no means uniform with the others.” on pop literature. For example, to illustrate is to develop a physics of society. Even the term ‘statistics’originated in this the abruptness and long-range character of In his fascinating new book Critical Mass, movement to quantify social phenomena.In phase transitions, he invokes Kurt Vonne- Philip Ball tells the story of this research in 1749, the German scholar Gottfried Achen- gut’s Grand Ah-Whoom from Cat’s Cradle, a comprehensive and often captivating way. wall suggested it as a name for the numerical in which a shard of ice-nine seeds the freez- But what I liked best about the book is that study of the states of society, which were just ing of all the world’s oceans. Ball delves far beyond today’s headlines. In beginning to be summarized in terms of On the other hand, I did find Ball’s treat- particular, I was surprised to learn that this death rates, birth rates and other population ment of the discovery process a bit disem- intellectual thread has a long history that measures. bodied. Or to say that more positively, this is the exact reverse of what is generally Critical Mass is full of historical tidbits is a book about ideas, not about scientists supposed. Statistical physics did not spawn like this. Ball particularly enjoys setting the and their struggles. You won’t learn much social physics;it was the other way around. record straight on matters of nomenclature, about how the pioneers in this field made In the 1870s, when James Clerk Maxwell priority and the like. Thus we learn that their breakthroughs, or about what drove was developing the kinetic theory of gases, Adam Smith argued that a hidden hand, them, or what they were like. But perhaps he knew it would be impossible to track the not an invisible one, ruled free markets, that would be asking too much. The book motion of countless particles ricocheting in and that Maxwell’s demon was so named by is already massive, and its heft made me a hot gas. But he drew hope from an earlier William Thomson, much to the chagrin of wonder if its title were a pun. generation of social statisticians, who had the devoutly religious Maxwell. Maxwell Speaking of which, the title Critical Mass: demonstrated that enormous populations of had described his hypothetical creature How One Thing Leads to Another struck me people — who were individually every bit as more lovingly, as “a very observant and as uncannily (or perhaps intentionally?) capricious as gas molecules — could never- neat-figured being”. close to the title of Malcolm Gladwell’s theless display precise regularities. After This book is impressively clear and recent bestseller, The Tipping Point: How reading Henry Thomas Buckle’s History of breathtaking in scope. Ball explains the Little Things Can Make a Big Difference.Both Civilization in England,which espoused this basics of every subject he treats, from the books deal with collective phenomena in

NATURE | VOL 428 | 25 MARCH 2004 | www.nature.com/nature 367 © 2004 Nature Publishing Group books and arts society, or what Gladwell calls social epi- demics, but the resemblance ends there. Gladwell’s book is fluffy and full of entertain- ing anecdotes, but is often unconvincing. Ball’s is substantial, impeccably researched and generally more persuasive. For anyone who would like to learn about the intellectual ferment at the surprising junction of physics and social science, Critical Mass is the place to start. ■ Steven Strogatz is in the Center for Applied Mathematics and the Department of Theoretical and Applied Mechanics, Cornell University, Ithaca, New York 14853-1503, USA.

Chemistry between man and insect which they can For Love of Insects feed on aphids by Thomas Eisner while avoiding the Harvard University Press: 2003. 464 pp. ants guarding the $29.95, £19.95 aphid colony. We Jeremy N. McNeil also learn of the ingenious ways in Chemical ecology, the study of how natural which defences chemicals affect the interactions between may be overcome, organisms, has become a well-recognized such as the herbi- discipline in the past 50 years. A number of vorous insects that remarkable scientists have moulded the can feed on milk- field, many of whom mix their love of sci- weed leaves by ence with an appreciation for the arts and severing the veins life in general, making them true Renais- that carry latex to sance individuals. Thomas Eisner, author of the leaf, or how For Love of Insects, is one such person: as some predators eat well as a world-renowned scientist and only part of their teacher, he is an excellent photographer and prey to avoid tox- Termites (top) and blister beetles secrete chemicals with very different effects. talented musician. ins. And if one is For Love of Insects is not your typical bio- on the lookout for research projects, Eisner Jeff Dean and 17 others who identified logical text integrating information from provides some pointers. For example, what and acquired biological material, provided the literature on a topic. Instead this book is the role of the second soldier caste in the insights into chemical defences and the role is a series of vignettes. Each of the ten chap- termite Nasutitermes exitiosus? Are blister of benzoquinones in particular, and worked EISNER, CORNELL UNIV. T. ters discusses a different chemical ecology beetles the only source of cantharidin, a out how to photograph the spectacular project that Eisner and his colleagues have defence chemical that is better known as the defensive behaviour. carried out over the past 40 years. The book pre-Viagra Spanish fly,for cantharidiphilous Two collaborators stand out. Eisner has is well written and beautifully illustrated insects? a long-standing association with chemist with colour photographs,the majority taken Second, the author shows what is needed Jerry Meinwald, who identified many of by the author. I would strongly recommend to produce first-class ecological research. the compounds involved in the interactions this book for several reasons. Like Eisner,one must combine keen observa- observed. The other is his wife, Maria First, each chapter introduces the reader tion with a desire to understand the phe- Eisner,who took the excellent scanning elec- to different facets of the wonderful world of nomena observed. Furthermore, one needs tron micrographs found in this book and insects. We learn how many insects defend to design elegant and simple experiments in many of their publications. Both share themselves by spraying aggressors with nox- to answer ‘why?’ and ‘how?’ questions. Eisner’s love of music, and I have heard ious secretions,how they hit their target,and This underscores the importance of creative Tom and Jerry perform at several scientific how they keep themselves from suffering the thinking — a timely reminder in the post- meetings. effects of the compounds they deploy. We genomic world that good science is not just a Throughout the text one is reminded of discover that insects may produce their own matter of fancy equipment and sophisticated the pleasure that the author derives from compounds or get them from their food. In techniques. discovery. Anyone reading this book will some instances, females may receive them Collaboration is another key ingredient. themselves embark upon a journey of dis- from males during mating, and the quantity Eisner’s research into the bombardier beetle, covery and come to share, if only at arm’s of a male’s defence compounds may deter- an insect that defends itself with boiling length,Tom’s love of insects and the wonders mine whether he gets a mate. secretions — rather like a castle’s soldiers of nature. ■ Some insects defend themselves with pouring hot oil on the heads of besiegers — Jeremy N. McNeil is in the Department of Biology, their excrement, or disguise themselves with spanned more than 30 years. He credits his University of Western Ontario, London, flower petals. Others adopt a strategy by graduate students Daniel Aneshansley and Ontario N6A 5B7, Canada.