Six leading scholars reflect on the enduring influence of Charles Darwin and demonstrate how his work continues to inspire scientists across disciplines.
The theory of natural selection that Charles Darwin first articulated in print in 1859 is still the fundamental idea on which all modern biological studies are based today. But Darwin's observations of evolution and sexual selection among humans and other species have reached far beyond the field of biology, to influence, inspire, and inform nearly every scientific discipline.
To ring in the year in which the world will celebrate the 200th anniversary of Darwin's birth, on February 12, and the 150th anniversary of the publication of his most famous work, On the Origin of Species, nine months later, we pay tribute to a legendary member of the New York Academy of Sciences with a collection of essays. The NYAS Magazine invited six leading scholars—historian Janet Browne, psychologist Paul Ekman, astronomist-physicist-biochemist Stuart Kauffman, theoretical physicist-environmental scientist Lord Robert May, geneticist-botanist-philosopher and "evo-devo" pioneer Massimo Pigliucci, and physician-behavioral scientist Charles Raison—to write about how Darwin's observations influenced their own work or field of research. Their respective comments illuminate the wide and varying influence Darwin continues to have on science and on humanity.
Over the years Darwin has become a real person to me. As a historian of science and as one of his several biographers, I have spent some 15 years in his company, often on a daily basis. Through his letters I accompanied him on his Beagle adventures, followed the development of his theories, observed his anxieties about marriage, watched his family grow up, worried with him about illness, and felt heartbroken at the death of his daughter Annie.
Darwin was a traveler, a family man, a thinker, a much-loved husband, father, friend, and neighbor—a likeable and genial figure, as expressive in his letters as he must have been in life. Although his theories were first conceived in the smoky atmosphere of London, just after his return from the Beagle in 1836, his major books and articles were all researched and constructed in the domestic setting of his home at Down House in Kent. There he lived for 40 years with his wife Emma Wedgwood and 10 children, of whom only seven survived to adulthood. The house still exists and is now a museum restored to show how it was in Darwin's time. It is an inspiring place to visit, quiet and rural, and one can almost imagine Darwin stepping in through a doorway. Visitors used to record how he would greet them with an outstretched hand.
So behind that large white Victorian beard, there existed a friendly, stimulating, often enigmatic personality, who still intrigues all those who come into contact with him. How could such a modest and retiring figure come up with the theory that made the modern world?
Writing about the famous has many advantages. Darwin was an eager and regular correspondent with a wide variety of people, and left a copious record of his activities, both personal and scientific. by the time of his death he had become an international celebrity and many manuscripts were preserved by friends and family. Encountering this rich and varied archive—now primarily located in Cambridge University Library—was a major influence on the way that I began to reconceptualize the role that biography might play in the history of science.
For too long, scientific biographies have been regarded as the lighter end of history, suitable mostly for bedtime reading. Instead, biography can also tell us much about the actual creation of science, from the first stirrings of new concepts in an individual's mind, followed by the careful documentation and experiment that usually supports a scientific claim, on to the eventual publication and public response to fresh ideas. Biography can, in fact, illuminate what historians are beginning to call the circulation and accreditation of ideas.
It seemed to me that this modern rethink provided a good opportunity to explore the movement of Darwin's evolutionary ideas from the privacy of his own mind, as expressed in letters and notebooks during his most creative years on the Beagle voyage and immediately afterwards, to the extraordinary public controversy that erupted after he published the Origin of Species. From private to public, from the Beagle years to The Origin of Species: Darwin's intellectual trajectory provided me with a way to investigate the generation and acceptance of new ideas.
Janet Browne is Aramont Professor of the History of Science at Harvard University, and author of the highly acclaimed two-volume biographical study, Charles Darwin: Voyaging, published in 1995, and Charles Darwin: The Power of Place, published in 2002, which won the James Tait Black award for non-fiction in 2004, the W.H. Heinemann Prize from the Royal Literary Society, and the Pfizer Prize from the History of Science Society. She was also associate editor of the early volumes of The Correspondence of Charles Darwin.
A few years ago I had a series of conversations with the Dalai Lama about the nature of emotion and compassion reported in our book Emotional Awareness.1 I explained recent research in which a monkey could get food only by delivering a shock to another monkey. If it was a familiar monkey, the hungry monkey did not attempt to get food for many days. The amount of delayed gratification decreased if it was an unfamiliar monkey, and even more if it was a monkey from a different species.2 Nevertheless, even when the monkey who would suffer was unfamiliar and from another species, there was still some delay in responding to hunger.
This is consistent with Darwin's report of such compassionate actions: "Many animals, however, certainly sympathize with each other's distress or danger. ... I have myself, seen a dog who never passed a great friend of his, a cat, without giving her a few licks with his tongue, a sure sign of a kind feeling in a dog. For with those animals which were benefited by living in close association, the individuals which took the greatest pleasure in society would best escape dangers. Whilst those that cared least for their comrades and lived solitarily would perish in great numbers."3
Importantly Darwin draws our attention to the benefits of compassionate behavior, what de Waals calls reciprocity.2
The Dalai Lama responded to these examples saying: "I fully agree ... in those animals, like turtles, [that do not interact] with the mother, I do not think they have the capacity to show affection. ... Affection [between mother and offspring] brings them together. Without affection, there is no force to develop ... [the necessary] willpower to face difficulties."1
Darwin explained the origin of compassionate actions as follows: "The sight of another person enduring hunger, cold, fatigue revives in us some recollection of these states, which are painful even in the idea. And we are thus impelled to relieve the suffering of another in order that our own painful feelings may be at the same time relieved."1 The Dalai Lama completely agreed, pointing out that when he acts compassionately it helps him at least as much as he helps the person suffering.
Darwin also expressed exactly the same ethic that can be found in Buddhism scripts from centuries earlier: "As man advances in civilization, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all the members of the same nation, though personally unknown to him. ... there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races. If indeed such men are separated from him by great differences in appearance or habits, experience, unfortunately, shows us how long it is before we look at them as our fellow creatures. Sympathy beyond the confines of man, that is, humanity to the lower animals seems to be one of the latest moral acquisitions. This virtue, one of the noblest with which man is endowed, seems to arise incidentally from our sympathies, becoming more tender and widely diffused, until they are extended to all sentient beings."1
Hearing this, the Dalai Lama pronounced himself a Darwinian!
Paul Ekman is Professor Emeritus of Psychology at the University of California, San Francisco, Medical School and director of the Paul Ekman Group, a small company that produces training devices relevant to emotional skills, and is initiating new research relevant to national security and law enforcement. Ekman is author and co-author of numerous books on the evolution of human facial expression, and was editor of Darwin and Facial Expression (1973), of the third edition of Darwin's The Expression of the Emotions in Man and Animals (1998), and of the Annals of the New York Academy of Sciences, Vol. 1000, Dec. 2003, Emotions Inside Out: 130 Years after Darwin's The Expression of the Emotions in Man and Animals.
Charles Darwin made what may be the greatest change in Western thinking. With him, history and historical processes emerge as a central focus for scientific thought. With Darwin, heritable variation, and natural selection, we have for the first time a start for understanding the becoming of the biosphere. In this short article I want to discuss two features of evolution that remain largely outside mainstream discussion of evolution, yet are of central importance to the evolution of the biosphere.
First, Darwin did not know of self-organization. Today, in part due to the sciences of complexity and the computer as a kind of "macroscope," we begin to see such self-organization. Does it play a role in evolution? If so, how does it mingle with selection as interwoven sources of order in organisms down the evolutionary pathways?
Second, what is the physical basis of the historical processes of which Darwin made us aware? We shall see that we are led to issues of the "open universe."
Darwin did not know about self-organization. Physicists do, of course, as in Benard cells and the Zhabotinski reaction. Snow flakes show six-fold exquisite symmetry without benefit of natural selection. Cholesterol dissolved in water forms liposomes, bi-lipid-layered vesicles that must be the origins of cell membranes, but arise without selection. For many years I have studied models of genetic regulatory networks, where I have modeled genes as binary, on/off devices and studied random Boolean nets.
More than 40 years of work shows that such networks behave in either an ordered or a chaotic regime, separated by a critical phase transition. The ordered regime and the critical phase transition demonstrate astonishing order. I will not describe it here, other than to say that our intuitions about the requirements for dynamical order have been drastically wrong. And I add two thoughts. Such networks have dynamical attractors. I have long thought that cell types correspond to attractors. I have also hoped that cells are dynamically critical, poised at the edge of chaos. Some recent evidence suggests this may be true.
If cells are critical, then we have before us an example of the marriage of self-organization and selection. In a parameter space concerning features such as the mean number of inputs per gene, the input distribution, biases on Boolean functions used in the network and so forth, critical networks are extremely rare, occurring on a critical surface in parameter space separating ordered from chaotic behavior. But if cells are critical, this suggests both that the generic self-organized behavior of complex non-linear dynamical systems such as genetic regulatory networks may readily afford "ordered" and critical dynamics without much selection, but that substantial selection is required to achieve and maintain critical behavior.
This means that we must rethink evolution. Selection is not the sole source of order in organisms. Neither is self-organization. We must understand both and their marriage. Somehow I think Darwin would have been delighted.
II. The Open Universe
Consider first the set of all possible proteins with a length of 200 amino acids. Since there are 20 kinds of amino acids, there are 20200 such proteins. We can easily now synthesize any one of them.
Now consider that the universe is 1017 seconds old and has about 1080 particles in it. It is easy to calculate that, were the universe doing nothing on the Planck time scale of 10−43 seconds but making proteins length 200, it would take 1039 times the current lifetime of the universe to make all these proteins just once.
This means that once we are above the level of complexity of atoms, where all possible atoms exist in the universe, the universe is on a unique trajectory. We will never make all possible proteins, complex molecules, organs, organisms, social systems. The universe is indefinitely open "upward" in complexity. More, when the space of the possible is vastly larger than the space of the actual, history enters. Here is the root of Darwin's historicity.
Now, suppose reductionism were right. Suppose that, when the science shall have been done, there really is some final theory as Nobel Laureate Steven Weinberg hoped, and perhaps still hopes, despite 10500 string theories. Then all that exists in the universe would be entailed by that final theory, and the existence of all that exists would be explained.
What if reductionism fails? What if the becoming of the biosphere is partially lawless? Then the very existence of many things becomes a matter to conjure upon. Consider a hummingbird and a field of flowers. The hummingbird puts her beak into one flower to eat the nectar, some pollen rubs onto her beak and sticks to it. She flies to the next flower, eats some nectar, and the pollen from the first flower rubs off on the stamen of the second flower. The hummingbird pollinates the flower.
Suppose before the hummingbird left the first flower, all the pollen fell off her beak, or she regularly flew to a nearby tree to eat the nectar. Pollenization would not occur.
Thus it is by the quixotic fact of the stickiness of the hummingbird's beak that both the hummingbird and the flowers exist and, with insects, have co-evolved for millions of years. They are conditions of one another's physical existence in the universe by virtue of a mutualism. But the biosphere as a whole is a vast mutualism in the sense that all in it exist with only sunshine and a few minerals, making natural games by which all live and evolve. The ground of our existence, then, is not to be found in physics alone, but also in the partially lawless becoming of the biosphere, econosphere, culture that we self-consistently co-construct. (See Reinventing the Sacred, Kauffman, 2008.)
Darwin, it appears, started us down a path now beyond his stunning dreams.
Stuart Kauffman is Professor in the Department of Biological Sciences and Physics and Astronomy at the University of Calgary. He is also Director of the Institute for Biocomplexity and Informatics, as well as an Emeritus Professor of Biochemistry at the University of Pennsylvania, a MacArthur Fellow, and an external professor at the Santa Fe Institute. Originally a medical doctor, Kauffman's primary work has been as a theoretical biologist studying the origin of life and molecular organization. He is the author of The Origins of Order, At Home in the Universe: The Search for the Laws of Self-Organization, Investigations, and Reinventing the Sacred: A New View of Science, Reason, and Religion.
In his own time, Darwin's theory of evolution had serious scientific difficulties. For one thing, the physics of his day put a limit of a few million years both on the life of the sun, and for planet Earth to have cooled from a molten ball to a frozen mass. For another, prevailing ideas about "blending inheritance" were irreconcilable with preserving variability within populations, upon which his theory was based. Happily, the former difficulty was resolved by our discovery of nuclear forces (which fuel the sun, and warm the earth by radioactive decays within its core) and the latter by rediscovery of Mendel (and consequent understanding of particulate inheritance by Hardy and Weinberg).
Darwin, however, arguably saw his most important unsolved problem as explaining how cooperative behavior among animals evolved. At first glance, the answer seems easy. You pay some small cost to gather a much larger cooperative benefit. For example, a prairie dog takes a personal risk in giving an alarm call, but all the colony benefits and, by taking turns as alarm giver, each individual's group benefit exceeds the occasional risk. But any such arrangement is immediately vulnerable to cheats who enjoy the benefits without paying the risk-taking dues. In evolutionary terms, such cheats have a selective advantage (today we would say their survivorship advantage means their genes are more represented in the next generation). So it is unclear how such observed cooperative phenomena can arise or be maintained.
Following work on "kin selection" by Hamilton and others a century after Darwin, we now understand how such cooperative associations can evolve and persist in relatively small groups of sufficiently closely related individuals. Moreover, these conditions could apply to humans when we were small bands of hunter-gatherers. But for large aggregations of essentially unrelated individuals, as developed once agriculture appeared and cities began, the origin of cooperative associations—with group benefits which exceed the "cost of membership"—remains as puzzling today as it was for Darwin.
Nor is this some abstract, academic problem. The past 150 years had seen the human population increase sevenfold, and the ecological footprint of the average individual also increase sevenfold, for an overall 50-fold rise in our impacts on the planet. And still these impacts are increasing. There are consequently huge and global problems—climate change, loss of biological diversity, pressure on water supplies, and much else—which demand globally cooperative solutions. These problems are further compounded by the fact that nations must cooperate, but in equitable proportions.
These problems have recently received much attention in the scholarly literature, employing a variety of metaphors: the Tragedy of the Commons; the Free-Rider problem; the Prisoner's Dilemma; and others. These metaphors are allied to artificial games in which the subjects (usually undergraduates) trade small sums of money to test limits to altruism and tolerance of cheating. Incidentally, essentially none of this work involves the costs and benefits varying among the players, as it usually does in the real world.
My own speculation about how cooperative human societies evolved is both less academic and analytic, and more gloomy. Once we move out of the mists of pre-history, we find stories of dreamtime, creation myths, ceremonies and initiation rites, spirits and gods, with a unifying theme that all seek simultaneously to help explain the external world and also to provide a "stabilization matrix" for a cohesive society. There are, moreover, some striking and unexplained similarities in belief systems and rituals from different times and places. Conscience, a simple word for a complex concept which helps foster behaviour in accord with society's professed norms, has been memorably defined by H. L. Mencken as "the inner voice which warns us that somebody might be looking." And how helpful it is if that somebody is an all-seeing, all-knowing supernatural entity.
Common to these conjectured "stabilizing forces" in essentially all earlier societies are hierarchical structures, serving and interpreting the divine being or pantheon, along with unquestioning respect for authority. In such systems, faith trumps evidence.
But if indeed this is broadly the explanation for how cooperative behaviour has evolved and been maintained in human society, it could be Bad News. That is because, although such authoritarian systems seem to be good at preserving social coherence and an orderly society, they are, by the same token, not good at adapting to change. Diamond's book, Collapse: How Societies Choose to Fail or Survive, provides striking examples.
A fundamental principle emerging from the Neo-Darwinian Revolution of around a century ago is Fisher's Fundamental Theorem, which states that a population's potential rate of change of gene frequency (which measures its ability to adapt to changing circumstances) is proportional to the variance in gene frequency, which will be small if essentially all individuals are well-adapted to their current environment. That is, there is an inherent tension between adaptedness and adaptability. If there is any substance in my speculations about the answer to Darwin's problem in explaining cooperation in human societies, we again have a fundamental tension—at the level of the entire society—between, on the one hand, "ties that bind" and permit stably cooperative aggregations, and, on the other hand, ability to respond effectively to changing environmental circumstances. It could even be argued that the recent rise of fundamentalism, in both East and West, is an illustration of this meta-level version of Fisher's Fundamental Theorem, as complex faiths are reduced to intolerant ideologies to resist the challenge of societal change. It is a pity that Darwin is not here to help us.
Robert, Lord May of Oxford, holds a Professorship jointly at Oxford University and Imperial College, London, and is a Fellow of Merton College, Oxford. He was President of The Royal Society (2000–2005), and Chief Scientific Adviser to the UK Government and Head of the UK Office of Science and Technology (1995–2000). He has also served as a Personal Chair in Physics at Sydney University, Professor of Zoology and Chairman of the Research Board at Princeton, and Royal Society Research Professor at Oxford and Imperial College. He is recognized for his research into how populations are structured and respond to change, particularly with respect to infectious diseases and biodiversity.
Charles Darwin had a difficult job in 1859: although a few people before him had floated the idea that living organisms evolve, no known mechanism for their alleged change over time was known, and little compelling evidence was available in favor of it. Indeed, the dominant paradigm among naturalists was William Paley's inference from biological complexity to supernatural intelligent design.
Darwin met the challenge by amassing an impressive amount of observations that pointed to two conclusions: all living organisms share common ancestors, and the chief mechanism responsible for the appearance of a "fit" between organisms and their environment is natural selection. Part of the evidence brought into play by Darwin and other early evolutionary biologists in favor of the view of evolution came from developmental biology, and consisted in showing that organisms of the same type (say, vertebrates) have very similar stages of development before they differentiate into the variety of adult forms that distinguish birds from reptiles and from mammals. This similarity in development was the result of their evolution from a common ancestor.
Despite its early central role in evolutionary biology, developmental biology remained a separate field of inquiry for most of the 20th century, especially during the 1940s, which brought us the mature version of evolutionary theory known as the Modern Synthesis. For decades biologists drew a sharp distinction between proximate and ultimate causes of the phenomena they studied: proximate causes deal with how living organisms are built (e.g., the molecules and developmental mechanisms that form the eye), while ultimate causes inquire into why they are built in a particular way (e.g., the eye's function to capture information about the environment aids survival and reproduction). Developmental biology (as well as genetics and molecular biology) was thought to address the first type of question, while evolutionary biology remained focused on the second one.
This neat distinction between types of causes would have seemed strange to Darwin, and it is being rejected by modern evolutionary developmental biologists, who are inspired by Darwin's comprehensive approach to understanding life's history and diversity. During the 1990s a new field of research, nicknamed "evo-devo" for "evolution of development" emerged with the declared goal of once again joining the two disciplines. The fundamental idea was that knowing how organisms develop also gives us invaluable clues to how they evolved: the "how" question informs the "why" question.
My own interest in this area has been spurred by the possibility that taking development seriously will usher in an expanded version of evolutionary theory, one where new phenomena—unknown to Darwin and to 20th century biologists—play an hitherto unexpected role. Take for instance the idea of "evolvability." The term refers to the fact that the ability of different lineages of organisms to evolve itself changes over time; that is, the very capacity for evolution evolves! We have known for some time that populations of organisms with different amounts of genetic variation respond at different rates to natural selection, some evolving faster than others because they harbor more genetic variants that can be selected. But recent discoveries in developmental biology have presented us with the intriguing possibility that natural selection may favor the evolution of particular molecules (called "capacitors" of evolution), or arrangements of gene networks, that make it easier for a population to evolve in response to new environmental challenges.
The implications of these views are still being worked out, and both theoretical and empirical biologists feel the excitement of the opening up of new vistas on the process of evolution, a prospect that would have delighted Darwin. As he famously put it: "There is grandeur in this view of life ... that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved."
Massimo Pigliucci is a Professor of Ecology and Evolution and of Philosophy at Stony Brook University in New York. He holds doctorate degrees in genetics, botany, and philosophy, and is known as an outspoken opponent of creationism and staunch supporter of science education. In 1997, he received the Dobzhansky Prize from the Society for the Study of Evolution, which recognizes the accomplishments and future promise of an outstanding young evolutionary biologist. His essays can be found at rationallyspeaking.org
During recent lectures, I've found myself invoking the old saying "all roads lead to Rome" as a way of embarking on discussions of the multiple ways in which innate immune, autonomic, and neuroendocrine pathways converge to promote depression in response to stress and sickness. This old chestnut about Rome is also profoundly true in regards to evolution. The great Roman edifices of evolution are survival and reproduction: as long as the road a species is on eventually passes through these portals, that road is a viable path, regardless of the length of the journey or the variety of scenery on the way.
An immediate implication of this truth is that all things not prohibited by the twin mandates of survival and reproduction are allowed. Indeed, some theoreticians suggest that all things not prohibited must be manifested somewhere across the vast reaches of universal space and time.4 Whether this is true or not, even a cursory look at the manifold pathways walked by life on earth makes the head spin. As I type this I am sitting in the Arizona-Sonora Desert Museum, and just from the last hour of wandering I have seen species that have survived by being aggressive or being peaceful, by being solitary or being highly social, by killing other beings to eat or eating sunlight directly. Evolution is not mandatorily about egregious selfishness. It is about anything that works, and even our one fragile planet testifies to the fact that almost anything—done well—can represent a viable strategy through the vast labyrinth of evolutionary design space. Increasingly, evolutionary scientists are recognizing that cooperation, connection, and reciprocity represent one such strategy, giving the lie to older simplistic ideas that evolution mandates a natural order that is unremittingly "red in tooth and claw".5
My realization of this truth while reading Darwin's Dangerous Idea, by Daniel Dennett, permanently altered the direction of my research.6 Prior to this realization, my professional work on inflammatory processes and psychiatric symptoms seemed unrelated to my hobbyist's interest in the health benefits of psychosocial connectivity.7 After the realization, I began to wonder whether social integration and inflammation might not be intimately locked in a struggle for the soul of humanity. Fueled by a hunger to bring evolutionary causality into my work with proximal mechanisms, I retooled my life's work.
My idea of an unsuspected evolutionary link between love and immunity seems odd at first blush, but makes much more sense when seen against the backdrop of our phylogeny, in which natural selection has favored robust brain-body danger response pathways (including inflammation) to protect individuals against predators and pathogens—whether encountered through killing, being killed, or being infected.8 In the modern world, however, with its laws, medical expertise, and sanitary practices, our need for such hair-trigger inflammatory pathways has waned, because civilization has become an extended phenotype for battling pathogens against which our one reliable defense used to be inflammation. Nonetheless, the old biology persists, with nothing better to do most of the time than fire off in response to all sorts of novel situations against which it is of little use. Hence the contribution of stress-induced inflammation to all the major modern maladies—and the potential for social embeddedness to shoulder at least some of its burden.
This line of reasoning made me begin to wonder whether training people to re-envision their social surround in more positive terms might enhance health by attenuating inflammatory responses to psychosocial stress—a thought I never would have had without an evolutionary perspective. But we reasoned that if people could be encouraged to see the social world as being less threatening and more supportive they would be less likely to activate danger pathways that trigger inflammatory responses, resulting in reduced wear and tear on the body and brain over time.
To test this idea my colleagues and I selected what appears to me to be the most radical program ever designed to change how we view our social connections—Tibetan Buddhist compassion meditation. And indeed, data from our group do suggest that when people learn through compassion meditation to see the world more realistically (i.e. as a safer, more nurturing place) than our old danger pathways would advise, inflammatory responses are reduced and people become less upset in the face of the types of social stressors that provide such rich fodder for illness in the modern world.9 Of course, to tolerate reduced inflammation safely we must keep civilization in tact, which in turn links compassion to odd bed partners like waste disposal and vaccines. But I am far over my word limit and these realities, alas, represent other roads to Rome.
Charles Raison is Assistant Professor in the Department of Psychiatry and Behavioral Sciences and Co-Director of Collaborative for Contemplative Studies at Emory University in Atlanta. He is a physician whose research ranges from immune system effects on central nervous system functioning to the application of compassion meditation as a strategy to prevent depressive symptoms in college students via reduction in stress-related inflammatory activity. He is also internationally recognized for his expertise in the diagnosis and treatment of interferon-alpha-induced depression and anxiety.
- Emotional Awareness, Dalai Lama & Paul Ekman, New York: Henry Holt, 2008.
- Primates and Philosophers, Frans B.M. de Waal, Princeton, NJ: Princeton University Press, 2006.
- The Descent of Man, Charles Darwin, New York: Appleton and Company, 1871.
- Deutsch D. The Fabric of Reality. New York: Penguin; 1997.
- Wilson DL. Evolution for Everyone. New York: Delacorte Press; 2007.
- Dennett DD. Darwin's Dangerous Idea: Evolution and the Meanings of Life. New York: Penguin Science; 1995.
- Cacioppo JT. Loneliness: Human Nature and the Need for Social Connection. New York: W.W. Norton; 2008.
- Raison CL, Capuron L, Miller AH. Cytokines sing the blues: inflammation and the pathogenesis of major depression. Trends in Immunology 2006;27(1):24-31.
- Pace TWW, Negi LT, Adame DD, Cole SP, Sivilli TS, Brown T, Issa MJ, Raison CL. Effect of compassion meditation on neuroendocrine, innate immune and behavioral responses to psychosocial stress. Psychoneuroendocrinology 2008; Epub.