COMMENTARY

The Tracks of Our Tears: Why We Cry

Michael R. Trimble, MD, FRCP, FRCPsych

Disclosures

October 18, 2016

Introduction

In the words of literary critic William Hazlitt (1788-1830), "to explain the nature of laughter and tears is to account for the condition of human life; for it is in a manner compounded of these two! It is a tragedy or a comedy—sad or merry, as it happens."[1]

It is the case that many mammals shed tears, especially in response to pain or irritation of the eye, and that tears protect the eye by keeping it moist, but it is a truism, observed by many, including Charles Darwin, that humans are the only living species that cries for emotional reasons. It is also the case that seeing tears flow from the eyes of another evokes prosocial behaviors, and that (mostly) people feel better after crying. Therefore, there must have been a point in the evolution of Homo sapiens when tears became a way to express the mental state of the crier. This exaptation, or change in use of a biologically adapted feature (tears), must have been associated with changes in the social and cultural milieus of our ancestors, and with changes in the neuroanatomy of the human brain, compared with our nearest primate relatives, such as the chimpanzees and bonobos.

It is not clear how far back in evolutionary time one has to go to explore the development of our cultural achievements. The earliest hominids appeared on earth perhaps four and a half million years ago, but many experts believe that there was a "symbolic revolution" some 150,000 years ago. The early hunter-gatherer smallish foraging and social groups were displaced by more sophisticated communities with kinship organizations, ritualized ceremonies, and an unsophisticated propositional language far adrift from the syntactical metaphor-rich languages we use today. At some point after that, Homo sapiens emerged, and human cultures, as we know them today, arose, with symbolism, storytelling, religion and myth, music, and eventually the stage art of tragedy. Somewhere along the way, we started to cry tears of emotion.

Crying is most frequently associated with events of loss, especially bereavement, although we also experience tears of joy and tears evoked by the arts, especially music and stories. This is relevant to our understanding of the evolutionary and social importance of crying. The awareness of the emotions of others (via mirror neurons)—which appeared at some point after the rise of consciousness, and almost certainly consequent to the development of linguistic skills—was signified by the use of deictic words (I, here, and now) that intone individuality, and perhaps dreaming. This allowed for the development of empathy, which is the embodiment of those feelings. It could be that the death of a member of a closely knit social group and the appearance of that person in a dream led to the enhancement of communal activities, such as attempts to find or visit another world, which, in turn, led to storytelling, religious rituals, and ideas of life after death.

During the evolutionary process, the development of the facial muscles allowed for much greater expression in Homo sapiens than in other primates, and the eyes underwent astonishing alterations. Look into the eyes of any other living primate and you will see that the sclera (the area around the iris) is dark. In the human eye, the sclera is white, so changes in the size of the iris that accompany shifting emotions are visible. The psychiatrist Stephen Porges connects[2] the emergence of empathy in primates with the increasing complexity of the autonomic nervous system (which is related to the moment-to-moment tuning of our emotional state) and with an increased sophistication in the social engagement system, which confers the ability to empathize. The ability to feel the sadness of others was a critical part of the development of Homo sapiens, and is directly related to neurobiologic changes that occurred in the central nervous system during the evolutionary process. Recent research in the field of neuroscience has revealed that certain brain circuits are activated, rapidly and unconsciously, when we see another in emotional distress, but the induced prosocial feelings are enhanced considerably, not only by facial expression, but also by the shedding of tears.[3]

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