Michael Trimble, a British professor at the Institute of Neurology in London, begins his new book with Gana the gorilla. In the summer of 2009, 11-year-old Gana gave birth to a boy at a Muenster zoo. But one day in August, the baby suddenly and mysteriously died. Gana held up her son in front of her, staring at his limp body. She held him close, stroking him. To onlookers it appeared that Gana was trying to reawaken him, and, as the hours passed, that she was mourning his passing. Some at the zoo that day cried. But Gana did not. Humans, Trimble tells us, are the only creatures who cry for emotional reasons. “Why Humans Like to Cry” is an exploration of why this would be so, a neuroanatomical “where do tears come from.” It’s also a meditation on human psychology. Many distinctions have been offered between humans and the rest of the animal world, and to this list Trimble adds another: the anguished tear, the apprehension that life is tragic. Trimble answered questions from Mind Matters editor Gareth Cook.

Cook: How did you first become interested in crying?
Trimble: Of course, because I cry, and some things bring tears quite easily, notably music, and opera with the power of the human voice.
Crying tears, for emotional reasons, is unique to humans. There has been a game of catch me if you can, which has been played by those interested in finding attributes or behaviours which separate humans from our nearest living relatives – namely the chimpanzees and bonobos. Certainly our propositional language is very special, but primate communities have very sophisticated ways of communicating. Other contenders, such as play, using tools, or having what is called theory of mind (the sense that I know that others have a mind very like mine, with similar inclinations and intentions) have all been argued as unique to our species, but all these have been demonstrated, in some form, to be found in other primates. Emotional crying makes us human.

Cook: What is known about crying in the animal world?
Trimble: Tears are necessary to keep the eyeball moist, and contain proteins and other substances which maintain the eye healthy and to combat infection. Tearing occurs in many animals in response to irritants which get in the eye, and in some settings tears fall for simple anatomical facts. When an elephant is standing, tears run down the trunk, but when lying down, the flow is impeded and tears may be seen coming from the eyes. It may be that animals that are abused shed tears, from pain, although observations of this are rare.

Cook: How is crying different in humans?
Trimble: Humans cry for many reasons, but crying for emotional reasons and crying in response to aesthetic experiences are unique to us. The former is most associated with loss and bereavement, and the art forms that are most associated with tears are music, literature and poetry. There are very few people who cry looking at paintings, sculptures or lovely buildings. But we also have tears of joy the associated feelings of which last a shorter time than crying in the other circumstances.

Cook: What do you find most interesting about the neuroscience of crying?
Trimble: If it is the case that only humans cry emotionally, then there must have been a time in human evolution when tears took on an additional meaning to their hitherto biological functions , namely as a signal of distress, and a cipher for suffering. In my book I discuss at when in the past our ancestors may come to possess this trait. I suggest that this is connected with the dawning of self-consciousness, with the development of theory of mind, and the realisation that the self and others can disappear. Attachment emotionally to others, with the development of sophisticated facial gestures associated with suffering, and with loss and bereavement ensued. All this before the development of our elegant propositional language. The emotional responses became largely unconscious and innate, and identification of tears as a signal for such distress was an important addition the so called Social brain, the circuitry of which can now be identified in the human brain.
I also discuss the differences between the neuroanatomy of the human brain and that of chimpanzees and other closely related primates, which may explain our ability to respond emotionally with tears to the arts. The brain areas involved are widespread, but link our cerebral cortex especially anteriorly with those areas associated with the representation of emotion – so called limbic structures and our autonomic system. The latter co-ordinates heart rate, breathing, and vocal output, all of which collaborate in the expression of emotion with tears.

Cook: You mention "theory of mind" and crying. Can you tell me more about the connection between the two?
Trimble: Theory of mind refers to an area of social cognition which has developed hugely in humans, although similar abilities in much more limited forms have been shown in chimpanzees. The ability to feel compassion, the embodiment of which relates to our capacity for empathy, is triggered by what the neurologist Antonio Damasio refers to as emotionally competent stimuli. The responses are automatic, unconscious and bound in with our personal memories. Seeing facial expressions of sadness trigger the neuronal circuits related to theory of mind and empathy, which to some extent overlap, and involve, in part, those brain areas that give us our visceral, emotional feelings noted above. The tear, as part of the expression of suffering, became an emblem embroidering the expression. The tear, mythological linked with purity with a pearl shape has provided an image which, over time, has come by itself to symbolise sadness, grief, but also joy in music, poetry and the visual arts.

Cook: What lesson do you think this holds for us?
Trimble: Tears are a natural response to not only suffering, but also to feeling compassion for someone who is shedding tears. There has been much reluctance, especially on behalf of men, to admit to crying, and to crying in public. Yet Greek heroes such as Agamemnon and Achilles cried, and 2012 has seen many public tears, from the winners and losers in the Olympic games, to President Obama who cried after his re-election victory. We should not be afraid of our emotions, especially those related to compassion, since our ability to feel empathy and with that to cry tears, is the foundation of a morality and culture which is exclusively human.

Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist at the Boston Globe. He can be reached at garethideas AT gmail.com or Twitter @garethideas.