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A sick baby cries long and plaintively in the night. A young woman sobs in despair and disbelief at her husband’s death in the September 11 attacks. Under cover of darkness, furtive tears swell in the eyes of a grown man watching the movie Titanic. As others grin and congratulate a bride, her mother weeps uncontrollably. Crying is at once among the most familiar and the most mysterious of human behaviors. Certainly human facial anatomy and physiology are intricately engineered for both the discharge of tears and the facial and vocal expressions that accompany them—but for what purpose?
Tears have been around a very long time. All animals with mobile eyes have nictitating membranes, or inner eyelids, and tears that aid in opening, closing, washing, lubricating, and protecting the eye’s delicate, transparent, adaptive lenses. Only humans, however, have evolved the tearing mechanism into a means of complex emotional expression, one of the behaviors that make us unique among animals, even primates. Because every innate behavior is explainable in terms of adaptation for survival, we know that emotional crying must serve some important function in humans.
Observing that we shed tears on our face—the part of our body most visible and conspicuous to our social companions—some people have theorized that crying evolved as one of the many facial movements by which we communicate. Facial sensations, movements, and lacrimal secretions (tears) are all controlled by the same facial nerve. The human embryo’s branchial arch, which becomes the lower part of the face, the neck, the lungs, and the heart, controls chewing, swallowing, breathing, and vocalizing. Two of our most basic emotional expressions, laughing and crying, occur in this part of our body, use the same muscles, and are synchronized with strong respiratory movements and vocalization. They do, however, involve strikingly different patterns of activity to achieve their unique forms of communication.
Crying may have evolved to call attention to our distress by exaggerating the normal tearing that occurs when the facial nerve is activated. We can see this normal tearing when yawning or laughing too hard makes our eyes water, or when irritation of our eyes and the area around them stimulates tearing. It seems likely that, in our distant past, tearing, together with vocalization, greatly increased the chances for survival of those infants who had acquired this capacity through random genetic mutation. As a result of the advantage it conferred, the behavior became lodged in our genome, and tears became a sign, and a symbol, of suffering.
As a mechanism of evolution, this kind of anatomical or physiological exaggeration is well known. The peacock’s spectacular tail and the male caribou’s gigantic antlers are both the work of a genetic positive feedback mechanism, increasing the size of the feature a little every generation because evolution favored the reproductive advantage that it conferred. Shedding tears could be a product of this same process.
Crying Rats, Weeping Elephants
Although complex emotional tears may be uniquely human, virtually all baby birds and mammals have peculiar, high-pitched, repetitive calls for help. This “separation cry” for their caretaker has obvious survival advantages, not only eliciting rescue, help, and feeding, but also signaling location. High-frequency, repetitive calls, much like a siren, transmit effectively and over long distances.
Psychobiologist Jaak Panksepp notes that infant rats emit separation cries that could be interpreted as a sign of emotional distress as well as a signal for help and retrieval by the mother. While dog pups whine in a range we can hear, rat pups’ calls are in the ultrasound (30-50 KHz) range, too high for us to pick up but perfect for communicating at a distance with mother rats. Rat and dog puppies, just like human babies, quiet down immediately when placed in close contact with the body of a caretaker or littermate.
Neurophysiologist Paul MacLean suggests that separation cries may be an ancient form of vocalizing in hominids, preceding speech. He reports that these cries are similar in species from marmosets to humans. Since the earliest species of mammals were probably nocturnal forest dwellers, crying favored survival of infants by helping parents ﬁnd them quickly at night, in caves, and in holes.
So animals cry, but do they express emotion in doing so? Jeffrey Masson, author of When Elephants Weep: The Emotional Lives of Animals,1 argues that many animals display sadness. Charles Darwin noted this, as well, in Expression of Emotions in Animals and Man, even comparing emotional facial expressions among several species; but he concluded that none of them cry in the same sense that humans cry. The title of Masson’s book is based on popular accounts of elephants that were observed shedding abundant tears in stressful or painful situations. Darwin, too, wrote that the keepers of Indian elephants at the London Zoo reported that the animals would sometimes weep from sorrow.
Still, the phenomenon, if it exists at all, is so uncommon in animals that we are driven to seek an explanation for its centrality and universality in humans. Alas, tears do not leave fossils; it may be impossible to know for sure why a random genetic mutation in the neural system that controls tears conferred a selective advantage on early hominid ancestors who wept to express feelings. But we can begin to seek understanding by exploring crying in the behavior, brains, and culture of present-day Homo sapiens.
Crying to Communicate
A baby’s face reddens; he closes his eyes; several muscles in his face and brow contract. He opens his mouth and emits a continuous, repetitive, wailing sound that can be counted on to distress every adult within earshot. Someone, most probably his mother, comes to the rescue. A comforting cuddle and a paciﬁer thrust into his mouth are enough to stop the crying.
Here, crying is clearly communication. Unable as yet to convey their feelings and urges in words, infants cry for help and care. In babies and adults, crying almost always means that something is wrong: hunger, discomfort, pain, frustration, grief, sleepiness, helplessness, anxiety, or fear. Children quickly learn that crying gets attention.
While most fathers barely stir in their sleep when their babies cry, mothers awaken instantly. This selective attention starts with a subconscious perception in the brain stem of the highly distinctive sound pattern of crying and then, controlled by hormones, activates the cerebral cortex. Other sounds do not arouse a mother from her sleep as effectively. A recent study by Jeffrey P. Loberbaum and his colleagues at the Medical University of South Carolina and the National Institute of Child Health and Human Development used functional magnetic resonance imaging (fMRI) to compare the activation of women’s brains in response to the crying of a baby and to artiﬁcial noises that held no meaning.2 Two areas of the women’s brains, the cingulate gyrus and the right medial prefrontal cortex, were activated by the crying baby, but not by the random noises. In our development as a species, the cingulate area is an older part of the brain, related to our emotions; in many mammals, it controls parenting behavior. The right prefrontal cortex is involved in our assessment of negative feelings. Another interesting observation is that the crying of an infant stimulates lactation in the mother, probably via the hypothalamus and the pituitary.
We say, “He cried like a baby”—or, condescendingly, “He’s a crybaby”—but is crying a persistence of juvenile characteristics throughout life? Both the pattern and frequency of crying change as we grow from infancy to adulthood. Later in life, for example, crying functions as communication less than it does in infants. What remains is emotionally expressive crying in response to pain, sadness, and grief. This crying is usually not as frequent or as intense as in children. Weeping is the most frequent type of adult crying, an attenuated form of crying, without the vocalizations and movements of the whole body present in children. In adulthood, also, we usually prefer to cry alone.
Why do we continue to shed tears as adults? One answer may lie in brain development. Humans are said to have a “neotenic” brain, neoteny being the persistence of infant anatomical and physiological characteristics into, and beyond, reproductive age. A related phenomenon is “pedomorphism,” the preservation of infant features in adulthood. Like babies, human adults have a large, rounded cranium that is disproportionate to our body size; a ﬂat face; large, prominent eyes; teeth of small size and delayed eruption; almost hairless skin; and genital organs on the front of our bodies. So evident is the persistence of these infant characteristics that some argue we evolved by retaining the juvenile features of our ancestors. As Louis Bolk, a German anatomist, has put it: “humans are fetuses who are capable of reproduction.” Indeed, the anatomical evolution of hominids from Australopithecus to Homo sapiens reveals a clear trend of increasing pedomorphism.
The retardation of human development is a scientiﬁc fact. According to the late noted evolutionary biologist Stephen Jay Gould, the bodies of mice and humans develop essentially in the same order, but the corresponding stages of development in humans take up to 15 times longer. Humans reach puberty at 60 percent to 70 percent of their ﬁnal body weight; other mammals usually reach it at only 30 percent.
Moreover, since we have such a large, complex brain, most of it develops outside the womb, after birth. Were this not so, gestation would be extraordinarily long and the baby’s head could not pass through its mother’s birth channel at delivery, both of which would reduce the mother’s chances of survival. Therefore we are, in effect, born prematurely, with an incomplete brain— almost blind, deaf, dumb, and without autonomy of posture, movement, coordination, or perception. Primates depend totally on their mothers for life-sustaining care for at least two years, and partially for many more. Neotenic brains remain highly plastic and susceptible to modiﬁcation by the environment for many years after birth, probably until 15 to 16 years of age in humans. This is crucial for our development of intelligence, conscience, memory, and other higher cognitive, motor, and integrative functions.
All this suggests the possibility that not only juvenile anatomical features but also many behavioral ones are carried into later adolescence and beyond. Anthropologist Ashley Montague, who ﬁrst presented the neotenic hypothesis in his book Growing Young, insisted that crying and laughing are chief among the traits of children that we preserve for life.
Relief or Arousal?
What are the effects of crying? In 1963, American psychiatrist Karl Menninger wrote in The Vital Balance that “weeping is perhaps the most human and most universal of all relief measures.” The observation applies well to both classic divisions of emotional tearing: tears associated with positive feelings such as joy, and tears associated with negative feelings such as grief. Crying is commonly thought to release emotional tension. Theologian Albert Richard Smith said that “tears are the safety valves of the heart when too much pressure is laid upon it.” It is often said that we cry when some internal barrier, a kind of dam, breaks, as in the expression a “well of tears.” Psychologist Dalbir Bindra found that crying episodes soften or dissipate the initial emotional state that triggered them, again suggesting a mood-relieving function.
Is crying, then, an adaptive response to stress? Do we feel better when we cry at extreme frustration, tension, stress, nervousness, or grief? A survey by William Frey suggested that this might be true: 85 percent of women and 73 percent of men reported feeling better after crying.3 In another study, scientists sought a common denominator in all the origins and types of emotional tearing. The only one they found, after analyzing 465 different episodes of crying, was a relationship with requesting or offering help. This would seem to reinforce the evolutionary, or biological, origin of crying behavior in humans. Children feeling hunger, fear, pain, or other discomfort ask for help by crying and feel relief that help is forthcoming. The authors go on to speculate that emotional tearing was acceptable as a sign of suffering because reﬂex tearing already showed nonemotional trauma or inﬂammation of the eyes. They were, however, less successful in explaining why tears are associated with positive feelings, as well. One possibility is that joy or relief liberates us to recognize and react to stored-up pain and sadness, so that our tears are actually a reaction to sadness, not happiness.
So crying could be considered a kind of psychic homeostatic mechanism, returning the body to an emotional equilibrium that has been upset. This “recovery” notion hypothesizes that crying alleviates negative emotions, providing cathartic relief for depression and sadness. Thus, tearing could be thought of as a rebound phenomenon, an exaggerated response in a direction opposite that of an initial, destabilizing reaction to an emotion or a need. A similar process is common in cases of strong activation of the sympathetic nervous system, for example, the parasympathetic release of urinary and anal sphincters under extreme stress. Since the lacrimal gland that produces tears is fed by the parasympathetic branches of the seventh cranial nerve, tearing could be a similar rebound effect.
Unfortunately for this hypothesis, there is contradictory evidence. During tearing episodes, our heart rate and sweating increase, suggesting that—far from producing relief—crying occurs in a state of physiological arousal. An arousal hypothesis about the function of crying postulates that tearing is used to communicate to a social group that something is not well, that an aversive response is taking place in the organism. This would put crying in the same category as other sympathetic responses to acute stress, such as when we become pale, our pupils dilate, and our hair rises. Additional evidence for the arousal hypothesis is that prolonged crying seems to be exhausting, leaving us drained of energy as well as emotion.
Both theories have pros and cons and, as is often the case, the truth could lie at their intersection. Perhaps during crying we are emotionally aroused in response to particular thoughts or emotions, but, shortly afterward, our crying becomes a rebound or relief phenomenon. A two-factor theory like this is proposed by psychologists Jay S. Efran and Thomas Spangler. In the ﬁrst stage of crying, they say, tension and emotional arousal (either positive or negative) are created. In the second stage, there is a shift to recovery. Does this imply that two distinct psychophysiological responses are at work? Scientists do not yet know, but ordinary language expresses our sense that there are different types of emotional tearing, for example crying from extreme apprehension and sobbing with relief.
Tears and Tears
Whatever the causes of crying, our tears are an excretion of the lacrimal glands, which are situated above the cranial orbits, laterally to the eyes. Tears are 98 percent water, with several electrolytes (giving them their salty taste) and small amounts of 45 different proteins.They also contain immunoglobulins, which are natural proteins produced during an immune response. This suggests that tears have a function in ﬁghting infection, viruses, and bacteria.
Tears have three functions: basic (or basal, or “continuous”), reﬂex, and emotional (“psychogenic”). Basal tears lubricate our eyes. They are excreted continuously in small amounts, spreading across the exposed surface of the eyes with the help of the eyelids. Reﬂex tears are stimulated in larger than normal amounts by irritation or trauma to the eye and have a protective function. For example, when a speck of dust lands on the conjunctiva (the white of the eyes), the ﬂow of tears increases, helping to dislodge the intruder and keep the eyes from being scratched. Chemical irritants (such as those in onions) are washed out by reﬂex tearing. Tears also carry a potent antibiotic (lysozyme) that protects the eyes against bacterial infection. Psychogenic tears ﬂow in response to alterations in our mood and emotions. While basal and reﬂex tears have distinct physiological functions (and exist in practically all animals that have complex eyes), we have seen that it is much harder to comprehend fully the functions of psychogenic tearing.
The three types of tears are different in composition (for instance, emotional tears are richer in manganese and proteins), but it is not clear why. William Frey and Muriel Langseth proposed in 1985 that excess stress-related toxins might be excreted in emotional tears, but there is no evidence that a person who fails to produce tears when under stress will develop an internal chemical imbalance, such as that which can occur when other excretory homeostatic systems such as respiration and urination are impeded.
Differences in the chemical composition of tears may have another explanation: different patterns of neural activation of the lacrimal glands. The lacrimal glands are connected to a branch of the facial nerve, a part of the parasympathetic division of the autonomic nervous system, which is responsible for controlling our internal machinery such as the heart, glands, vascular system, and bladder.
Inside the Crying Brain
Despite the importance of crying as a powerful human emotional expression, there have been few experimental or clinical studies of its basis in our brain and nervous system. At this point, we know far more about the low-level neural control of shedding tears than about the upper brain structures involved with the emotional aspects. The ﬁgure above shows that both emotional and reflex production of tears are brought about by activation of the pair of facial nerves, which in turn have their neuron cell bodies within the brain stem, in the pons and medulla. It is the brain stem that controls our most basic physiological functions—heartbeat, respiration, blood pressure, and so on—and, in terms of evolution, that is the brain’s oldest part.
Some branches of the facial nerve handle sensory input (for example, pain sensors in the eyelids); others have a somatic motor function (activating the muscles of the face that contract during crying); and still others an autonomic nervous function. The two main divisions of the autonomic nervous system, the sympathetic and the parasympathetic nervous systems, are responsible for visceral expression of emotional responses. A study monitoring the psychophysiological reactions of subjects exposed to a sad motion picture revealed how this happens. As subjects began to cry, their respiratory muscles were activated to ﬁll and expel air from the lungs, and movements of the lips, mouth, tongue, larynx, and vocal cords produced crying’s typical vocalization. Sobbing requires a convulsive inhaling of air, with spasms of many of these muscles.
Other parts of the sympathetic and parasympathetic nervous systems account for our red face (by dilation of tiny skin arteries), sweating, increase in blood pressure and heart rate, secretion of adrenaline and noradrenaline, and activation of the adrenal glands through stimulation of the hypophysis by the brain. This is remarkably similar to the general defense reaction that we see in response to stress, but a puzzling question is whether these autonomic and hormonal changes occur as a direct reaction or indirectly, as a result of increased somatic activity.4
While purely basal or reﬂex tearing takes place at the level of our peripheral nerves and brain stem (and occurs even in severely brain-damaged children), emotional crying seems to call on our higher brain structures. One clue comes from clinical studies of the exaggerated or pathological crying that accompanies brain lesions. Vikram Patel of London’s Maudsley Hospital reports that these studies have helped locate the areas of the cortex that may mediate the emotional component of crying behavior.5 He thinks that there must be an interaction between the higher cortical and subcortical structures that mediate mood and the cranial nerve nuclei in the lower brain stem that we have discussed.
In humans, one of the subcortical structures that may be involved in crying is the limbic system, which is central in motivation and emotional behavior. Experiments in which lesions are made in mammal brains suggest that the cingulate gyrus, an evolutionarily newer part of the limbic system, is involved both in infant crying and in maternal behavior.
In 1980, Panksepp and his group observed the effect of several psychoactive drugs on the distress calls of rat pups. Drugs such as chlorpromazine, reserpine, meprobamate, diazepam, alcohol, pento-barbital, and amphetamine did not reduce these distress calls when the rat pups were separated from their mothers, but social stimuli such as contact with siblings did inhibit them. Opiates such as morphine, however, were strong inhibitors. According to Panksepp, “if you give opiates to animals, they do not cry at all.” He suggests that, in inhibiting distress, opioids may act on the same brain system as do social contacts.
Through research on the brain basis of emotions such as fear and aggression, we know that there are many pathways from the limbic system to the hypothalamus, mesencephalon, and brain stem. We may speculate that these pathways are also involved in the expression of crying.
Crying on the Right?
Ever since French anatomist and neurologist Pierre Paul Broca discovered that the motor control of language is lateralized, with the brain’s left hemisphere predominant, scientists have wanted to know whether emotions are lateralized, too. For more than a century, for example, we have known that when patients sustain damage to one hemisphere of the brain, their emotional reactions change markedly. An intriguing ﬁnding comes from Richard Davidson’s 1982 studies of the prefrontal cortex, a brain structure crucial in feelings and emotions. Using several brain-imaging technologies, Davidson found that the left prefrontal cortex participates with other structures in a circuit important for certain types of positive emotion.6 By contrast, activation patterns in the right prefrontal cortex are more often associated with negative emotions, including uncontrollable crying.
Davidson and a co-researcher, Nathan A. Fox, discovered that the degree of frontal brain asymmetry predicts the nature of an infant’s responses to maternal separation. The 10-month old babies who cried during brief separation from their mothers showed greater EEG activation in the right frontal brain, in comparison with the babies who did not cry. The researchers suggest that “frontal brain activation asymmetry may be a marker for individual differences in threshold for reactivity to stressful events and vulnerability for particular emotions.”
Crying Across Cultures
Crying is a universal human phenomenon, but researchers have repeatedly shown that where, when, and how people cry varies not only with their sex, ethnic and cultural backgrounds, and individual psychology, but also with socioeconomic conditions, level of education, family traditions, religious beliefs, and even occupation.
For author and psychotherapist Jeffrey A. Kottler, “The language of tears is hardly a universal form of communication in every part of the world. People from different places speak unique dialects in their tearfulness and have different attitudes toward emotional expression…Some cultures encourage tearful expression as healthy and socially appropriate in certain circumstances, while others suppress crying with a vengeance.”7 His expression “the language of tears” implies that crying—a nonverbal, primitive means of expressing ourselves in emotional situations—is a kind of language for communication, and thus learned.
To a large degree, cultural rules learned early in our childhood govern our control over our emotions. It is widely observed, for example, that people from the Mediterranean and Near East (such as Italians, Arabs, and Jews) tend to be less restrained in their emotions than people from Northern latitudes (such as Anglo-Saxons and Scandinavians). Among the latter, showing tears in public is deterred by the cultural assumption that weeping and crying are manifestations of weakness and inferiority. An extreme example of this inhibition of crying can be found in Asia, where the Minangkabu people of Indonesia are absolutely forbidden to show any signs of sadness. They never cry.
Even within a culture, social rules permitting crying may vary.8 In some African cultures, boys undergoing circumcision are forbidden to cry because they must demonstrate courage and manliness; but when they experience any other kind of pain, they are encouraged by their mothers to wail their lungs out to signal danger and call for help.
In all cultures, however, there is strong polarization by sex, epitomized in the phrase “big boys don’t cry.” This shows how important learning is to the emotional inhibition of men. Generally, women are far less socially controlled in expressing grief and pain by crying and are encouraged to do so in many social situations, such as funerals. In the Middle East and southern Italy, for instance, incessant wailing, beating at the head and breast, and tearing at the hair are dramatic postures and behaviors of bereavement that the women at a funeral will show, and are so stereotyped as to seem to outside observers artiﬁcial or forced. For these women, crying is a cultural event, expected to occur. If a widow does not cry at the funeral of her husband, or a mother at her child’s burial, her social group concludes that she did not love them or even that she welcomed their death.
Studies of cultural differences in expressing emotion, such as Paul Rosenblatt’s Parent Grief: Narratives of Loss and Relationship, a seminal work on grief and mourning in 78 cultures, reveal that the most effective and universal stimulus for crying is bereavement, particularly grief at losing a loved one. With few exceptions, funeral rites in all cultures include open crying. Inhabitants of Bali, however, rarely cry, even in bereavement and pain. Children in Bali may make crying noises, but do not shed tears. There are great differences in the ways the predominant religions in the West and East treat death. In the United States, for example, about 20 percent of crying is related to bereavement; in Japan, only 5 percent.
Styles of crying range from silent, expressionless tears, to loud weeping and wailing, to continuous or convulsive sobbing. Culture appears to dictate these styles, and when they are used. Kottler cites the Makonde, a Bantu tribe of Tanzania, who cry in explosive, sirenlike bursts. Thus the way people cry appears to be a kind of learned language, an “extension of a culture’s native tongue.” In fact, American researchers Sara Harkness and Charles Supper, who carried out cross-cultural studies of crying in East Africa, observed that learning when and how to cry can be said to parallel how language is learned. Upon a biological basis of emotional expression (the lexicon) are imposed the social rules and norms (the grammar) of crying.
Crying and Laughing
What strikes us most about what we know and do not know about crying is that this important human phenomenon has won scant attention from scientists. We have no credible, overarching model for understanding crying, and, so far, research on the brain mechanisms of crying has been pitifully small.
As a step toward one possible model, let us propose an intriguing link between crying and the concepts we discussed in “Our Ancient Laughing Brain” (Cerebrum, Fall 2000). Ancient Greek culture symbolized the theater with two masks, one laughing, one crying. Indeed, these are the dominant emotional expressions in human beings, and both have a communicative function, one positive, one negative. But what do they have in common?
Crying and laughing share central and peripheral expressive mechanisms in our brains and bodies. Both involve a complex interaction among the prefrontal cortex, limbic system, and the muscles and glands of the embryonic third branchial arch. Both also emerge as nonverbal communication by babies and later, in modiﬁed form, are incorporated into adult behavior. Neither appears abruptly in primate evolution; in nonhuman primates there are analogous behaviors, such as an ape’s reaction to tickling and a rat pup’s separation cry—although humor and shedding tears to express complex emotion are unique to human beings.
The communicative role of crying and tears, as of smiling and laughing, is underlined by how easily they are understood by the intended receivers. Konrad Lorenz, a founder of the study of behavior, proposed that the immature anatomical features of children make them cute and likable to adults by means of patterns that have powerful emotional and help-eliciting effects. Cartoon animal characters, such as Mickey Mouse, make use of these facial patterns, as Stephen Jay Gould wryly observed. German ethologist Irenãus Eibl-Eibesfeldt noted that the facial expressions of laughing and crying may be innate “releasing” stimuli, since they can be represented and recognized based on very few lines (essentially the shape of the mouth and eyes), as Greek theater masks show.
Laughter and crying may converge in the early development of our brains. We require many years to install intelligence, thought, and language in the immature brain with which we are born. Throughout this extended period, adults are the main source of life experiences, teaching, and supervising; children tend to remain receptive to this tutelage, pliable and ﬂexible, until reaching puberty. Natural evolution may have favored children with those traits because they enhanced survival and reproduction.
Crying and laughing persist into later life because they are indispensable in expressing positive and negative feelings, inhibiting aggression, promoting social contact, and eliciting cooperative and helpful behavior. In this may lie the overarching explanation of both. What really matters about crying and laughing is understanding their roles in our lives. They are the unique human way of expressing strong emotions and convey a sense of commonality among all human beings.
- Masson, J. When Elephants Weep: The Emotional Lives of Animals. New York. Delacorte Press, 1995
- Lorberbaum, JP, Newman, JD, Dubno, JR, Horwitz, AR, Nahas, Z, Teneback, CC, Bloomer, CW, Bohning, DE, Vincent, D, Johnson, MR, Emmanuel, N, Brawman-Mintzer, O, Book, SW, Lydiard, RB, Ballenger, JC, George, MS. Feasibility of using fMRI to study mothers responding to infant cries. Depression and Anxiety. 1999; 10(3): 99-104.
- Frey, WH, Langseth, M. Crying: The Mystery of Tears. Minneapolis. Winston Press, 1985.
- Gross, JJ, Fredrickson, BF, Levenson, RW. The psychophysiology of crying. Psychophysiology 1994; 31: 460-468
- Patel, V. Crying Behavior and Psychiatric Disorder in Adults: A Review. Comprehensive Psychiatry 1994; 34(3): 206-211.
- Davidson, RJ, Schwartz, GE, Saron, C, Bennett, J, Goleman, DJ. Frontal versus parietal EEG asymmetry during positive and negative affect. Psychophysiology 1979; 16: 202-203
- Kottler, JA. The Language of Tears. New York. Jossey-Bass, 1996.
- Luts, T. Crying: The Natural & Cultural History of Tears. New York. W.W. Norton and Co., 1999.