The Intelligent Movement Machine: An Ethological Perspective on the Primate Motor System is a clearly written history of motor cortex research from its discovery to the present, a discussion of the major issues in motor cortex research, and an account of recent experiments that led to the “action map” view. Michael Graziano, the author, has spent his formative research years at Princeton University, focusing on the sensory guidance of movement. In the excerpt below Graziano looks at the act of crying.
The act of crying, more than any other human social behavior, looks like simulation of a defensive reaction. the behavior under discussion here is not a distress call such as many animals including humans make, or the wailing of a human infant that presumably falls into the category of a distress call. Instead the behavior under discussion is a squinting of the eyes, an excretion of tears, a lifting of the upper lip that results in an upward bunching of the cheeks toward the eyes, a ducking of the head, a shrugging of the shoulders, a forward curving of the torso, a flexion of the hips and knees, a pulling of the arms across the torso or upward over the face, and sharp vocal exhalation. These components point-for-point resemble or are an exaggeration of an extreme defensive reaction.
It is interesting that the components of crying resemble the components of extreme laughter so closely that it is mainly social context that allows humans to distinguish the two states. This bizarre similarity between two social displays that have apparently opposite meaning was noted at least as far back as Homer, who, in The Odyssey, famously compared the laughter of a group of men to the crying they were about to do on being killed by Odysseus.
This similarity between crying and laughing suggests a further speculation. Crying could plausibly be an evolutionary modification of tickle-evoked laughter. In the hypothesis…tickle-evoked laughter evolved from play fighting in which a strong defensive reaction broadcasts that one animal has succeeded in penetration the defenses of another animal and has contracted a vulnerable body part. The signal is not all-or-nothing. It is a graded signal, in which a stronger or more intense signal is evoked by a greater degree of violation of personal space. An extreme signal suggests such violation of personal space that injury may have occurred. In a play fight, injury is clearly not an adaptive goal to either participant. An extreme defensive reaction could serve as a useful signal for the fight to stop and the winner to comfort the loser, to reestablish social amity. Because of its adaptive value, the signal is therefore put under evolutionary pressure. Two kinds of adaptations are expected. First, it would be adaptive for humans to evolve a strong and immediate response to the signal that includes comforting and providing help to the person whom one has accidentally injured. In this way, useful social amity is preserved. Second, it would be adaptive for humans to exploit the signal, using it to gain comfort and help even outside the context of play fight and from individual who were not responsible for the injury. The hypothesis proposed here, therefore, is not that crying is an extreme form of tickle-evoked laughter, but that it is an evolutionary modification of tickle-evoked laughter. The signal has taken on its own role but physically retains its resemblance to its close ancestor, the tickle-evoked laughter.