Vocalizations are important components of social behaviour in many vertebrate species, including our own. Less well-understood are the hormonal mechanisms involved in response to vocal cues, and how these systems may influence the course of behavioural evolution. The neurohormone oxytocin (OT) partly governs a number of biological and social processes critical to fitness, such as attachment between mothers and their young, and suppression of the stress response after contact with trusted conspecfics. Rodent studies suggest that OT's release is contingent upon direct tactile contact with such individuals, but we hypothesized that vocalizations might be capable of producing the same effect. To test our hypothesis, we chose human mother -daughter dyads and applied a social stressor to the children, following which we randomly assigned participants into complete contact, speech-only or no-contact conditions. Children receiving a full complement of comfort including physical, vocal and non-verbal contact showed the highest levels of OT and the swiftest return to baseline of a biological marker of stress (salivary cortisol), but a strikingly similar hormonal profile emerged in children comforted solely by their mother's voice. Our results suggest that vocalizations may be as important as touch to the neuroendocrine regulation of social bonding in our species.

Child maltreatment often has a negative impact on the development of social behavior and health. The biobehavioral mechanisms through which these adverse outcomes emerge, however, are unclear. To better understand the ways in which early life adversity affect subsequent social behavior, changes in the neuropeptide oxytocin in children (n=73) aged 8.1–11.5 years following a laboratory stressor were examined. Girls with histories of physical abuse have higher levels of urinary oxytocin and lower levels of salivary cortisol following the stressor when compared to controls. Abused and control boys, however, do not differ in their hormonal responses. These data suggest that early adversity may disrupt the development of the stress regulation system in girls by middle childhood.

Human speech evidently conveys an adaptive advantage, given its apparently rapid dissemination through the ancient world and global use today. As such, speech must be capable of altering human biology in a positive way, possibly through those neuroendocrine mechanisms responsible for strengthening the social bonds between individuals. Indeed, speech between trusted individuals is capable of reducing levels of salivary cortisol, often considered a biomarker of stress, and increasing levels of urinary oxytocin, a hormone involved in the formation and maintenance of positive relationships. It is not clear, however, whether it is the uniquely human grammar, syntax, content and/or choice of words that causes these physiological changes, or whether the prosodic elements of speech, which are present in the vocal cues of many other species, are responsible. In order to tease apart these elements of human communication, we examined the hormonal responses of female children who instant messaged their mothers after undergoing a stressor. We discovered that unlike children interacting with their mothers in person or over the phone, girls who instant messaged did not release oxytocin; instead, these participants showed levels of salivary cortisol as high as control subjects who did not interact with their parents at all. We conclude that the comforting sound of a familiar voice is responsible for the hormonal differences observed and, hence, that similar differences may be seen in other species using vocal cues to communicate.

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Chronic and severe stress exposure in early childhood is associated with the development of psychiatric disorders. Yet, the molecular mechanisms underlying this relationship remain poorly understood. Here, we profile molecular marks (DNA methylation and gene expression) throughout the human genome to determine the associations between childhood stress exposure and gene regulation. To do so, we collected saliva tissue from prepubertal girls (mean age 10.9 ± 1.26 years) who had experienced different levels of childhood adversity, ranging from mild to severe. We found 122 differentially methylated genes (FDR P-value < 0.05) associated with high childhood stress exposures that affect brain development. Of these differentially methylated genes, 12 also differed in gene expression. To further investigate the potential effects of stress exposure on gene regulation, we examined the DNA sequences flanking all the differentially methylated loci. This analysis revealed enrichment of known binding sites for transcription factors, suggesting that DNA methylation may regulate gene expression by mediating transcription factor binding on these genes. Together, these findings indicate a possible neuromolecular mechanism linking children’s social experiences with risk for anxiety and depressive disorders.

Comparing adults and infants of our own species must be considered as carefully as comparisons between humans and other primates. While many of the same neuroendocrine systems are present in each, there are also important behavioral and adaptive distinctions which, if not precisely defined, may lead to erroneous conclusions about evolutionary history.