If the sound of a baby crying gets you red in the face, you're not just imagining things. A new study has found that an infant's cries – particularly those that signal pain – really do trigger a hot flush in adults' faces, perhaps as a way of launching us into action.
Babies' wailing is, in a sense, unbearable on purpose. Without language, it's an infant's best shot at getting the help they need from their caregivers. The vibrations that erupt from a baby in serious distress are quite literally more chaotic than the cries of low-level discomfort.
Cries of pain are formed differently than other sounds a baby makes: they forcefully contract their ribcage, pushing high-pressure bursts of air through their vocal cords to produce variable pitches and disharmonious sounds known to acousticians as 'nonlinear phenomena', or NLPs.
Related: Sorry, Parents: New Research Shows That Infant Crying Doesn't Peak After 6 Weeks
"It has been established that NLP are reliable markers of the level of distress and/or pain expressed by the baby," Jean Monnet University bioacoustician Lény Lego and colleagues write.
"Our results demonstrate that the level of NLP in a cry modulates the temporal dynamics of the facial thermal response in listeners, independent of their sex."
Caregivers can usually differentiate between cries of ordinary discomfort and those blood-curdling NLPs that signal serious pain, but it's unclear exactly how our bodies arrive at this higher-order cognitive effect.
A team led by researchers from Jean Monnet University and the University of Saint-Etienne, both in France, wanted to know how these sounds affect adults' nervous systems on a subconscious level and the physiological effects they elicit.
They tested the reactions of 41 participants (21 men, 20 women, aged 35 years old on average) to the sounds of babies crying. These adults listened to a selection of 23 audio tracks, recorded from 16 different infants while they were either experiencing the benign discomfort of a bath, or the pain of a vaccine jab at the doctor.
As the participants listened, a thermal camera tracked changes to their face temperature. An increase in heat from this part of the body is a response from the autonomic nervous system, the largely unconscious part of our nervous system that controls internal body functions like heart rate, breathing, and digestion.
After listening, the participants also reported whether they thought they had heard sounds of either discomfort or pain.
"Variations in a listener's facial temperature, a marker of the autonomic emotional response, reflect the pain expressed by a baby's cry," the authors write.
A cry with a higher level of NLP elicited a stronger flush response from listeners' faces, suggesting that these chaotic sounds are much more effective at getting the attention of adults, on a physiological level, than cries with little or no NLP.
This response to high-NLP cries was seen equally in male and female participants, adding to the researcher's previous findings that both sexes can reliably identify pain in a baby's cry.
"While our results sound interesting and new, this study remains rather preliminary and raises a number of questions, both in terms of interpreting the results and from a methodological point of view," the authors note.
For one, the participants had little or no experience with babies, so these results don't necessarily reflect the physiological response of seasoned parents. Future studies could reveal how physiological responses to NLP differ based on experience.
The authors also note the cries played in this study are natural sounds, a messy smorgasbord of acoustic phenomena. They're yet to unpack exactly which NLP elicit a thermal response, or if, perhaps, it's their unsettling combination that sends such a powerful SOS.
This research was published in the Journal of the Royal Society Interface.