• Jordan Raine is a PhD Researcher, Nature and Function of Human Nonverbal Vocalisations, University of Sussex.
Have you ever been sat on a flight with a crying baby in your vicinity, wondering more and more with each successive wail how much longer you can stand the sound?
Or maybe you've been a parent, barely able to resist for a second before running to soothe your precious infant's ear-piercing distress? Most of us have been there at some point in our lives. But what exactly is it about a baby's cry that makes it so hard to ignore?
First, it is important to draw a distinction between crying and tears. Many species produce cries, but we appear to be the only animals that send emotional droplets streaming down from our tear ducts. While tears often accompany cry vocalisations in older age, they are by no means a prerequisite of crying - newborns cry from birth but don't produce tears until they are two to three months in age.
It also turns out that these early cries have evolutionary roots separate from the more cultural, learned "emotional crying" that we develop in later life.
Crying is a primitive behaviour shared across mammals, whose governing mechanisms are rooted in the evolutionarily ancient brain stem - infant rats, cats, and humans have all been shown to be able to cry even when the forebrain, which evolved much later, is absent.
Indeed, the cries of many human and non-human mammal infants are highly similar in both acoustic structure and in the contexts in which they occur - across the mammal kingdom, infants cry primarily when they're hungry, when they're in pain, and when they're alone.
But why cry? As is the case with any primal vocalisation, crying evolved to have a specific impact on listeners.
Plentiful research has shown these calls to specifically activate adults' brain regions important for attention and empathy.
This makes them highly effective at grabbing the attention of caregivers and orienting them to provide company, safety, food, or comfort.
While research is in its early stages, oxytocin - popularly termed the "love hormone" and central to the fostering of social bonds - seems to be at the neurochemical heart of this attention-grabbing behaviour.
Infant distress results in reduced oxytocin and opioid levels, and evidence suggests that this then triggers and escalates crying.
When a mother hears these cries, this in turn causes an increase in her oxytocin levels and encourages care-giving behaviour.
What little we know about father-infant bonding suggests a similar role for oxytocin. Additionally, cries cause a dip in testosterone in empathetic men, facilitating nurturing behaviour.
In fact, oxytocin may even amplify the brain's response to cries, making us more likely to hear them and respond appropriately. Finally, when social contact is established, this stimulates oxytocin release in the infant, and crying behaviour ceases. Sometimes.
The boy who cried deer
Pitch is certainly important in drawing a response from caregivers - species of deer only come running to isolation cries possessing a pitch within a species-specific frequency range.
But this frequency range is actually surprisingly wide - deer will respond to the cries of infant seals, cats, and humans, and even bats and marmots if the pitch of the call is manipulated to fall within that frequency range.
The response of deer to other species from which their evolutionary lineage diverged as much as 90 million years ago isn't as amazing as you might first think though - it really just illuminates our shared ancient history.
Mammals all descended from the same common ancestor, so the mammalian larynx (which produces pitch) is remarkably similar across species until puberty, when species-specific environmental pressures lead to major differentiation along sex and species lines in voice characteristics and vocal repertoires.
Before that point, there is no evolutionary reason for any mammal to differentiate their voices from any other.
This similarity in calls affects the approach of caregivers. Many distress calls occur before mothers have had time to learn the specific vocal signature of their offspring through contact calls.
With the succession of your genes possibly at stake, it therefore makes sense to respond to any cry that vaguely resembles your child. This and the substantial variation within members of the same species in cry pitch have oriented cries towards casting as wide a net of influence as possible.
While we can distinguish cries from other vocalisations, we're pretty bad at identifying the specific motivation behind a cry without accompanying contextual information - perhaps because there don't appear to be reliable acoustic differences between pained howls, hungry whines, and lonely wails.
What is represented, however, is the level of distress. As urgency increases, so do the maximum pitch and loudness, while the pause length between cries decreases.
In addition, more of the sound wave's energy becomes concentrated in higher frequencies, towards the range where both adult hearing is most sensitive, and where sound diminishes least rapidly in the environment.
Across cultures, we use these same acoustic attributes to accurately track distress, and this influences the urgency of our responses.
What really puts paid to ignorance, though, is unpredictability. Studies show that when babies are really distressed, their cries start to deviate from their predictable, tonal quality.
Be it in the form of chaos, otherwise known as turbulence or "roughness", where the voice contains energy at random frequencies and has a scratchy quality (think white noise); biphonation, where the voice has two pitches; or high variation in pitch during a call, these vocal attributes are representative of a voice pushed to the limit.
This vocal regime is segregated from other signals, enabling faster and more accurate localisation of the sound source and engaging brain structures critical to rapidly appraising danger. It has been suggested too that this unpredictability makes cries harder to habituate to and ignore - which can you imagine falling asleep to more easily, a tonal cry or a chaotic one?
When an infant is in serious pain or grave danger, it will do everything in its power to make its voice heard.
So now the next time you hear one of those delightful cries for help, you'll understand a little better how it's piercing its way into your brain, and just how deeply your discomfort is hardwired into you by evolution. Will that make it any easier to bear? Somehow, I doubt it.