Tears of Sadness: The Neurochemistry of Human Crying

An exploration of how the brain creates tears from emotions.

Oct 01, 2024

Hand-Drawn - Copyright © 2024 Nicholas Kircher

“Heaven knows we need never be ashamed of our tears, for they are rain upon the blinding dust of earth, overlying our hard hearts. I was better after I had cried, than before--more sorry, more aware of my own ingratitude, more gentle.”

Charles Dickens: Great Expectations.

Note: This post is not well suited to the visually impaired, as it involves testing functions that use the human visual cortex.

If you want to skip the game and go straight to the science, just click here!

This was a subject requested by

Tina Marsan

, and it is a good one.

Let’s play a little game.

I want you to look at the following picture:

Creative Commons 4.0 With Attribution - Picó A, Gadea M (2021) https://doi.org/10.1371/journal.pone.0251083

Now, write down a number between 1 and 10 that represents how friendly the animal in the image is, based on their facial expression, where 1 is least and 10 is most friendly.

…

Done? Ok, good.

I’ve got another one for you:

Again, 1 to 10, least friendly to most friendly.

Go.

…

Got it? Great.

Now a cat:

Same thing, put that number down.

…

Fabulous!

I’ve got one last one for you.

Ready?

Seem familiar? That’s ok, don’t worry about that. Once you’ve put your number down for this one, compare that number to what you wrote for the first picture.

It’s different, isn’t it?

If you marked the last picture as more friendly than the first picture, you, my friend, might be a human being.

Congratulations!

If you got a different result, I’m so sorry you had to find out like this, but I’m pretty sure you’re adopted.

Share your results in the comments for some additional fun!

What is going on here? What tom-foolery is this?

Many creatures throughout the animal kingdom have unique behaviours and responses to all manner of stimuli. For humans, one of our most intriguing and unique responses to emotional stimuli is to produce tears.

“Emotional tears are a special form of non-verbal communication that are unique to humans and which favour the inference of emotional states.”
Picó, A., & Gadea, M. (2021). When animals cry: The effect of adding tears to animal expressions on human judgment. PloS one, 16(5), e0251083.

We also have fairly unique responses to seeing tears in other humans, as well as other animals (despite emotional tears being an exclusively human thing).

What is rather surprising is that there isn’t a huge body of understanding around the neurochemical pathways that produce this response. Recently, that has begun to change.

First, lets talk about the evolutionary crying response.

When newborn babies cry, it’s typically signalling a need: safety, hunger, thirst, discomfort, pain. We (as in, myself and all my other newborn compatriots, obviously) share this behaviour with most mammals, and birdies too. There is no prior socialisation, training or teaching required: it just happens™, much like a reflex.

In fact, the acoustics of newborn crying are remarkably similar across all primate species. It suggests very little change to the sound we make has occurred since the early days of primate evolution.

However, in the paper “The Neurobiology of Human Crying”, the authors cite works which show how it developed beyond just these distress calls, when certain unique challenges may have encouraged the development of tears as a social signal.

“Emotional crying in humans seems to have its evolutionary basis in these animal distress calls, which is evident in its solicitation of help-provisioning and nurturing behavior.”
[…]
“This coupling at least suggests that humans at some point in their evolution were confronted with unique challenges for which the shedding of tears proved to be advantageous.”
Bylsma, L. M., Gračanin, A., & Vingerhoets, A. J. J. M. (2019). The neurobiology of human crying. Clinical autonomic research : official journal of the Clinical Autonomic Research Society, 29(1), 63–73.

Here’s what is perhaps most curious: what evolutionary advantage do tears provide, anyway?

Well first of all, in infants, this question is pretty straight forward to answer:

“[Infant crying] is believed to have evolved as an attachment behavior that functions to unite infant and mother when the infant is in need of care”

This is true of all species where an infant vocalises distress.

So, what about when baby is all grown up?

We can find some idea from the many, many studies on how tears influence our perception.

When we see visible tears on a face - whether a human one or otherwise - it automatically increases our perception of friendliness and decreases perceived aggression. It also stimulates empathy, and the desire to help and provide care to the crier.

“The effectiveness of tears in conveying sadness and eliciting sympathy was greatest for images of adults, intermediate for images of children, and least potent for images of infants. These findings suggest that the signal value of tears varies with the age of the crier.”
Zeifman, D. M., & Brown, S. A. (2011). Age-Related Changes in the Signal Value of Tears. Evolutionary Psychology, 9(3), 313-324.

So for us, it’s basically a short-cut to informing others of our submission, our open vulnerability, and our need for support or assistance. Canines and felines can give off a similar signal by rolling over and showing their belly, placing themselves in a vulnerable position deliberately for reasons of social order.

If you’ve ever made someone cry when you were angry, and suddenly felt that need to flip to a more supportive, care-giving, empathetic and optimistic tone and attitude, that is an instinctual built-in short-cut activating in your brain from the sight of tears. If the person had just said the words “I need you to be more supportive, care-giving, empathetic and optimistic with me right now because I am fragile” instead, it is unlikely to have had the same effect. The production of tears is a supremely powerful social signal.

So then why do sad movies make us cry?

It’s likely because sadness evolved from the needs of our complex social lives, and when coupled with empathy and mirror-neurons, our automatic reflection of the appropriate or apparent psychological state from what we observe kicks in, generating emotions in the Amygdala.

When the protagonist in the movie is sad, we reflect that apparent mental state, and feel sad too. When something bad happens to them, we might feel sad on their behalf (or any other appropriate emotion, such as anger). In fact, humans can produce tears from just about any emotion, if it’s intense enough.

Also, when crying evolved, there was nothing like movies that evolution could have excluded from this physiological response, so although evolutionarily there’s no point to crying at a sad movie, our genes haven’t had time to come to terms with that. Might need a few million years.

Yeah great but what about the chemistry?

I was getting to that.

To begin with, an emotion of sadness would be charged up in the Amygdala - commonly the source of many emotions in the brain - and Anterior Cingulate Cortex (ACC). That signal, if above a certain threshold, then over-flows into the Limbic system, dispatching the Acetylcholine neurotransmitter which floods the autonomic nervous system; this activates the lacrimal glands, on the inside edge of each eye, producing a flow of tears.

Eventually, the parasympathetic nervous system will kick in to calm you down, and this involves a lot of Enkephalins, which are a type of Endorphin (hence the feeling of “a good cry”).

Here’s where the chemistry gets kinda odd, though. Did you know that your tears also contain decent quantities of endorphins? Endorphins are a type of “Endogenous Opioid”, (Endogenous means “produced by the body”, as opposed to exogenous, meaning “from outside the body”) and inhibit pain perception. Why are they in our tears?

Fascinatingly enough, there are 3 types of tears that humans produce:

Basal tears, which keep the eye constantly moisturised and lubricated, and contain water, lipids, glucose, a bunch of other proteins, enzymes, and antibodies from the immune system to protect the eyes from pathogens,
Reflex tears, produced by irritants like onion vapours, tear gas and pepper spray; 1/4 of all the protein in Reflex tears is Lipocalin, a hydrophobic-material-binding agent,
Emotional tears, which contain much higher quantities of various hormones, like prolactin, ACTH (a hormone that tells the body to produce more cortisol), adrenaline, and “Leu-Enkephalin” (an Endorphin), but also more potassium and even manganese!

The idea for expelling adrenaline and ACTH through tears kinda makes some sense, as crying can be a response to a major stressor event, which could significantly elevate cortisol; hence, tears might be a way of bringing those stress levels back down again by getting rid of excess stress hormones.

However, I don’t really understand why we would expel Endorphins. My thinking is that we need those Endorphins to help calm us down again, and to ease the emotional pain we might feel. I haven’t found a really solid claim for what they’re doing there, but one theory is that perhaps it’s re-absorbed through the skin; that seems remarkably inefficient though, and evolution trends towards efficiency in all things.

So that’s it!

Now you have your next start-up idea: harvesting hormones, neurotransmitters and manganese from the tears of all the haters. Remember to send me a portion of your profits.

Hit those comments with your scores from the picture game! Did your score change when seeing the added tears? Or did my experiment fail miserably?