I remember when I was as young as 3 years old, I had consciously decided to burst into tears when faced with raised voices. There didn't seem to be a direct threat to my existence, however, up until the age of 23, I would continue to burst into tears if anyone I am engaging a conversation with turned up their volume. And it forever baffled me, as to why was I indulging in such a behavior. I had chalked it off to me being thin-skinned, easily triggered, highly stimulated and a Highly Sensitive Person (HSP).
But the explanations weren't enough. They didn't explain the whole body tumultuous experience, one of the likes of an anxiety attack. Perhaps, raised voices in my childhood were equated to an unstable environment. Perhaps, they were equated to the split within my family and some how it got translated as a precursor to an impending divide within my relational attachments.
And then I can across this study, one that put everything into perspective. Flawed as it may seem, it still is a breakthrough in analyzing "Why we cry?" especially in a highly emotional situation where in vulnerability could be construed as a weakness. What do we gain from being so vulnerable?

The study delved into analyzing the correlative functioning of a Tear Signaling Protein by the likes of ESP, specifically ESP-1. It observed that when released, ESP-1 triggers the V2 Rp5-expressing VNO neurons in females so as to display sexually receptive behaviors. Research has also shown that if the female rat is pregnant and gets a whiff of a passerby male rat's ESP-1 (male who is not the father of the child), she will spontaneously miscarry to then display sexually receptive behaviors towards the new male. So it is safe to say that in rats, ESP plays the role of a pheromone.
When we look at other species, its purpose shifts. In blind mole rats, it is released to reduce the male aggression from the environment so that the mole rat has a better chance of surviving. In mice, it takes the form of ESP 22 to reduce male sexual aggression towards them. In humans, the paper we are about to dissect, finds that sniffing tears reduces aggression by 43.7%.

The one glaring limitation the study went with was an overgeneralization conducted with a sample size off 6 women, predominantly within the reproductive age-range and ideally white. This reductionist approach to incorporating a hyper-focused lens on white representation in a sample size, to set an ideal for the rest of the world to follow has been the racist and sexist bias for the majority of the 19th century research. It begs one to question what biases are the researchers operating under so as to utilize a tried and failed racist ideology and reintroduce it to the 21st century. Furthermore, this hyper-focus on the White Woman's Tears also limits the study's gender-expansive audience participation. Are results based on a Binary, truly the best way to observe "tearing" and its impact on aggression levels when the latter clearly impacts quality of life of all genders?

Having established the limitations of the study and curbed our expectations, let us look into the study design. There exists 2 experiments. However, before we delve in the researchers described the 2 different types of stimuli obtained: one was tears collected into a text tube, and the other was saline collected after they trickled down the cheek. The logic behind the use of the latter was to eliminate any impact of the dead skin cells, proteins, and other molecules that the trickled saline might pick up on their way down.
Another limitation here is that since they have used trickled saline and not trickled tears, any elimination of unwanted molecules could be with respect to saline and not with respect to tears themselves. In other words, even if tears were to pick up cheek molecules on their journey, they may react differently with these said molecules than saline.
Experiment 1
A sample size of 25 reproductively active and in their prime men, were subjected to the sniff test. It consisted of 3 sniffs of Saline (which is used as a Blank/Control), followed by 10 sniffs of a Stimulus (Trickled Saline/Tears). The sniff test was rated on the odor intensity, pleasantness and familiarity. Following which, a 100 microliters of stimulus was applied to the upper lip for continuous exposure as these men played a game.

Game Time
The Point Subtraction Aggression Paradigm is a monetary game designed to create high levels of aggression with "property loss" so as to rate the observed Aggression Provocation Ratio (APR). Higher the APR, higher the Aggression. Here's where the number 43.7% comes in.

Experiment 2
Use of Hana 3A Cells
Purpose: Hana 3A cells, derived from HEK 293T cells, are engineered to express olfactory receptors (ORs) on their surface. They are widely used for studying how ORs respond to various chemical stimuli.
Relevance: These cells provide a controlled environment to test which ORs respond specifically to emotional tears.
Luciferase Assay
Purpose: The luciferase assay is a sensitive biochemical technique that detects OR activation through light emission. When a receptor is activated by its specific ligand (e.g., molecules in tears), it triggers a signaling cascade that activates the luciferase enzyme, resulting in luminescence.
Why Used: This assay quantifies receptor activation, making it easy to identify and compare which ORs respond to the stimulus.
Dilution of Stimuli (Tears/Saline)
Purpose: Emotional tears were diluted to specific concentrations to simulate realistic exposure conditions and test receptor sensitivity at different stimulus levels.
Why Saline: Saline serves as a control to ensure that the response observed is specific to emotional tears and not due to nonspecific activation.
Initial Observation of 21 ORs
Purpose: The first screening aimed to identify all olfactory receptors that were activated by components in emotional tears. Detecting 21 ORs indicates that multiple receptors may initially respond.
Serial Dilutions with Emotional Tears
Purpose: This step narrowed down the results by identifying receptors that consistently respond across a range of tear concentrations, a hallmark of a true sensory response profile. It ruled out receptors that were weakly or nonspecifically activated.
Outcome: Only 4 ORs (OR2J2, OR11H6, OR5A1, OR2AG2) demonstrated robust and reproducible activation across multiple dilutions, suggesting these are the primary receptors involved in detecting emotional tears.
Overall Objective of the Experiment
The study aimed to:
Identify the olfactory receptors activated by emotional tears.
Narrow down the specific receptors with a sensory response profile.
Provide insights into the molecular mechanisms by which emotional tears are perceived as a signal, possibly contributing to social or emotional communication.
This type of research helps unravel the biochemical pathways involved in human chemosensory perception.

Experiment 3
With regards to the brain regions corresponding to this aggression style, it was observed in the left Anterior Insula Cortex and the Bilateral Prefrontal Cortex. However, only the projections from AIC lit up. These projected towards the Temporal Pole, followed by the Right Amygdala and finally the Piriform Cortex. With the introduction of Tears, it increased the functional connectivity in networks of olfaction and aggression.

While functional connectivity in neuronal networks controlling aggression was increased, it might suggest that these were neurons involved in interpretation of tears as a social signal. Which might suggest why there existed a downregulation of aggression post sniffing the tears.

Like mentioned earlier, this study does come with its limitations.

Despite all its limitations, it is an interesting study that begs further questioning.

Perhaps, tears evolved as an evolutionary response to threatening stimuli. Perhaps it is the body's natural defense against increased aggression. Perhaps it could be considered as a survival instinct.
Agron, S., de March, C. A., Weissgross, R., Mishor, E., Gorodisky, L., Weiss, T., Furman-Haran, E., Matsunami, H., & Sobel, N. (2023). A chemical signal in human female tears lowers aggression in males. PLoS biology, 21(12), e3002442. https://doi.org/10.1371/journal.pbio.3002442