Why Do Humans Cry?

Updated on September 29, 2018
K S Lane profile image

K S Lane is a student of science and is deeply passionate about educating others on her favourite topics.

Human beings are the only animal on the entire planet that shed tears in response to emotions like sadness or happiness, and we can all agree that having a good cry can be a cathartic experience that helps to relieve stress. But why, exactly, do humans cry? Is there a biological or evolutionary basis behind why sad movies will make you start bawling? Are the tears you shed when you lose a loved one any different from the tears that well up when you cut onions? This article explores the science behind why you cry and answers all of these questions and more!

This article explores the science behind why humans cry.
This article explores the science behind why humans cry. | Source

How do the Tear Ducts Work?

'Tear Ducts' are the common name for the scientifically termed lacrimal glands. The lacrimal glands sit just underneath the skin of the upper eyelid. Their function is to secrete a salt/water mixture through tiny, porous openings in the upper eyelid. When you blink, this salty fluid gets spread over the surface of your eyeballs, maintaining a layer of moisture to protect the sensitive organs from drying out. This process is fairly easy to explain and has a clear benefit to us; that is, that our eyes don’t shrivel up and stop working. The process of crying when we feel sad uses the same kind of mechanism, but the reasons for it are much more complex.

The lacrimal glands, more commonly known as the tear ducts, sit just under the upper eyelid and are responsible for producing tears.
The lacrimal glands, more commonly known as the tear ducts, sit just under the upper eyelid and are responsible for producing tears. | Source

Are There Different Types of Crying?

According to science, there are three different types of crying. The first is a process that most people wouldn’t consider to be real crying, which is the shedding of tears in order to maintain the layer of moisture over our eyes. These tears are called Basal tears, and their purpose is to stop your eyes from drying out and being damaged.

The second type of tears are shed in response to irritants entering the eye. The classical example of this is what happens when you’re cutting onions. The gas released when you slice into the vegetable mixes with other gases in the air and creates sulphur gas, which irritates the eyes. In response to this, the brain sends signals to the tear ducts to start trying to flush your eyes out (I’ll explain this process in more depth a little later). The tears produced in response to this are called reflex tears.

The third kind of crying is the type that’s unique to humans, and the type that’s the most scientifically interesting. When humans experience strong emotions, particularly sadness and pain but also sometimes excitement or joy, we suddenly get all weepy. The tears produced in response to strong emotions are called psychic tears.

Cutting onions produces sulphur gas, which irritates the eyes and causes the release of reflex tears
Cutting onions produces sulphur gas, which irritates the eyes and causes the release of reflex tears | Source

What Goes on in the Brain When we cry?

When we experience powerful emotions, certain areas of our brain light up with activity. These areas are all part of the limbic system, which functions as a sort of emotion-processing centre. One such area is the hypothalamus, which controls the body’s emotional response system. The hypothalamus happens to be directly connected to the lacrimal glands. Signals are sent from the hypothalamus to the glands to start producing tears, which they promptly do. This signalling is done via the autonomic nervous system, which is the branch of the nervous system that controls our involuntary responses. So next time someone tells you to get over it and stop crying, you can remind them (through your uncontrollable sobbing) that your autonomic nervous system has taken over and you physically can’t make yourself stop.

The hypothalamus, which is part of the brain's limbic system, has a direct connection with the lacrimal glands.
The hypothalamus, which is part of the brain's limbic system, has a direct connection with the lacrimal glands. | Source

Why do Humans Cry?

So, we now know how humans cry. When parts of the limbic system, specifically the hypothalamus, are activated in response to strong emotions a signal is sent to the lacrimal ducts to start producing more moisture than normal. But we still haven’t touched on why we cry. What’s the point of this strange response, in which salty water starts dripping out of our sight-holes? There’s no definitive answer, but there are two predominating theories and it’s fair to say that the answer probably lies somewhere right in between them.

The Stress-Relief Theory:

It’s been found that psychic tears contain a higher level of certain proteins than reflex or basal tears, specifically adrenocorticotropic hormones. These hormones are responsible for some of the symptoms associated with high stress levels, particularly through their effect of increasing the production of cortisol, which is the principle stress hormone. Thus, it could be argued that crying directly helps to decrease your stress levels by flushing your body of these hormones. There has also been some research that suggests that psychic tears actually contain a natural pain-reliever, called leucine enkephalin, which is a further chemical hint at why humans evolved the crying response. Research in this area is still limited, but even half-developed the theory definitely seems promising.

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The Evolutionary Theory:

On first glance, crying doesn’t make all that much evolutionary sense. Most innate instinct and reflexes programmed into our bodies are there because they helped us survive in one way or another; our heart races when we’re nervous to increase blood flow to the muscles, we feel sick because the process of digestion stops to allow more energy for things like running and fighting, and so on. But crying? As I’m sure you know, tears make your vision blurry and the sobbing that sometimes accompanies them can hardly be considered threatening to potential predators. What, then, could be the point?

Many researchers have theorised that tears are designed to be a literal cry for help from others in our surroundings; a way of instantly communicating our distress and calling others to our aid. This idea is supported by a study from the journal of Evolutionary Psychology, which showed participants photos of crying people, and then the same photos but with the tears photoshopped out. The study found that the people in the photos with tears were much more likely to be rated as being in more distress than the people with their tears removed, who were sometimes confused as having 'puzzled' or 'shocked' expressions rather than conveying sadness.

Another similar theory argues that the very nature of tears is meant to convey helplessness to others and instil in them a desire to help and protect. As mentioned before, tears make taking any defensive or offensive action harder. If you’re faced with a sabre-toothed tiger and immediately burst into tears, wrestling it to the ground or even running away is going to prove difficult with your watery vision and dripping nose. This signals to other humans in the area that you’re not going to be able to deal with the situation by yourself and that you need help now. Of course, in the modern world you’re unlikely to run into a sabre-toothed tiger, but the pathetic helplessness that crying projects can be useful in other situations; if you spontaneously start crying in front of a friend, they’re more than likely going to drop everything and try to help you.

The present findings suggest that tears communicate sadness and sincerity, but not anger. Tears also elicit sympathy, inhibit irritation among observers, and provoke interventions aimed at comforting criers.

— Zeifman, D. & Brown, S. (2011). Age-Related Changes in the Signal Value of Tears.
Most babies cry without tears until the age of about 2 months.
Most babies cry without tears until the age of about 2 months. | Source

In Conclusion:

We shed three different types of tears; basal, reflex and psychic. Basal tears are designed to help keep our eyes moist and reflex tears flush irritants out. Psychic tears, however, are more interesting. In response to strong emotions, parts of the limbic system including the hypothalamus sent signals to the lacrimal glands to start producing tears. There are two different prevailing theories that attempt to explain this. This first is that crying helps us to project helplessness and signal to other people nearby that we need help, and that tears are a quick and unmistakable way of conveying our distress to others. The second is that crying literally helps us to de-stress because it flushes the body of stress hormones, which are found in higher levels in psychic tears than other types of tears, and because tears contain leucine enkephalin, a natural pain-reliever. Both of these theories have their merits, and the real answer to why we cry most likely lies somewhere in between the two. Knowing this probably won’t make you feel any better the next time you’re sitting in bed weeping for a lost love or sobbing at the end of a sad movie, but at least you’ll be able to impress your friends with your knowledge between sobs.

Sources and further reading:

  • https://sciencebob.com/why-do-we-cry-when-we-chop-onions/
  • https://www.youtube.com/watch?v=QGdHJSIr1Z0
  • https://my.clevelandclinic.org/health/diseases/17540-tear-system
  • https://psychneuro.wordpress.com/2014/03/14/the-biochemecial-purpose-of-crying/
  • Zeifman, D., & Brown, S. (2011). Age-Related Changes in the Signal Value of Tears. Evolutionary Psychology, 9(3), 313-324 (https://www.ncbi.nlm.nih.gov/pubmed/22947977)


Questions & Answers

    © 2018 K S Lane

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      • Pamela99 profile image

        Pamela Oglesby 

        10 months ago from Sunny Florida

        This is a very intereting article. I definitely didn't know about the three types of tears. I had a friend that use to cry at the drop of a hat, and she really didn't know why. She was such a tender hearted sweet person, but it was hard for me to understand as I cry occasionally, but not a lot.

        I respect the way you presented this information as it is so well organized and understandable.

      • Sneha Sunny profile image

        Sneha Sunny 

        10 months ago from India

        This was such an awesome read! Great job! I didn't know that tears shed cortisol and the natural pain-reliever. And I also didn't know that they are called psychic tears. Shared!

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