The Science of Tears: Why Do We Cry?

You're cutting an onion. Your eyes sting. Water streams down your face.

You stub your toe. Your eyes well up.

You watch a heart-wrenching movie. The tears flow freely.

Same body part. Same output. But fundamentally different processes. The tears from chopping onions, the tears from stubbing your toe, and the tears from a fictional character's death are biochemically distinct — and only one of them is unique to humans.

Here's what's going on behind your eyes.

Three Different Kinds of Tears

Your lacrimal glands (sitting just above each eyeball, under your brow bone) can produce three chemically distinct fluids:

1. Basal Tears — The Constant Lubrication

Your eyes are never dry. Every time you blink, a thin film of basal tears spreads across your cornea. These tears are 98.2% water, plus a precise blend of:

Mucins — spread water evenly across the eye surface (without them, water would bead up like on a waxed car)
Lysozyme — an enzyme that dissolves bacterial cell walls
Lactoferrin — binds iron that bacteria need to grow
Lipids — an oily outer layer that prevents evaporation

Your basal tear film has three layers (mucous, aqueous, lipid) and is replaced roughly every 5–10 minutes. You produce about 5–10 ounces of basal tears per day, most of which drains through your nasolacrimal ducts into your nasal cavity. That's why your nose runs when you cry.

2. Reflex Tears — The Eye's Emergency Response

When your eyes encounter an irritant — onion fumes, smoke, a stray eyelash, a splash of hot sauce — your trigeminal nerve detects the threat and signals your lacrimal glands to flush the eye. Hard.

Reflex tears arrive fast and in volume. They contain higher concentrations of antibodies (IgA) than basal tears, because the irritant might have introduced pathogens. They also have a different electrolyte balance — more sodium and chloride — making them saltier and more watery, optimized for flushing rather than for cushioning.

The onion effect deserves its own explanation: when you cut an onion, you rupture cells that release an enzyme called alliinase. It converts sulfoxides into syn-propanethial-S-oxide (a sulfur-containing gas). This gas wafts up to your eyes, dissolves in your basal tears, and forms tiny amounts of sulfuric acid. Your eyes say "irritant detected, flush protocol activated."

3. Emotional Tears — The Human Mystery

And then there are emotional tears. The ones triggered not by an irritant, but by a feeling. A loss. A joy. A moment of overwhelming beauty.

We don't fully understand why humans evolved emotional tears. But we know this much: no other animal does it. Not chimpanzees, not dogs, not elephants. They all produce basal tears. They all produce reflex tears. But none of them cry at funerals.

Emotional tears have a different chemical profile than reflex or basal tears. Studies by Dr. William Frey at the Ramsey Medical Center (1980s) found that emotional tears contain:

Higher levels of manganese — a mineral involved in mood regulation
Higher levels of prolactin — a hormone linked to bonding and stress
More protein overall — including natural painkillers like leucine-enkephalin
ACTH — adrenocorticotropic hormone, a stress marker

The theory: emotional tears flush stress-related substances out of your body. When people cry after a stressful event, they're literally excreting stress chemicals. This might explain why people report feeling better after "a good cry" — the tears themselves may be removing stress hormones from your system.

The Evolutionary Question: Why Crying?

If emotional tears have a biological function, what is it? Scientists have proposed several competing theories:

The Excretion Theory

The oldest hypothesis: crying is just the body's way of dumping excess emotion-related compounds. Like a pressure release valve. Dr. Frey famously said, "Crying is an exocrine process — a way of removing toxic substances from the body that build up during stress."

The Signal Theory (currently most supported)

Emotional tears are primarily a social signal. They communicate vulnerability, which triggers caregiving behavior in others. Think about it: you can scream "I'm hurt!" from across the room, but tears are intimate. They work up close. They make people pause. They elicit comfort.

Research supports this: people are far more likely to offer help to someone who's crying. Even photos of crying faces activate brain regions related to empathy (the anterior cingulate and insula) more than neutral faces.

The Blurred Vision Theory

There's a darker theory: tears blur your vision, making it hard to fight. In a conflict, crying — genuine crying — signals: "I can't fight you right now. I'm defenseless." This might have evolved as a submission signal, de-escalating conflicts in close-knit groups.

The Infant Analogy

Human infants cry before they develop language. It's their only signal for distress. Some researchers believe adult crying is an extension of this infant behavior — a vestigial survival mechanism that evolved into a sophisticated social signal.

The Chemistry of a Good Cry

Here's what happens in your body when you have an emotional cry:

Your parasympathetic nervous system activates — the "rest and digest" branch
Your vagus nerve fires, connecting your brain stem to your face, throat, and chest
Your breathing changes — the characteristic sobbing pattern, which is actually hyperventilation mixed with breath-holding
Your lacrimal glands receive the signal from both your sympathetic (arousal) and parasympathetic (calming) systems simultaneously
Tears are produced with a different chemical mix than your usual tears
Your heart rate initially spikes with sobbing, then drops below baseline as you settle

People typically report feeling worse immediately after crying (red eyes, puffy face, headache, fatigue) but better about 15 minutes later. The mood improvement — if it comes — arrives after a delay.

Who Cries More?

Studies consistently find that women cry more than men — about 3.5 times per month versus 1.9 times for men. But the gap is smaller in cultures where emotional expression is normalized, suggesting social conditioning plays a role.

The average crying episode lasts about 6 minutes. About 65% of people feel better after crying. About 35% feel worse — particularly people who experience "embarrassed crying" or cried in a social context where they felt judged.

Why Some People Can't Cry

About 1 in 10 people report being unable to cry. This condition — sometimes called "dry eye syndrome" in the emotional sense — can be linked to:

Certain medications (antidepressants, particularly SSRIs like Prozac, are well-known for suppressing the ability to cry)
Dehydration (your body can't produce tears if it doesn't have the water)
Keratoconjunctivitis sicca — a medical condition affecting tear production
Emotional numbing associated with trauma or depression
Neurological conditions affecting the lacrimal nerve pathway

There's also a condition called crocodile tears syndrome (Bogorad's syndrome) where people tear up when eating — the result of nerve damage causing the salivary and lacrimal nerves to cross-wire during healing.

The Practical Upshot

A few things worth knowing:

Tears drain into your nose — that's why crying makes your nose run
Crying is physically exhausting — the vagal activation is literally tiring
Crocodile tears are real — crocodilians actually do secrete fluid when eating, but it's from their Harderian glands (similar to our third eyelid), not from emotion
You can cry in space — but without gravity, tears don't fall. They pool on your eyeball in a liquid contact lens that can actually sting after a while
Onions make you produce reflex tears, but they're slightly different in chemical composition from either basal or emotional tears (higher in sulfur compounds, lower in protein)
Contact lenses interfere with normal tear film dynamics, which is why your eyes might feel dry if you wear them too long

Why It Matters

Tears seem trivial — a nuisance when they come at inconvenient moments, an embarrassment when they come in public. But they're one of the most distinctive features of our species. Humans are the weeping animal. The mammal that broadcasts vulnerability instead of hiding it.

Somewhere in our evolutionary past, hiding pain stopped being optimal. Showing it — signaling "I need help" — became the winning strategy. And that changed everything. It's part of what allowed human groups to become more cooperative, more empathetic, more bonded than any other primate.

Your tears are a signal older than language, designed to reach across the gap between people and say: I'm hurting. Please stay.

And most of the time, it works.

Post #35 / 35 — part of the Sidebar Blog's series on the strange science of everyday life.