Why do our pupils dilate when we’re aroused? Anatomy experts explain

His gaze softens as he draws closer to you. With one hand around your waist and the other cradling your jaw, he pulls you in. You look into his eyes, and notice his pupils have grown large and hungry.

So the story goes in every other romance novel, where enlarged pupils are commonly enlisted as imagery to indicate sexual arousal. And it’s not unusual to read advice online suggesting dilated pupils are a sure sign someone you like also likes you back.

But what does the science say?

In fact, it’s true: our pupils really do tend to grow large when we’re aroused. Here’s why.

What is the pupil?

The pupil is an opening in the iris (the colored part of the eye) which directs light through the eyeball and onto the retina.

Typically this opening is 2–4 millimeters in diameter in bright light, and 4–8 millimeters in darkness.

The black color of the pupil is the color of the inside of your eye. Surrounding the pupil are two tiny muscles of the iris which are under separate control.

The muscle around the edge of the pupil acts like a sphincter. When stimulated by the parasympathetic nervous system (sometimes known as the “rest and digest” system), it contracts to close down the pupil.

On the outside of the sphincter, another muscle acts like the springs holding the trampoline mat.

When stimulated by the sympathetic nervous system (the “fight or flight” system), it shortens to enlarge the pupil.

Your pupils and the six ‘fs’

There are two different mechanisms to make the pupils dilate.

The first is by direct sympathetic nervous system stimulation causing the pupil to dilate (enlarge). This is triggered when you need or want to:

1. fight
2. flee
3. feed
4. fornicate
5. get a “fix” (of illicit drugs such as cocaine or methamphetamine)

The second is by stopping the signals of the parasympathetic nerves going to the sphincter muscle of the pupil. This is triggered when you need or want to focus (number 6).

Together, these are sometimes known as “the six f’s.”

So, is it the same for all of us?

A meta-analysis of 550 heterosexual men, 403 heterosexual women, 132 lesbian women, 124 bisexual men and 65 gay men reported that pupil dilation is related to your sex and your sexual preferences.

Overall, the study found men’s pupils dilate strictly according to their sexual preferences, and women’s pupils dilate more variably.

The study found that heterosexual men’s pupils dilated more in response to erotic imagery of women, and gay men’s pupils dilated more in response to erotic imagery of men.

However, lesbian women’s pupils also dilated more in response to erotic imagery of men, and heterosexual women’s pupils dilated for erotic imagery of men and women.

Are large pupils more attractive?

Interestingly, a study of 60 young adults (aged between 18 and 26) found pupils of 5 millimeter diameter most attractive.

A pupil of 5 millimeters is abnormal for situations in bright light. Could it be that we’re attracted to the types of pupils we’ve seen before in the relative darkness of an intimate setting?

The idea of large pupils being attractive isn’t new. During the Renaissance in Italy, women used eye drops made from a poisonous plant called Atropa belladonna (belladonna means “beautiful woman” in Italian) to make their pupils dilate. This gave them a wide-eyed, “seductive” look (it also, unfortunately, was rather dangerous).

The plant contains a chemical called atropine, which is still (safely) used today by ophthalmologists and optometrists to dilate the pupils for eye exams or surgery.

Getting in sync

Pupil dilation also plays a role in social and interpersonal interactions. Studies have found administration of oxytocin (a hormone associated with bonding and trust) enhances pupil responses to emotional expressions, suggesting increased sensitivity to social cues.

Pupil dilation synchrony between people has been linked to better teamwork and mutual attraction, reflecting shared arousal states.

This phenomenon, sometimes referred to as “pupil mimicry” or “pupil contagion,” aligns with other autonomic synchronizations such as heart rate.

It all goes to show that so much of connection and attraction is subconscious.

What else can make the pupils dilate?

Various substances and medical conditions can also affect pupil size. Stimulants such as Ritalin and Adderall, anticholinergics (often used to treat Parkinson’s disease and chronic obstructive pulmonary disease), and certain medications such as phenylephrine (Sudafed PE), and benzodiazepines such as alprazolam (Xanax) can all cause pupil dilation.

So too can illicit drugs such as cocaine, ketamine, MDMA, LSD and cannabis.

Some neurological conditions or closed angle glaucoma, as well as stressful situations, can cause the pupils to stay dilated (a condition known as mydriasis).

If you have prolonged dilation of your pupils, you should speak to your doctor.

Does intellectual or emotional arousal cause pupil dilation?

When you are trying to solve a mathematics problem, listening carefully as you take notes, or listening to your favorite singer’s music, your pupils will enlarge.

Anticipation of rewards, emotional conflict, and processing of emotionally charged stimuli—such as scary movies or certain trigger sounds—also lead to increased pupil size.

Anxiety, pain, and even conditions such as fibromyalgia have also been linked to dilated pupils.

Context is everything

It is crucial to emphasize pupil dilation doesn’t automatically mean someone is aroused. Interpreting pupil dilation requires context, and you can’t assume big pupils means the person is attracted to you.

Verbal consent and other behavioral cues are essential.

If you’re wondering if the other person likes you, why not just ask?

This article is republished from The Conversation under a Creative Commons license. Read the original article.