Many factors can influence pupil size. Typically, the pupils open up in low light situations to let in more light. However, some research suggests that attraction and mood might also cause pupil dilation.

The pupils are responsible for how much light enters the eyes. They will increase in size when the eyes need more light, and they will shrink to reduce the amount of light that enters.

The eyes are an important part of social interactions. For example, eye contact is central to most conversations.

Eye movements or changes can indicate various emotions, such as anxiety. Some research suggests that pupil size may even be an involuntary sign of attraction.

In this article, learn more about what might influence pupil size and dilation.

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Dilated pupils may indicate attraction.

The pupils are small circular openings that control how much light enters the eyes.

The colored ring around the pupil is the iris. This contains muscle fibers that control pupil size. The pupils get smaller or larger depending on how much light is necessary to allow for good vision.

For example, the pupils increase with dim lighting to allow more light in and improve vision. In bright settings, the pupils become smaller to prevent too much light from coming in.

When the pupils become smaller, this is known as pupil constriction. When they get larger, this is known as pupil dilation.

Changes in pupil size are involuntary. The autonomic nervous system controls pupil size. This is the same system that controls other automatic actions, such as breathing.

However, other factors can also cause pupils to increase in size, or dilate. Some examples of these factors include:

  • medications and other drugs, such as alcohol
  • eye injuries that affect the iris muscles
  • neurological conditions, such as Adie’s pupil
  • brain injury, such as from a stroke

In some cases, the pupils will dilate permanently. For example, someone with congenital aniridia will have larger pupils all the time, due to a deficiency of the surrounding iris.

In other cases, the pupil might change throughout the day.

For example, emotions such as anxiety may also cause changes in pupil size. This can be a result of the fight-or-flight response, which is involuntary.

Eye contact has long been central to human interactions. Changes in emotion might cause pupil dilation.

The autonomic nervous system triggers various involuntary responses during emotions, such as fear or arousal. Some research suggests that pupil dilation is one of these involuntary responses to arousal or attraction.

For example, one 2012 study recruited 325 men and women with varying sexual orientations. The researchers played erotic videos to the participants while monitoring their pupil size.

The study found that erotic videos led to pupil dilation in the participants. The team showed that bisexual men and heterosexual women had dilated pupils in response to erotic videos that included people of either sex.

Another study, this time in the journal Evolution and Human Behavior, focused on associations between pupil dilation and sexual attraction. The researchers found that it was possible to determine sexual interest from the size and darkness of the pupils.

Pupil size also appears to respond to hormonal changes. One study recruited 14 women to view arousing pictures on a computer screen on different occasions. Half of the women were taking hormonal birth control pills.

Women not taking birth control pills had dilated pupils while viewing arousing pictures during ovulation. This did not occur in women taking birth control pills.

The study therefore suggests that hormonal changes could influence arousal and pupil dilation.

There is also some evidence to suggest that pupil dilation may indicate trust. A study in the Proceedings of the National Academy of Sciences suggests that pupil size changes play a role in trust formation.

It remains unclear why pupil dilation might be involved in attraction. It could be a result of hormonal changes.

However, many other factors can also affect pupil dilation. For example, research also suggests that paying attention to something can influence pupil size. It could be easy to mistake paying attention for attraction.

More research in this area will be necessary to establish whether pupil dilation is a sign of attraction or a result of other factors, such as hormones or involuntary bodily responses.

Pupil dilation occurs for a variety of reasons. Mostly, pupils change size depending on how much light is necessary for proper vision.

Other factors can also influence pupil dilation, such as medication or drug use. However, some research suggests that emotions can also influence pupil dilation, including sexual attraction. This might be due to hormonal changes.

Related emotions might also cause changes in pupil size, such as trust.

More research in this area will help determine whether or not pupil dilation is a sign of attraction. It might also help to uncover why the pupil might change size in response to attraction.