Genetics

• Your eyelid design is the result of your genetics. There are many factors at play in determining whether you have double or single eyelids, including the structure of your face and eye area. Additionally, the level of the skull and shape of your lateral orbital rim in relation to the medial orbital rim determines the angle and width of your eyelids.
  
  

Types of Single and Double Eyelids in Asians

• The different types of Asian eyes include the single eyelid that has no crease, the eyelid with non-continuous or broken crease, the eyelid with a crease, eyelid with a partial crease, eyelid with a parallel crease, eyelid with a nasally tapered crease that widens on the outer end -- and the eye with multiples creases. The typical caucasian eyelid is called semi-lunar and has a crease.
  
  

Tissue

• Tissue layers differ in all patients, including those with and without double eyelids. The tarsus, which is a fibrous band of tissue, creates the straight and firm edge of the eyelid margin. You can see it when you pull down the lower lid or flip the upper eyelid inside out. In non-Asians the height of the average tarsus is more than 10 mm in the upper eyelid. However, in Asians the tarsus is closer to 8 mm.
  
  

Fat

• Asians have another layer of pretarsal fat below the orbicularis muscle. The insertion of the orbital septum is also lower and and they have a thicker layer of skin, which makes the pre-tarsal Asian eyelid much thicker than non-Asian. Whether or not you have an eye crease is determined by the thickness of the surrounding eyelid tissues and the strength and length of the attachments between the tarsas and the skin. One reason that Asians may have thicker eyelids is because they evolved in Northern Asia and needed a means to protect their eyes from the glare off of the snow and from the cold and wind.
  
  

Fibrous Attachments

• Fine fibrous attachments in the upper edge of the tarsus are stronger in those born with double eyelids. Double eyelid fibrous attachments are also stronger than the fibrous attachments found in those who aren't born with a crease but eventually develop one, as well as those who have single eyelids.
  
  

The Nose

• In many of Asian descent the nose bridge doesn't elevate as it does in other ethnicities. Because of this there is a prominent epicanthal fold, which has to be dealt with if the person opts to undergo double eyelid surgery.
  
  

Japanese Terms

• The Japanese call the two types of eyelids "hitoe," which means single crease, and "futae," which means double crease.