A lazy eye (amblyopia) is caused when something disrupts the normal development of vision.
How vision develops
It's often assumed that younger children have the same vision as adults, but this is not the case.
Children have to learn how to see – more specifically, their brains have to learn how to interpret the nerve signals sent from the eyes to the brain.
It usually takes around 3 to 5 years before children can see as clearly as adults, and up to 7 years before the vision pathways in the brain become fully developed.
If something affects one of the images the eye shows the brain as the brain develops, the signals become disrupted.
The brain starts to ignore the poor-quality images, resulting in a "lazy eye". For most children this happens because of a problem with the image in one eye. As a result, the brain becomes increasingly reliant on the stronger eye, which makes the weaker eye even lazier.