What causes astigmatism?

Although spectacles were invented in the 13th century, methods of testing for astigmatism have been developed only within the past one hundred years. Even today, the etiological factors responsible for corneal astigmatism are not well understood. A mechanism which may account for the existence of corneal astigmatism is proposed, on the basis of data concerning changes in astigmatism with age, racial variations in astigmatism and changes in astigmatism due to the wearing of contact lenses.