Myopia is a complex disease with a multi-factorial etiology. Twin studies and family studies have shown a high heritability for development of myopia, and more than 40 genetic loci have been identified. Outdoor activity seems to reduce the myopia progression. The shape of the eye may be of importance; axial growth makes the eye ellipsoidal which increase the peripheral defocus on the retina. Peripheral defocus has been suggested to induce further axial growth of the eyeball.