What is the main cause for child myopia?

From what I’ve read, progressive myopia or nearsightedness is mainly caused by genetics (it is one of the causes, but most of the time, the cause of myopia is difficult to identify). It occurs when the eyeball is too long, causing light rays to focus at a point in front of the retina, rather than directly on its surface. It can also be caused by the cornea and/or lens being too curved for the length of the eyeball. That being said, how a person uses his/her eyes (e.g., often having to performing detailed or up-close work) may also influence the progression of nearsightedness. For more information, do take a look at this informative article: http://www.allaboutvision.com/parents/myopia-causes.htm