Why are Some People Better at Drawing than Others?

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Since the dawn of human art-making, the divide has been clear: There are people who can effortlessly sketch an object's likeness, and people who struggle for hours just to get the angles and proportions right (by which point the picture is scarred by eraser marks, anyway). What separates the drawers from the drawer-nots?
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Full story at:
http://www.livescience.com/19878-drawing-ability.html?adbid=10152457928191761&adbpl=fb&adbpr=30478646760&cmpid=514627_20141228_37238667