Why are we short-sighted?

Interesting article, thanks.

The author relates being told by his childhood ophthalmologist that he "could blame [his] genes and a love of reading" but I'm certainly no fonder of reading than either of my parents (though we are all quite bookish, my mother especially so) and they both made it to their 60s without glasses and now need reading glasses, whereas I became shortsighted in my early 20s - so that explanation has never sounded right to me either.