There are many factors behind the shoshika (the declining birth rate) trend. One is mistrust on the part of Japanese women toward child rearing. The feeling is: Why have children and divest the best years of one's life bringing them up when they're likely to metamorphose into shonen-hanzaisha (underage criminals), netto otaku (Internet nerds), geima (game freaks), toko kyohi (those who refuse to go to school) -- or all of the above -- some time after their 12th birthday?
Even if such a fate isn't in the works, "Dose hikikomori ni naruyo (in any case, they'll become recluses)" says my friend Tomomi, who has agreed with her husband not to have children, ever.
It's a sad development, but she has a point, of sorts. It's a heartbreaking task to be the parent of a hikikomori (someone who has withdrawn from society), and there's no guarantee it won't happen, no matter how isshokenmei (hardworking) the parent is.