As birth rates decline in many modernized countries around the world, it's interesting to think about what's driving that in each place, since the factors are never exactly the same.
In Japan, which is famous for having one of the lowest birthrates in the world (Germany has the lowest rate), a large part of it may be attributed to what is known as the "celibacy syndrome":
sekkusu shinai shōkōgun セックスしない症候群 (literally, "syndrome of not doing sex"; 39,100 ghits)
Incidentally, I was struck while typing this by what a wonderful example it is of the richness of Japanese orthography: katakana, hiragana, kanji, all in such a short space. If only there were Arabic numerals in there somewhere!
Nathan Hopson writes:
The verb suru, shinai する・しない ("do; not do") is always written with hiragana in contemporary Japanese. Historically, 為 was used (also as nasu 為す, which likewise means "do" and can also be written 成す), but this is never the case in contemporary Japanese outside of the occasional literary usage. I think it must have been part of the language reforms during the Occupation, but I have nothing to back that except my Spidey Sense.
Here are some journalistic and blog accounts of the "not doing sex syndrome":
[Thanks to Carley De Rosa, Nathan Hopson, and Cecilia Segawa Seigle]