I'm pretty sure they figured out they were a race in decline during their second golden age, 1500'ish years of plenty, during which time some even thought the dark elves had simply faded away. If we go back to real-world demographics, the better-off countries (generally "the western world") have lower birthrates than poorer countries, such as in the african continent; http://upload.wikimedia.org/wikipedi..._countries.PNG paradoxically enough though, in the richer countries birthrates tend to decline during economic downturns, while poorer countries have higher birthrates. Then again infant mortality rates are higher in poorer countries, and so of the many children not all make it to adulthood. As I am not very knowledgable on the subject, I'll just venture a guess that it is simply a 'cultural thing', in that it is usual for people in western countries to have a handful of kids, 2-3, because that is the general mindset, but also because almost all kids survive, so having a dozen is really expensive. This may be similar for the high elves, along with a slower path where a couple takes its time before having children, and after having a few and after these have grown up and moved out they feel no need to have some more children even though they have many centuries of their lifespan still ahead.