fertility is dropping in the west (and other areas)
http://www.nber.org/digest/jul09/w14820.html
[h=2]The Cost of Low Fertility in Europe[/h] 
  In the long run, low rates of fertility are associated with diminished economic growth. 
    As in many parts of the world, Europe has seen a rapid decline in  fertility. In 1960, Estonia was the only European nation whose total  fertility rate was less than two. Today, only two European countries --  Albania and Iceland — have fertility rates above two. Several factors  are thought to be driving that decline in Western Europe: socioeconomic  incentives to delay childbearing; a decline in the desired number of  children; and institutional factors, such as labor market rigidities,  lack of child care, and changing gender roles. Also, the nations in  Eastern Europe have gone through major economic, political, and social  change.
 In the long run, low rates of fertility are associated with  diminished economic growth, according to a new study by NBER Research  Associate 
David Bloom and his co-authors 
David Canning, 
Günther Fink, and 
Jocelyn Finlay. In 
The Cost of Low Fertility in Europe (NBER Working Paper No. 
 14820),  they observe that in the short term, low fertility rates raise per  capita income by lowering families’ costs of child-rearing and boosting  the share of working-age people. But as that working-age population  moves into retirement, the number of workers who replace them will  shrink. So, whatever short-term boon European nations may have gained  from low youth dependency will be overwhelmed eventually by the economic  burdens of old-age dependency.
 If fertility rates stay at current levels and life expectancy  averages 80 years, this study implies that Europe’s share of working-age  people will fall from about 70 percent today to somewhere between 50  and 55 percent in the long run. That would suggest a 25 percent drop in  the number of workers per capita, assuming that labor participation  rates stay the same. 
 There are several ways to analyze the effects of fertility on  economic growth -- these authors choose to concentrate on age structure.  The idea is that fertility, mortality, and net migration together  determine the size of a nation’s working-age population. The bigger is  that group relative to the total population, the more workers there are,  and thus the more income the nation is likely to generate. The smaller  is that working-age group relative to total population, the smaller is  output per capita in equilibrium.
 Of course, small changes in any one of several variables can alter  the picture dramatically. In France, for example, where life expectancy  is 80, the fertility rate that would maximize the working-age share of  France’s population would be 2.1 if young people started working at age  20 and retired at age 60. With retirement at age 55, the working-age  share-maximizing fertility rate would have to rise to 3.1. With  retirement delayed until 70, that rate would drop to two. 
The same dynamic works at the other end of the working-age  spectrum. If young people entered the workforce at age 15, the fertility  rate necessary to keep everything in balance would rise to 2.6. If they  entered at 25, then fertility only would need to be at 1.8 (below  replacement level) to maximize the working-age share of the population.
 Another factor is immigration, which typically helps to boost the  size of a nation’s working-age population. But its impact is usually  quite small. Austria, for example, has Europe’s third-highest net  migration relative to its overall population, but over the past 45 years  the absence of migration barely would have changed its share of  working-age people, this study finds. Even if it did, political  resistance to immigration is high.
 “In short, migration is highly unlikely to have a major effect on  falling working-age shares in Western European countries over the next  decades,” the authors write. “The size of the economic repercussions of  declining working-age shares on economic development, however, will  critically depend on individual behavior.” 
Previous research has shown that for every extra child that a  woman has, her labor participation falls on average 1.9 years over her  lifetime. So as fertility falls, women tend to spend more time working,  which allows them to accumulate more savings, more experience, and  possibly a better-paying job. This accumulation of physical and human  capital may offset some of the overall long-term income decline that low  fertility suggests.
     
--  Laurent Belsie