Lower birthrates mean fewer young people, which means a shrinking workforce and more difficult economic growth. But there are other reasons to hope for more children, no less real for being more intangible. Mass reluctance to bring a new generation into the (admittedly troubled) world could signify growing pessimism in a society that has historically thrived on optimism. A disproportionately elderly society could be a wiser but less dynamic one, with fewer young people to take risks, contribute new ideas and — yes — provide youthful joie de vivre.