Demographic challenges for the 21st century : a state of the art in demography : conference organized as a tribute to the continuing endeavours of Prof. Dr. Em. Ron Lesthaeghe in the field of demography - Surkyn, J.; Deboosere, Patrick; Bavel, Jan Van; Willekens, Frans; Koloriet, Sterrebeek (VUBPress, 2008)