reiner klingholz is one of germany's leading population researchers.ince the 1990's, he's been looking into the question of how many people can earth support. >> we live on a planet that has limitations. the resources s required to guarantee people a liviveable le are finite. whether we're talking about energy, infrastructure, roads,, housing, or health care. at s some point, the global populaonon will so o hugehat this will be a problem. >> it already is a problem inh populaonon will so o hugehat ratetes. demogrgraphics reseaearchers y that a country can only develop if the birth rate is low enough. harsh as it sounds, children don't contribute to wealth eation..that means y >> t there needs t to be a falg birthrhrate and a rerelativelyh number of young adulults reachg a working age and actually finding jobs. only then have we seen progress. >> this development has already taken n place in many countrie, which is why the worldwide birthrate has declined significantly since the 1960's. today, women have an average 2.4 children. in chad and mali, the fe