Over half of the world 's population live in countries with `` medium human development '' ( 51 % ) , while less than a fifth ( 18 % ) populate countries falling in the `` low human development '' category .
Over half of the world 's population live in countries with `` medium human development '' ( 51 % ) , while less than a fifth ( 18 % ) of the world 's population are in countries that are in the `` low human development '' category .