This paper argues that the demographic transition has been central to the creation of the modern world, and that its role in this respect has generally been underestimated. The paper proposes a partial, causal theory – operating at the ‘super-macro’ level, and over the very long run – for several major aspects of world ‘development’. Key elements of the argument are that sustained mortality decline causes both urbanisation and fertility decline. In turn, urbanisation leads inevitably to an increased division of labour and the wider distribution of political power in society. Fertility and mortality decline contribute to reduced gender differentiation and women become more like men. Population ageing probably contributes to the rise of modern democracy. Unless development theory puts the demographic transition at its core, it cannot adequately account for key social–structural transformations which are integral to the concept of development. Copyright © 2001 John Wiley & Sons, Ltd.