We propose a new theory of the demographic transition based on the
evidence from natural sciences that physical development during
childhood affects adult life expectancy positively. This theory is
embodied in a continuous time OLG framework where fertility,
longevity and education all result from individual decisions. We
conclude that a sustained improvement in physical development is at
the basis of the observed demographic transition in today's
developed countries and may be an important factor in explaining
the slow transition from the Malthusian towards the Modern era. The
dynamics of the proposed model reproduce the key features of the
demographic transition, including sustained improvements in child
physical development, permanent increase in life expectancy, a
hump-shaped evolution of both population growth and fertility and
late increases in secondary educational attainments.