This paper demonstrates that, to a close approximation, the intrinsic growth rate of a population is equal to the mean of age-specific growth rates below age T, the mean length of a generation. This mean is normally close to the growth rate of the entire population block below age T. Therefore, when a disparity exists between the intrinsic growth rate and the actual growth rate of a population (whether or not net migration is included in both rates), it must be attributable to an unusual growth rate of the population block above age T. One implication is that the ‘momentum of population growth’ is entirely confined to the age span above T, approximately age 28 in developing countries to-day.