The interwar period witnessed the emergence, diffusion and international adoption of population forecasting in the form of the cohort-component projection methodology. The interaction of population structure by age and sex and age-sex specific rates of the components of population change, mortality, fertility and migration, was used to indicate the most likely future course of national populations. However, right from the beginning the demographic approach was challenged by a revival of the belief that the future course of population is governed by a law. The belief was based on the (re)discovery of a homeostatic model, the so called law of logistic population growth. The logistic approach to population forecasting was advocated by the American geneticist Raymond Pearl, who introduced it in the 1920s. It replaced the 19th century Malthusian law of geometrical population growth. The decade of the 1920s witnessed the confrontation of the “logistic law” and “demographic” approaches. This article discusses the background and context of the confrontation of the two approaches, the issues at stake and the outcome of the contest. The debate on the method of forecasting population was initially a debate between biology and demography. The controversy was played out in conferences, articles and books, on the sidelines of the field where the technical innovations were made. The cohort-component method was easily applied in planning. It provided detailed insights into the factors accounting for the dynamics of population, and yielded details on the future population by age and sex.