A special case of diffusion in which the probability of a population member adopting an innovation is proportional to the number of current adopters within the population. It is a mathematical representation of “keeping up with the Joneses.” If the number of adopters is *Y*_{t}_{ }and *a*
is the saturation level, then the equation

describes the growth of the number
of adopters of the innovation over time (*b* and *c* are constants
controlling the rate of growth). For a discussion of the logistic and related
diffusion curves for forecasting, see Meade and Islam (2001).