While everyone is familiar with Darwin's theories about the evolution of living species, it is not generally recognized that the development of the brain itself, both during the embryonic stage and then after birth, constitutes an evolution in the course of which the connections among nerve cells are themselves subject to a process of selection. The fact that this process continues through further stages of evolution, at higher levels of organization, may account for the unfolding of thought and mathematical reasoning, perhaps even for imagination. Given a widely spread innumeracy of the population, it is natural to suspect the environment in which brain's evolution takes place. Instead, they generally retrieve the result from a memorized table. Retrieval time from the memory table increases as the operands get larger.
Several reasons are available to explain dependency of the retrieval time on the problem size.
A very plausible explanation stems from the drilling practices. Researchers have tallied up how often each addition or multiplication problem appears in children's textbooks. The hypothesis that memory plays a central role in adult mental arithmetic is now universally accepted. It does mean, however, that a major upheaval in the mental arithmetic system occurs during preschool years. Children suddenly shift from an intuitive understanding of numerical quantities, supported by simple counting strategies, to a rote learning of arithmetic.