The most long-standing theory, and the most widespread until recently, considered as cause of the colonization the problem of overpopulation in the Greek mainland. It should be taken into account that the rise in population is relative and constitutes mainly a sign of decrease in the mortality factor. Moreover, the view that colonies were a way of defusing the phenomenon of overpopulation at that time is not certain. In several cases of overpopulation, it has been noticed that nature took care of the problem (for example, outbtreak of epidemics that caused a decrease of population). Of course the human factor can not be ignored, which looks for solutions that can be applied as instantly and easily as possible. Therefore, if there were redundant citizens, research would have brought to light something of the above mentioned and the expedition to the colonies would be the ultimate solution. If there were a population explosion in the Greek mainland, nature and human resourcefulness would take over to deal with and control the problem. Therefore, there would not be any redundant citizens to be sent to colonies.

An example that oppposes this opinion is the case of Athens. Here, a rise in population is observed during the Archaic period. Nevertheless, the city, even though it did not have enough fertile soil, it did not participate in colonization. On the contrary it expanded internally, that is from the coast to the inland of Attica, in order to exploit new cultivable soils.