The demography of Greece is a very difficult subject to investigate because of the shortage of relevant statistical data. Ancient authors did not write any books about demography and give hardly any figures for population sizes, and none at all for vital rates. Owing to the emphasis on war in ancient historiography, most ancient demographic estimates relate to the size of military forces or to the manpower available for military purposes—i.e., to adult males only. Total population sizes must be extrapolated from such information because women, children, and slaves were usually not enumerated at all. Moreover, literary authors were prone to exaggeration—with respect to the size of Persian armies, for example—although Thucydides (2) was a notable exception to this rule. Even in Classical Athens, for which the sources are relatively abundant, it seems unlikely that there was a central register of hoplites in addition to the deme registers. In general, Greek states did not have taxes payable by all inhabitants that would have required the maintenance of detailed records for financial purposes, and censuses of citizens were rare in the ancient Greek world. It is certain, however, that both mortality and fertility in ancient Greece were high by the standards of modern developed countries. Human mobility, whether voluntary or involuntary, was also an important factor in the population history of individual cities.