Mass killings under communist regimes occurred through a variety of means during the 20th century, including executions, famine, deaths through forced labour, deportation, and imprisonment. Some of these events have been classified as genocides or crimes against humanity. Other terms have been used to describe these events, including classicide, democide, red holocaust, and politicide. The mass killings have been studied by authors and academics and several of them have postulated the potential causes of these killings along with the factors which were associated with them. Some authors have tabulated a total death toll, consisting of all of the excess deaths which cumulatively occurred under the rule of Communist states, but these death toll estimates have been criticized. Most frequently