By far, the consensus figure for those that Joseph Stalin murdered when he ruled the Soviet Union is 20,000,000. You probably have come across this many times. Just to see how numerous this total is, look up “Stalin” and “20 million” in Google, and you will get 183,000 links. Not all settle just on the 20,000,000. Some links will make this the upper and some the lower limit in a range. Yet, virtually no one who uses this estimate has gone to the source, for if they did and knew something about Soviet history, they would realize that the 20,000,000 is a gross under estimate of what is likely the Stalin's true human toll.
The figure comes from the book by Robert Conquest, The Great Terror: Stalin’s Purge of the Thirties (Macmillan 1968). In his appendix on casualty figures, he reviews a number of estimates of those that were killed under Stalin, and calculates that the number of executions 1936 to 1938 was probably about 1,000,000; that from 1936 to 1950 about 12,000,000 died in the camps; and 3,500,000 died in the 1930-1936 collectivization. Overall, he concludes:
Thus we get a figure of 20 million dead, which is almost certainly too low and might require an increase of 50 percent or so, as the debit balance of the Stalin regime for twenty-three years.
In all the times I've seen Conquest’s 20,000,000 reported, not once do I recall seeing his qualification attached to it.
Considering that Stalin died in 1953, note what Conquest did not include -- camp deaths after 1950, and before 1936; executions 1939-53; the vast deportation of the people of captive nations into the camps, and their deaths 1939-1953; the massive deportation within the Soviet Union of minorities 1941-1944; and their deaths; and those the Soviet Red Army and secret police executed throughout Eastern Europe after their conquest during 1944-1945 is omitted. Moreover, omitted is the deadly Ukrainian famine Stalin purposely imposed on the region and that killed 5 million in 1932-1934. So, Conquest’s estimates are spotty and incomplete.
I did a comprehensive overview of available estimates, including those by Conquest, and wrote a book, Lethal Politics, on Soviet democide to provide understanding and context for my figures. I calculate that the Communist regime, 1917-1987, murdered about 62,000,000 people, around 55,000,000 of them citizens (see Table 1.1 for a periodization of the deaths).
As for Stalin, when the holes in Conquest’s estimates are filled in, I calculate that Stalin murdered about 43,000,000 citizens and foreigners, over twice Conquest’s total. Therefore, the usual estimate of 20 million killed in Soviet democide is far off for the Soviet Union per se, and even less than half of the total Stalin alone murdered.
