There was a mass exodus between 1925 and 1930, the number who left is unknown but memorial attendence dropped by at least 80% (e.g. 1925: 90,434; 1926: 89,278; 1927: (unreleased); 1928: 17,380; see Your Will Be Done, 1958, p. 337 and the 1958 Yearbook, p. 284). That was the genesis of Rutherford's "remnant" doctrine about the anointed class (cf. 15 July 1927 Watchtower, p. 216; 15 September 1927 Watchtower, p. 280; 1 December 1927 Watchtower, p. 355; 15 April 1931 Watchtower, p. 119; 15 November 1930 Watchtower, p. 342; 1931 Yearbook, p. 57), he had to come up with some Bible-based explanation for the serious losses in membership. Then the numbers began to rebound during the Great Depression in the early 1930s with new converts being attracted to the movement (memorial attendance in 1935 was 32,795, see 1936 Yearbook, p. 53; 15 December 1988 Watchtower, p. 12). That posed a potential contradiction with his existing "remnant" doctrine, so he declared that these new converts belonged to a new class (the Jonadabs), which would increase and expand with Jehovah's blessing while the "anointed" class would continue to decrease. In 1935, the existing membership of the "anointed" class was effectively capped, although the facts of history show that a continual small number of converts continued to flow to this supposedly-closed class.
There was also an exodus between 1975 and 1980. Although it was a much smaller proportion of the overall membership compared to the situation in 1925-1930 (i.e. a decrease of only a few percentage points, but since there were also new converts, the number leaving was greater than the decrease), since the JWs were a vastly larger movement in the early 1970s when compared to the early 1920s, it is possible that this was a larger exodus.
