Examples of Soviet cheating in FIDE competition: Petrosian-Korchnoi match, 1971

I greatly admire Bronstein.  He was severely punished for not signing a letter against Korchnoi in 1976.  He had integrity.

I believe Spassky also refused signing this letter ? 

They were Bronstein, Gulko, Spassky and (fasten your belts) Botvinnik.

But he alleged he didn't use to sign colective letters.

"These numbers allowed the Soviet players to pre-arrange easy draws among themselves, to conserve strength for games against the Non-Soviet players"

Keres won three of the four games against Tal when the latter won the Candidates (and lost the fourth), three of Tal's four games against Smyslov were decisive, Spassky and Kotov won against Smyslov when he won the Candidates, Bronstein was beaten by Smyslov in 1950, etc.

Keres, Petrosian, and Geller were part of the "drawing cabal" in Curacao.

A 2006 statistical study, by Moul and Nye, demonstrated that Soviet master had a higher rate of draws among themselves in international competition than in all-Soviet competition.

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=905612

Spassky left for France that year, as I recall.

By 1992, Karpov was free to speak his mind and give an insider account in his autobiography.

Fischer wrote in Chess Review, [As early as 1953] there has been undeniable collusion by the Russians to freeze out Weastern competition."

At the time, the New Yorks Times opined aboy "... possible collusion between Soviet players to help one [Soviet] win a tourney, as against a non-Soviet opponent."

Elliot Hearst noted, "Reports from Curacao indicate an overwhelming number of the foreign correspondents and local orgsnizers also believed that the Russians were soft on each other ... to assure a Soviet victory."

"the Russians were soft on each other ... to assure a Soviet victory"

Neither Keres, Geller nor Petrosian were Russian, but in any case it is rarely mentioned that Keres and Petrosian scored +0-0=15 against each other from before Curacao to the end of their careers. Keres was almost 30 years older than Fischer and given that the exhausting format was much better suited for the younger players he may have wanted to save some energy where it was possible, without caring about helping anyone else than himself to win. No one ever suggested that the only Russian in Curacao (Korchnoi) or the tournament favourite (Tal) were involved in some drawing pact to ensure a Soviet victory, so a more limited and "selfish" pact seems considerably more believable to me.

It was a common colloquialism to call anyone from the Soviet Union a "Russian."  Technically incorrect, but widely understood.

Fischer though Korchnoi intentionally lost games to the Soviet leaders, but I personally do not think that was the case.  And Tal was obviously ill.

The charge that Keres, Geller, and Petrosian formed a cabal at Curacao originated with Korchnoi.  In the Moul & Nye article (linked below), two footnotes point to Korchnoi:

4 After his defection, Korchnoi claimed the the trio of Geller, Keres, and Petrosian had agreed upon a private arrangement to draw amongst themselves.

5 After the tournament, Soviet writers took pains to dsmiss Fischer's allegations (for example, see discussion by Flohr and Boleslavsky in Plietsky and Voronov, 2005 [Russians versus Fischer], p 105). Forty years later, though, Averbakh and Korchnoi clashed when Averbakh continued to deny the allegations while Korchnoi claimed that indeed, a private pact had been agreed to (pp 105-109).

http://www.fsb.muohio.edu/moulcc/sovietchesscartel.pdf

"Averbakh and Korchnoi clashed when Averbakh continued to deny the allegations while Korchnoi claimed that indeed, a private pact had been agreed to"

Fischer's allegations, that were denied by both Korchnoi and Averbakh, were that there was a Soviet drawing pact in Curacao to stop him from winning. What Korchnoi claimed was that his own (and Fischer's) opponents Keres, Geller and Petrosian drew the games against each other without making much efforts in them.

You would think so but stylistic rock, paper, scissors plays a role too.  Some players handle different styles differently.  It's why some 1900 rated players seem very tough while other 1900s seem easy.  

Korchnoi, at the time, and later, has denied he intentionally lost games in the 1962 Candidates tournament.  I believe him.

Korchnoi also later related that three Soviets had a drawing agreement.  In 1962, during the tournament, that was also alleged, and just afterward by Fischer.  I believe Korchnoi here too.

However, Averbakh had been less forthright.  His statements have been circumspect.

Korchnoi's wife was being held in the Soviet Union, against her will, as a political prisoner - solely to influence the outcome of the 1978 match between Korchnoi and Karpov.

It was the type of crime against humanity for which the Eastern Bloc was infamous.

The FIDE, had it not been a pawn of the Soviets, should have cancelled the match, tossed out the Soviet Union from its membership, and declared Korchnoi the Champion.

She could not leave the freely the country - which was a giant gulag in the first place.