Here http://www.abc.net.au/radionational/programs/hindsight/inside-robbers-cave/4515060 is a radio show on ABC radio Australia (roughly equivalent to the US PBS), casts a new light on the Robbers Cave experiment.

It is claimed that

1. Those conducing the experiment came in with a preconceived agenda which they wished to prove ie that conflict easily arises based on quite trivial group differences etc.

2. There had been two previous failed experiments in which attempts had been made to create the conflicts described in this study and involving in some cases quite blatant attempts to foment conflict (eg false flag attacks) (see at 12:15 in the audio).

3. In this third experiment conditions were artfully manipulated to create and then defuse conflict. It appears the key issue in creating conflict is that the two groups must not be permitted to get to know each other and become friendly, and that intense competitive situations are needed, preferably with zero or negative sum outcomes. To then defuse the conflict, allowing socialization no longer sufficed and it was necessary to create a common threat or difficulty to bring the groups together again.

So the statement in the main post that

Well, the 22 boys were divided into two groups of 11 campers, and— —and that turned out to be quite sufficient.

Is perhaps overstated significantly.

It is a very interesting listen.

I don't know if the claims are true. Given how 'right' the results of the original experiment feel, and did feel after WWII, one should be on guard.

Edit; corrections - the experiment was not rigged to quite the degree I originally said. But still I would argue not quite as advertised.