Here is a good reason to think that people would help others less even when everybody is in agreement:

https://www.youtube.com/watch?v=mwzBJF5Q7Vw#t=3m29s

This is Dan Ariely describing "The public goods game" and the instability of 'cheating'. Cheating in scare quotes because the conclusion stands -- in fact is enhanced -- even when the 'defect' choice is regarded as a choice that members of society are free to take (in fact that is the case he discusses).