Isolated groups that find ways (preferably institutional) to match people who can serve each other’s needs could be expected to be more content, on average, than those that are unsuccessful. It should be noted that violence would not necessarily disappear in successful groups; it would just reach a level that everyone considers acceptable. Also, if environmental conditions caused an increase in stress, the successful group would need to be able to adapt.
Interacting groups would be forced to find new ways that include everyone as part of a new group. The world is currently experiencing the ultimate version of this, as all groups are drawn into the creation of a final, global community. The success of this effort will literally determine the future of our entire species.
Meanwhile, the environmental changes that have accompanied the merging of humanity, coupled with the depletion of critical resources, threaten to tear the world apart again, introducing epic stresses on the entire population that could lead to our complete demise. It is therefore critical that any solutions to maximizing happiness (and conversely, minimizing stress) be flexible enough to apply in any group, making the disintegration (if we are unsuccessful in avoiding it) as painless as possible.