I begin with the acknowledgment of individuals. To facilitate discussion, I will talk about these most naturally as single organisms but, in principle, I think it could be any entity with a given set of properties existing in the world. I further acknowledge that individuals can come together to form groups composed of multiple individuals. For the time being, I'm not going to fret too much about how that group is defined or how it's bounded, though that's clearly something that needs to be considered. I should also point out that I'm not concerned with the fact that individuals can be thought of as collectives of individual entities. That simply moves the level of analysis to differing levels of complexity without affecting the broad theoretical framework I hope to develop.
I propose that, once a particular group is constituted, it becomes an entity in the world in its own right. If this is so, then a group, just like an individual, can become subject to selection pressure. As an aside and in a nod to Ian's theoretical work, my intuition tells me this will hold true regardless of whether we think of selection for or selection against. However, I haven't come close to thinking through any of that so I'll leave it for now.
This brings us to a crucial point. Namely, individuals who are members of groups are themselves still subject to natural selection. This means that natural selection can act at all levels of organization. This, I think, is obvious but somehow it's escaped our attention so far as we've tried to think our way through what environmental factors might be favorable to tight or loose groups. Furthermore, I think this may be absolutely key. To see why, consider that there is no requirement that the selective pressures at both levels or organization be at all closely aligned. I expect that groups can evoke emergent properties that make them subject to pressures not necessarily felt by individuals. Whether the converse is true, whether individuals can be subject to selection pressures not felt by the group they belong to, is a bit thornier so I'll set that aside for now. At the same time, even though selection pressures at different levels need not be aligned, it's not unreasonable to imagine that they often are.
This is all still a bit sketchier than I'd like it to be, but I think it sets up some interesting predictions. For instance, individual variation in any particular trait should be inversely proportional to selective pressure working on that trait. If there is strong selection for a suite of traits, then variation in those traits should be relatively low. If selection is weak, then variation should be greater. (And here I suppose the selection for vs. selection against changes the syntax a bit. Whether it changes things more substantively is an open question.)
If all this holds true, then we can start asking interesting questions. The simplest is, what happens when selection is weak at both the individual and group level? The prediction in this case is that such a circumstance would allow high degrees of variation at the individual and group level. Many different kinds of groups should be able to thrive in such an environment and the variation of individuals within each groups should also be high. All groups would likely become loose groups, with high levels of variation.
What if selection pressure at the group level is high, but selection at the individual level is low? Under these conditions, we'd expect less variation in the kinds of groups that exist in an appropriate level of analysis, or perhaps more precisely, we'd expect groups existing within that context to exhibit less variation in those traits that are adaptive to the pressures being exerted. On the other hand, the traits exhibited by individuals within each group could show considerable variation as long as the composition of individuals reliably evoked the group level adaptive traits needed for the group to survive. Given that the group is under strong pressure in this scenario, we might expect there to be some core traits that are in some sense inviolable; without them, the group could not survive. Again, groups facing this kind of differential selection pressure at the group and individual level should be loose.
Tight groups, then, should emerge any time that selection pressure at the individual and group levels are relatively high. One thing I need to think more deeply about is what happens when selection pressures at the group level are weak relative to selection pressures at the individual level. My sense right now is that this would not affect tightness so much as it would the variation in the kinds of groups that might emerge within the environment, but I'm not sure that really follows.
However that may turn out, it seems to me that this basic framework, sketchy as it is, does at the very least accounts for many of our existing observations. For instance, fundamentalism, which neatly fits our tight category, is associated with existential insecurity. In other words, when individuals are living on the edge of existence, when they are experiencing strong selection pressures, they form tight groups. In the most extreme circumstances, the survival of the group and the survival of the individuals within the group are very closely aligned in that the traits that are individually adaptive within that context are likely very similar to group level traits that are similarly adaptive. An interesting prediction from this that I don't think I've heard before is that because selection pressure at the group level is also high, you would expect to see a reduced variety of groups operating within that environment since relatively few kinds of groups will be adaptive.
In times of existential security, by contrast, selection pressures at the level of the individual are greatly reduced. This should lead to a greater variety of groups and to a greater tolerance of diversity within those groups. If these groups end up in competition with one another for some resource, then the variety of groups should diminish in response to the selection pressures, but tolerance within each group for diversity should remain relatively high as long as the diversity does not affect those group level traits that are adaptive. This could account for why some of the loosest churches we've come across seem to have members who are middle class and uncharacteristically high levels of education. This lines up neatly with some of the strict/lax literature, though it reaches the same predictions from a very different direction. It also accounts for why many of those churches are struggling to survive. Although in many cases they seem oblivious to the fact, they are in fierce competition, not only with other religious groups but, as David is so fond of saying lately, also with secular alternatives. That they are failing to grow or even hold steady likely reflects that they have not found, despite (or perhaps because of) all their internal variation they have not hit upon the "trick" of surviving in the current social context.
Whew! I could probably go on, but I doubt anyone has made it this far. Hell, I don't even have the energy left to go back and proofread what I just wrote. Nevertheless, I'm glad to have laid all this out and I welcome feedback. I'm not quite there yet, but I think I'm getting closer and I think we're getting close to being able to make some novel predictions that we can test.