I suspect that the methodology reflects regional and perhaps ideological bias.

Are religious tithes included in charitable giving ?  (I know that local Baptist churches do not report their offerings to anyone).  Is being a deacon or Sunday School teacher in the local church considered being a "club officer" ?

From distant memory, I recall that Mississippi had the highest per capita religious giving.  Not bad for a "white" state.

Newcomers often join local organizations in order to "fit in".  Those that already "fit in" by growing up there and knowing everybody only join if they want to/enjoy the activity.

Alan, you're probably thinking of the Urban Institute's study back in 2000. Bible Belt states (including, it should be noted, South Dakota) were at the top of the list in charitable giving. New England states were generally the worst.

I sympathize with your suspicious attitude. I'd want to know a lot more about the assumptions of this study before I trusted it. I think it's important to distinguish between cultures that are unfriendly to outsiders, but very protective of one another, versus cultures that are friendly and open to outsiders in a nominal sense, but wouldn't be willing to devote personal resources to aiding them. Small town rural America (particularly in the deep South) is at one extreme, and suburbia/exurbia is at the other.

I also doubt social capital is a universal function. It seems likely that relative demographic correlation issues are quite important. If you are a Baptist and can join the local Baptist church, I'd bet Baptist churches in Alabama are as good or better than Baptist churches in San Francisco, and there are way more of them to choose from. On the other hand, if you're a Buddhist, San Francisco is going to give you a much deeper pool of resources.

There is more to social capital than mere church attendance and charitable giving. If you are a die-in-the-wool southern Baptist/Muslim/Buddhist/Catholic/Rastafarian/Zeus worshipper and you belong to no voluntary associations that have members of other faiths, how likely are you to trust a person not of your faith? History shows that absent the presence of these organizations the chance is not very high.
Right, there is "bonding" social capital, which strengthens the bonds within a group like an extended family, religious group or tribe. However this can breed a in-group versus out-group mentality that creates the seeds of conflict.

Then there is "bridging" social capital which emphasizes links between different groups and communities. A bowling league or a military unit can have the same impact of making people who would have never come together meet each other and build a relationship based on a shared activity goal. These bridging links are especially good at reducing intolerance and creating more mutual understanding.

We will need a lot of both to survive Peak Oil, IMHO