following up on this post, here’s an oversimplified illustration of how inbreeding can make the spread of altruism genes (within a lineage?) easier.
here are two families that look like our non-inbred, randomly mating family from the previous post, but they are about to start inbreeding (oh noes!). here we have seven brides for seven brothers four sons (on the left) and four father’s brother’s daughters (on the right):
and here are all of the possible offspring of all the possible marriage arrangements they could make:
aa + aa
aa aa aa aa
aa + aA (x4)
aa aa aA aA
aa + AA (x2)
aA aA aA aA
aA + aA (x4)
aa aA aA AA
aA + AA (x4)
aA aA AA AA
AA + AA
AA AA AA AA
now, what happens in an already inbred lineage? lets say the two fathers (who are brothers) share the exact same altruism genes:
what are all the possible outcomes then?
aA + aA (x4)
aa aA aA AA
aA + AA (x8)
aA aA AA AA
AA + AA (x4)
AA AA AA AA
as you can see, in the second scenario (in the already inbred lineage), the possible offspring combinations are much narrower — and all the possible marriage outcomes involve at least one chance of the (grand)fathers’ altruism genes being passed on exactly (barring mutations, of course). that only happened in half (3 out of 6) of the possible marriage arrangements in the not-so-inbred lineage.
lather. rinse. repeat.
of course, this is only one possible example. just meant to be illustrative.
edit: actually, one could imagine that both of the families were just starting to inbreed, only the fathers/brothers in the second instance shared exactly the same altruism genes while the first brothers did not. going forward in time, you can see how continued inbreeding would mean that the second family’s gene pool would remain much narrower than the first. several sub-clans of this lineage could really wind up to be very inbred, having only AA members in their extended families, while only a handful would wind up that way in the first example.
previously: an over-simplification and technical stuff and four things
(note: comments do not require an email. time for lunch….)
hbd chick 
very interesting
1) say you take as a premise that:
group synergy ~ numbers x cohesion
where group synergy is a bonus to all group activity.
2) take a second premise that environments determine optimal group size i.e. environments fix the “numbers” variable.
3) then you get the conclusion that maximizing group synergy in each broad type of environment would mean maximizing the cohesion strategy for the group size determined by that environment.
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so if you look at pastoral nomadism in light of the above
http://countrystudies.us/mongolia/39.htm
“The demands on human labor mean that a single household is not the optimal unit for herding. The basic unit in Mongol pastoralism is a herding camp, composed of two to six households, that manages its flocks as a single integrated economic unit. In the past, the members of a herding camp were usually, though not necessarily, patrilineal kinsmen.”
and you add in the correlation between FBD marriage and pastoralism it does make me think that FBD marriage might be the cohesion maximizing strategy for groups of that sort of size in which case FBD might not be related to pastoralism directly but indirectly through the optimal size of pastoralist groups.
which comes back to your post.