This article attracted my attention today: Be your best friend if you'll be mine: Alliance Hypothesis for Human Friendship
Basically, it says that the mutual altruism model doesn't explain human behavior at all, when it comes to friendships and an Alliance based model makes a lot more sense. They compare it to nations who build alliances with other nations, just in case there's a conflict later (which kind of is a weird comparison, because that's still a human-friendship-relationship, and so it's comparing human friendship to human friendship, but whatever). It says, also, that the most valuable friends are the friends who have fewer other "alliances" or friends, because they can be counted upon more readily to support us in the instance of a conflict, whereas someone with lots of other strong relationships may not be able to support us totally, since their attention and time is divided.
I think that this kind of explains the single vs. married chasm**. Single people seem to have more single friends, and married people seem to be more interested in cultivating "couple" friends. But if this is about alliances, then for a single person, this makes a lot of sense-- you can't count on one half of a married party to ally with you. They always have an obligation to their mate that supersedes you, as a single entity. I would say that the only time this doesn't apply is when a single person is equally good friends with both halves of the couple in question, but traditionally, that doesn't seem to happen all that often. For example, My husband and I have several friends who are single and friends with both of us, but I have a lot more single friends who have no relationship with him at all. That being said, I would say that those single friends who are friends with both of us would divide in a conflict along gender lines, and as strong as those relationships are, they're not equally friends with both of us. The relationships are perhaps just MORE equal than others.
So. There you have it. All that drifting apart that occurs (or we're worried will occur) between single friends and their newly married buddies-- that's completely in line with the model. It's a natural consequence when we look at friendships with an Alliance based model. Mutual Altruism wouldn't really explain that either-- because whether you're married or single, the exchange of friendship services shouldn't really change that much, but Marriage is a swearing of allegiance, causing friends to fall in line as well. Now you're not only obligated in alliance to your friend, but also their spouse, in conflict. But your friend's first obligation will always be to their spouse, even if previously it was to their single friend.
Anyway, all of this is supposition without any real research backing it-- just my limited experience with human relationships and the world. But I think it's fascinating to think about, and even more interesting that this article doesn't touch on the single vs. married conflict, even though it seems kind of obvious to me.
**Disclaimer: By no means do I mean to say that by getting married, a person must eschew their single friends, or that a single person doesn't have married friends, and vice versa. It just seems to me that this is a popular trope of society and popular culture, if perhaps exaggerated. (Is trope the right word there? I'm leaving it anyway, because I feel like it applies.)