Weak Social Theories
Much of armchair social science is pondering "puzzles," i.e., social behavior that doesn't fit well with simple theories, such as yesterday's cashless Christmas wish-list puzzle. A few standard theories can be easily invoked to explain most any social puzzle:
Preferences – we just intrinsically want the specific outcomes produced by certain behavior; it is not instrumental toward other goals. E.g., folks pick their nose because it just feels good, not to clear out their noses.
Mistakes – typical mental heuristics and patterns of thought just consistently misjudge the situation. E.g., we buy mid octane gas because just we didn't think of mixing high and low octane gas.
Norms – we do the usual thing so we won't look weird; norm violators tend to be odd and poorly informed about social conventions. E.g., you don't want to hire a guy who doesn't wear a suit to an interview.
The problem is that it is way too easy to explain most anything with such theories, at least if most possible preferences, mistakes, and norms seem nearly equally plausible. We can get more mileage out of specific and constrained theories of what are plausible preferences, mistakes, and norms. But no doubt many puzzles are in fact explained by odd preferences, mistakes, and norms. So what do we do?
It seems to me that the right approach is to seek theories that, with only a few free parameters, can explain a wide range of diverse puzzles. Trying to explain each puzzle with a new ad hoc story just doesn't get you very far; much better to lay out lots of related puzzles on the table and then ask how to explain many of them all at once. For example, instead of making up an ad hoc theory to explain US Christmas giving, collect "stylized facts" about holiday, wedding, birthday, and other gifts from across diverse cultures, and then look for theories which might explain a lot of that variation all at once.
Christmas gifts are given in place of money because they are representative of the gifts given by the maji. We exchange gifts in remembrance of Jesus' birthday.
I have some thoughts about the Christmas gift phenomenon, if it is not too late to chime in. I think that a lot of people got pieces of the puzzle right, but I wanted to identify an additional component - we want to consume things ourselves, but at the same time, personal consumption looks bad because those are resources we are NOT using to signal our commitment to others. The gift-as-signaling theory definitely plays a part, in that we can be genuinely pleased with a surprisingly apt gift, but that doesn't explain why people give out gift lists. However, if the goal is to maximize our own consumption, while ALSO appearing to send commitment signals to others, we can mutually benefit from informed gift giving.
My guess is that there is also a prospect theory story going on here - we value resources that we earn for ourselves a lot more than resources given to us. We WANT to increase consumption without incurring the mental penalty of spending our money. Getting a gift, we can frame it as an unearned gain and consume it more readily, while spending our own money is framed as a loss. The mutual gifting process essentially allows us to get around our biased utility function.
The other components that people have mentioned definitely factor in as well - particularly how cash nets out, and so you'd see it in non-reciprocal relationships more frequently. The story about information and search costs I think is valid as well, and it fits in with my story just fine. You provide a list of things, and the gift-giver can either guarantee you a guaranteed utility gain by buying something on the list, OR they can optionally use your stated preferences and their personal information to present you with a gift that they predict you will like but that you simply did not know existed yet. As I said above, this pleasantly surprises us, since it is a successful forecast of our own preferences.
Hopefully this is not too weak of a social theory!