This theorum, which I am currently developing, goes something like this:
When parents come to visit their adult children, it is impossible for them to resist using their visit as an opportunity to clean our their own homes.
James and I have boatloads of proof for this theorum, all on display in the growing mountain of boxes in our basement. Many of them are a ginormous collection of baseball/magic: the gathering/other nerdy cards that James accrued in his childhood, left behind when he moved out, and recently re-acquired over several of his parents' visits. Others are boxes full of Christmas decorations, household knickknacks, and other various miscellanea from my mom, also acquired over a variety of her visits. I imagine the parental thought process to go something like this: finally my son/daughter has a house of his/her own! Not only can I now give him/her all this old shit of his/hers that's been taking up space in my house, but I can also pawn off all the old things that I feel bad getting rid of but that I don't actually want in my house! Certainly he/she will want them or know what to do with them!
Of course, the problem is that neither of us really know what to do with all of this stuff. When we bought this house, we were (and remain) determined not to collect a bunch of stuff just because we had room for it in our basement. We don't want to live here for 5 years or 15 years or whatever and realize that we own truckloads of things that we'd totally forgotten about just because they'd been tucked carefully away out of sight. But with this constant influx of things from our parents, it's harder.
Not to say that they don't bring things that we like and will use, of course. It's just that much of the random stuff that makes its way up here is stuff that we'd forgotten about for a reason: it's not that important to us. But once it's up here again, it's much harder to get rid of. Trips to Goodwill take planning and foresight when you're schlepping your things by bicycle (or renting a zipcar for an hour); not to say that it's impossible to take care of all this without a car, of course, but it's just one extra step that makes it easier to let all these boxes slowly take over the basement.
Clearly, we should have been more proactive about getting rid of all our old stuff in our parents' homes so they didn't have things to bring us--but I'm pretty convinced that they'd still bring stuff anyway, even if it wasn't necessarily ours. It's the parental way, right?