I'm starting to see some signs that Pepper's easing out of her honeymoon period after four days with us, which prompts this post.
When a dog arrives in a new environment -- especially if that dog is coming out of a chaotic, stressful place, as is the case for most shelter mutts -- it's likely to show inhibited behavior for a while. Basically, until the dog can figure out the manners and mores of this strange new land, it'll be a little more cautious and polite than usual, just as you or I would probably try to be extra-courteous when staying as the houseguest of someone we don't know well.
Once the dog gets more comfortable, it relaxes, and its real personality starts to shine through. Exactly when this happens varies from dog to dog. Shy or skittish dogs and those who have had a lot of instability in their backgrounds tend to take longer, but my experience has been that most of the foster dogs we get start relaxing within a few days and are completely comfortable in about two to three weeks. This is when you might start seeing some naughty behavior -- suddenly that previously-polite dog is raiding the garbage can, stealing your shoes, and making all kinds of trouble. Alas, the honeymoon is over.
But while it lasts, the honeymoon period is particularly relevant to foster care for two reasons:
(1) It operates as a "reset button" that allows you to impose new house rules and manage potential unwanted behaviors from the beginning, even before they manifest. Maybe the dog pooped in the closet and stole shoes at its old house, maybe it didn't. Doesn't matter. You can teach it from Day One that in this household, poop goes outside and chewing is restricted to dog toys (of which you should have a plentiful and interesting supply). For a few days, at least, you have a clean slate to make it clear what your rules and expectations are. Here's where canine limitations on generalizing actually work in your favor: shoes in your house are not the same as shoes in the old house, so it's not hard for the dog to learn to treat them differently.
As long as you're clear from Day One, the dog should be able to relax gradually into a structured, supportive environment where its normal doggy behaviors can be channeled into acceptable outlets -- and this is a great saver of time and trouble. Ounce of prevention/pound of cure, and all that. The honeymoon period gives you a chance to slip that prevention in there right at the start.
(NB: The "reset button" effect only applies to minor mischief and bad habits. Serious behavioral issues, like severe separation anxiety and crate soiling [where the dog isn't just clueless about potty training, but is actually accustomed to living in its own filth, as some hoarder cases and puppy mill survivors are], are NOT reset and may actually get worse as a result of a change in households. You can expect your dog to not bark at the UPS guy for the first couple of days; you can't expect him to brush off the effects of poor breeding or puppyhood undersocialization.)
(2) If you're only fostering the dog for a short period, you may not have enough time to get an accurate sense of what the dog's actual personality is like. This is especially true if the dog is stressed or shut down and thus displaying an energy/noise level higher or lower than its normal state of being. In some cases, mistaking the dog's honeymoon-phase personality for its real personality (or being ignorant of bad habits that only manifested themselves later) has led rescue groups to make inaccurate assessments and thus bad matches between dogs and adopters. This risk is one of the reasons that I strongly prefer to keep my foster dogs for at least a few weeks before placing them: I want to be sure that I have enough information to give prospective adopters.
My experience has been that our dogs don't drastically change their personalities past the honeymoon period, and don't even really display any major new vices, but to some extent this is probably influenced by good luck (we haven't yet had any real tough cases come through the foster program -- fingers crossed it stays that way!) and pre-emptive management. I try to use the honeymoon period to set them up for success, and so far it's worked pretty well. Not perfectly, by any stretch, but it's definitely been a help.
It'll be interesting to see what Pepper blossoms into.