You've probably proved this theory to yourself and groaned about it, but researchers at the Stanford Graduate School of Business can prove it—buying one item reduces the amount of deliberation that goes into buying the next item, and the next one, and so on. In other words, as Get Rich Slowly's J.D. puts it:
Once a person decides to buy one thing, this creates "shopping momentum," increasing the likelihood that he will buy additional items. If you pick up an impulse item (like a magazine or candy bar) as you enter a store, this can serve as a trigger to encourage you to buy more.
It's a good reason to find your impulse buy weaknesses and stay clear of them, if possible, to avoid arriving home with a long, unexplainable receipt in your pocket.