It Takes Two When it Used to Take One ...

According to this NY Times article, momentum requires at least two wins ...

Contrary to some pre-election analyses, there was no reason to expect that Obama's Iowa victory would have much effect on New Hampshire voters, according to scientists who study so-called preference cascades. In studies of cascades, people do tend to follow others’ lead against their own preferences -- but not until at least two others have made a choice, said Duncan J. Watts, a sociologist at Columbia and senior scientist a Yahoo Research. Mr. Obama needed to take New Hampshire to get the snowball rolling.