Appealing to popularity or the fact that many people do something as an attempted form of validation.
The flaw in this argument is that the popularity of an idea has absolutely no bearing on its validity.
If it did, then the Earth would have made itself flat for most of history to accommodate this popular belief.
Shamus pointed a drunken finger at Sean and asked him to explain how so many people could believe in leprechauns if they're only a silly old superstition. Sean, however, had had a few too many Guinness himself and fell off his chair.