According to a popular belief

[Rachel Adams]

Hello.

Should it be ''according to a popular belief'' or '' to the popular belief?''

''According to a popular belief mice take children's lost teeth and bring them money instead''.

 

[GoesStation]

According to popular belief is a semi-fixed phrase. As such, that's usually the way you'll see it. There's nothing wrong with adding the indefinite article, though. If you do that, you're thinking about one specific popular belief.

You can generally replace "a/an" with "one" to see how the indefinite article affects the meaning. You often won't get a natural sentence (though there's nothing wrong with it in this example), but it should be grammatical.

 

[Rover_KE]

In the UK, the Tooth Fairy does that.

 

[Tarheel]

In the UK, the Tooth Fairy does that.

Same here.