conquer city with pop 1?

[Dooke]

I have been playing civ for year now and i've always wondered the following:

When i attack and conquer an enemy city with the population of 1 i either:
- get to choose whether to keep or raize the city
- autoraize the city


My question is, why is there this difference, and how can i tell before attacking a city with pop 1 if i'm gonna autoraize it or if i'm gonna have the opportunity to keep it?

Not even my good friend google could help me, so i hope you guys can.

 

[Sporknife]

If the city has ever been over 1 pop, you get to choose. if not, it's autorazed.

 

[Popvox]

Not always, a city with any culture can be kept, so a pop 1 with 1 culture could be kept, but a pop 1 with no culture is autoraised