This dates from 3.5e (or earlier?) - the quote below is from the d20 SRD, and is the same in the PF SRD.
Dire flails, dwarven urgroshes, gnome hooked hammers, orc double axes, quarterstaffs, and two-bladed swords are double weapons. A character can fight with both ends of a double weapon as if fighting with two weapons, but he or she incurs all the normal attack penalties associated with two-weapon combat, just as though the character were wielding a one-handed weapon and a light weapon.
The character can also choose to use a double weapon two handed, attacking with only one end of it. A creature wielding a double weapon in one hand can’t use it as a double weapon—only one end of the weapon can be used in any given round.
This is open to interpretation, but my opinion is that, as double weapons are already a special case that differ from the general rules for two-handed weapons, that WotC intended for them to be wieldable one-handed as this implies, as well as the explicitly stated two-handed or two-weapon methods.
It appears that Paizo took a different view. There is a feat to specifically allow a character to wield a quarterstaff (a double weapon) one handed.
Quarterstaff Master (Combat)
You can wield a quarterstaff as either a two-handed or one-handed weapon.
Prerequisites: Weapon Focus (quarterstaff ), base attack bonus +5.
Benefit: By employing a number of different stances and techniques, you can wield a quarterstaff as a one-handed weapon. At the start of your turn, you decide whether or not you are going to wield the quarterstaff as a one-handed or two-handed weapon. When you wield it as a one-handed weapon, your other hand is free, and you cannot use the staff as a double weapon. You can take the feat Weapon Specialization in the quarterstaff even if you have no levels in fighter.
While the value of the feat is argued (due to the "WotC" interpretation of the double weapon rules), Paizo's official position is that you can't wield one handed without it.
Hey there folks,
You cannot normally use a double weapon in one hand unless it is sized smaller than you. This feat allows you to get around that restriction.