Duty is one aspect of Natural Thinkamancy.
All units are subject to Natural Thinkamancies: Obedience, Loyalty, Duty, and others. Natural Thinkamancy doesn't compel Units to do things they don't want to do, or forbid them from doing things they do want to do. Rather, it influences their desires, making them want to do certain things.
Duty affects Commanders only, which means it affects Warlords, Casters, and possibly Chiefs of Natural Allies. It has a higher affect on Warlords, highest on Chief Warlord. It makes them want to use their own initiative in the service of the Ruler. It also makes them reluctant to withhold information, unless they are convinced that it is in their Ruler's best interests for them to do so. For example, Parson withholds from Stanley his strong suspicions that Charlie is responsible for their failure to locate any Gobwins, because he's sure Stanley would react inappropriately if he knew that Charlie was up to something. Duty also makes them very reluctant to conspire against the Ruler. Maggie has stated that that "Duty, in fact, compels us to take actions to benefit our ruler and our side". 
Duty can backfire when a Unit truly believes their Ruler is too incompetent to make decisions or what they want would destroy the Side, and decide to overrule them. Maggie demonstrated this by using mind control on Stanley. Caesar and Benjamin refused to let Don King empty the treasury to make a loan to King Slately, even when the King threatened to disband them.
Rulers may be affected by some form of Duty that makes them pop or designate heirs, an action that is good for their side but has no advantages for them (and can even be a dangerous option in some cases).
- ^Natural Thinkamancies - TBFGK_84a, Maggie mentions the natural Thinkamancies.
- ^Withholding information - LIAB Prologue 14 Parson reasons why he should withhold information from Stanley.
- ^Duty versus Loyalty - First Intermission 26, Maggie makes a Suggestion... spell.
- ^Refusing an order - LIAB 59, Caesar convinces Benjamin to refuse an order for the good of the Side.