Some creatures are so dull of mind that even the battlefield can sometimes leave them rather distracted and confused.
Provided that they are not engaged in close combat, a unit that contains one or more models with the Stupidity special rule must take a Leadership test at the start of its Movement phase. If the test is passed, the unit will act normally this turn.
If the test is failed, it moves directly forwards using the Random Movement (D6) special rule in the Compulsory Movement sub-phase. The Stupid unit cannot take any further action that turn, so cannot declare charges or make a shooting attack. However, a unit that has failed a Stupidity test can still be forced to move, or perform any other action, by a spell, or other special rule/magic item.
Until they pass the Stupidity test again, models that have failed their Stupidity test have the Immunity (Psychology) and Random Movement (D6) special rule, except that they can only move directly forwards. They cannot choose to do a combat reform or choose any other charge reaction except Hold. In addition, Wizards cannot attempt to cast or channel power dice or dispel dice.