Simple doubles the effectiveness of stat stages. For example, if Defense Curl is used, the screen will display that the user's Defense has increased by one stage and the game will store the user's Defense stage as +1, but the effect of that +1 stage will be equal to the effect of two regular stages and the user's Defense will be doubled during damage calculation. Similarly, if Tail Whip is used on this Pokémon, its Defense stat will fall one stage, but the effect will be treated as two.
The effect of stat stages cannot exceed their usual cap. For example, a Pokémon with Simple that has obtained +3 Defense stages will have quadrupled their Defense stat in damage calculation (the usual effect of +6 Defense); they can still continue to use a move like Defense Curl up to three more times to boost their Defense stage to +6, but these three additional stages will have no greater effect during damage calculation. The same applies to -3 Defense.
Because the stage numbers in-game have not changed, a Pokémon that uses Psych Up on a target with Simple only copies the stored (not doubled) stat stages. If a Pokémon uses Baton Pass to switch to a target with Simple, the target's passed stat stages will essentially gain double effect; if a Pokémon with Simple uses Baton Pass to a target without Simple, the target will not obtain any doubled stages. Similarly, if a Pokémon boosts its stats and then acquires Simple through a move like Skill Swap, its pre-existing effective stat changes will be doubled (barring caps) until it loses Simple.
Simple now directly doubles the number of stages that a stat is increased or decreased when the move, Ability, or item is used. For example, if Defense Curl is used, the screen will display that the user's Defense has increased by two stages ("sharply rose") and the game will store the user's Defense stage as +2, doubling the user's Defense as expected. Similarly, Tail Whip targeting a Pokémon with Simple will cause its Defense to directly drop by two stages.
As a result, moves like Defense Curl can only be used up to three times before they will fail (as the user's Defense stage will be the maximum of +6). If Psych Up is used on a target with Simple, the doubled stages will be copied. Using Baton Pass to a recipient with Simple will not cause the recipient to receive stat stages of double effect; if a Simple Pokémon uses Baton Pass itself, it will transfer the doubled stat stages. Similarly, if a Pokémon boosts its stats and then receives Simple through a move like Skill Swap, its pre-existing stat changes will not be doubled and nothing further will happen if it loses Simple.
The move Simple Beam can change the target Pokémon's Ability to Simple.