Differential Effect is a limitation found on page 28 of GURPS Psionics. It is related to Accessibility but worth half as much for half the effect. The example given is Half Damage Against Men worth -10%.
Half is not the only option, it simply instructs to reduce the value of an accessibility based on the amount of reduction.
This is an extension of this guideline:
- "If the power is only weakened (half Power) instead of becoming useless, halve the value of the limitation."