 If they are very close to another, they can exchange energy by FRET and because the emission can occur from one of many independently moving (or oriented) molecules this results in a lower than expected anisotropy or a greater decorrelation.This type of homotransfer Förster resonance energy transfer is called energy migration FRET or emFRET.Steady-state fluorescence anisotropy only give an "average" anisotropy.