We study the effects of excess thermalized electron and hole populations on femtosecond carrier thermalization in optically excited GaAs quantum wells. In modulation-doped samples we find that the presence of 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ electrons produces an extraordinarily fast carrier thermalization (faster than 10 fs); however, the presence of the same density of holes yields a thermalization time close to that of undoped samples (\ensuremath{\cong}60 fs). These results give the first direct evidence that electron-electron interactions yield the dominant contributions to carrier thermalization in GaAs and also show that the electron distribution function itself influences thermalization dynamics.