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New analytical tools to calculate the variance due to cross-phase modulation (XPM) and four-wave mixing (FWM) induced intensity distortion are derived based on the Volterra series transfer function method. The analysis for both the XPM and FWM effects is based on the same system configuration with a continuous-wave (CW) probe channel plus modulated pump channels, which makes possible a fair comparison between the two nonlinear effects. Effective ways to reduce the XPM- and FWM-induced intensity distortion are given. The new results on the variance of the nonlinearity-induced intensity fluctuation also make it possible to study both synchronous wavelength-division multiplexing (WDM) systems with fixed channel delays and asynchronous WDM systems with random channel delays. The new analytical results provide accurate and efficient ways for system parameter optimization to reduce these two nonlinear effects.