If you find that income significantly varies on the basis of gender, you have main effect.
A main effect is the effect of just one of the independent variables on the dependent variable. There is one main effect to consider for each independent variable in the study. Thus, you don't always need main effects when there is an interaction.
For instance, in a design with age and gender as factors, the main effect of the gender would be the difference between the genders averaging across all ages used in the experiment.
Hence, main effects look at one variable at a time.
To learn more about main effect here:
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