What is main effect?
From PsychWiki - A Collaborative Psychology Wiki
Main Effect Main effect is the unique effect of an independent variable on an outcome variable. The effect of a variable taken in isolation is known as the main effect. The main effect of an independent variable is the effect of the variable averaging across all the levels of other variables in the experiment.
Example / Application
For example, we can test the main effect of alcohol on the selection of mates in two different genders. In one of the experiments it was found that when it came to mate selection, the effect of alcohol was different for male participants as opposed to female participants. In other words, alcohol had very little effect for women; the attractiveness level of the partners they selected did not change regardless their alcohol consumption. Men, on the other hand, dropped their standards after consuming a certain amount of alcohol. While overall results can indicate that alcohol significantly affects whom the participants would hit on, it can be misleading. Only after examining the interaction effect, we would see that this is only true for males, and the females remain unaffected. In the video, we can see people who are pretty wasted. They don’t show this in the video, but if the main effect of alcohol applied to them the same way as described in the example, they would probably not be as choosy as usual in their mate selection.
Field, A. (2006). Discovering Statistics Using SPSS: Second Edition. London. Thousand Oaks. New Delhi. Sage Publications.