Koordeman and colleagues (2011) conducted a study on the effect of commercials on alcohol consumption. Participants watched commercials for one of two types of beverages: alcoholic or nonalcoholic. Researchers then measured their consumption of alcohol during a 2-hour movie. Finally, participants completed a questionnaire on their usual weekly drinking habits. The results showed that typically heavy drinkers were influenced to drink more alcohol when they saw alcohol commercials, whereas the type of commercial did not influence the alcohol consumption of typically light drinkers. What is a factor in this study