A confound is something that biases the results of a study. It is a third, extraneous variable that accounts for the relationship between the two variables of interest. For example, much of the early literature on intelligence tests found that Americans of northern European descent had greater intelligence than immigrants from southern or eastern Europe. These results were confounded by language fluency as the immigrants were not fluent in English. We cannot conclude that the difference in test scores across ethnic groups is due to intelligence if it is confounded by language ability. There are statistical techniques to control for confounds, but they are not appropriate in all cases and it is always better, if possible, to avoid confounds in the first place.