A recently published study by Anna McAlister and T. Bettina Cornwell at the University of Michigan reports that smarter kids are more affected by branding and know more brands. A number of people are interested in this because the naive prediction might have been that smarter people (including kids) should be less impressionable and less susceptible to marketing, rather than more.
The study caught my eye because it is a nice example of a problem that developmental psychologists run into. One question a developmental psychologist might be interested in is at what age children acquire a particular ability (like susceptibility to branding). This type of research has implications for education, public policy, etc. But the age you get depends on the age of the kids you test.
It happens to be the case that the children who are most easily recruited into developmental psychology studies tend to be relatively advanced. This happens for a number of reasons. Developmental labs tend to be in universities, which tend to be surrounded by relatively affluent, well-educated communities. Even within a community, not all parents are equally likely to bring their kid into do a study, and those that do seem to often be the sorts of parents that have advanced children. Many studies may disproportionately test children of professors and graduate students. It's easy to imagine additional reasons.
Unfortunately, there is a problem with the direct link to the study. It should be the top study on this Google Scholar search.