Looks good on paper right? But on closer examination, this study is certainly flawed. Many longitudinal studies are, because they focus on one group of kids in one area, and they do no account for the prevailing culture of that area.
This study follows kids that grew up in the Berkeley area, which has been a den of leftist thought for many years now. In fact, many consider Berkeley the birthplace of American leftist ideology. So with the kids growing up in an area that is predominately on the left side of the spectrum, it's no wonder that they internalized the values, they did. Knowing the leftists that I do, they do not like to share the stage with dissenting opinions. The kids that were the most confident were most likely the kids whose parents were of the majority, therefore they wielded the power. And with kids being kids, that left the kids from conservative families feeling shut out and whining is the only thing that some kids know to do, when they perceive they are not being listened to.
I bet the the study would have turned out differently, if the study had been conducted with kids from, say, the midwest or the south. There, the predominant culture is conservative and they are the ones in power, at least for the most part.