As a member of this generation i am offended. I know equal numbers of dumbasses who were born both before and after 1991. In fact i would say that pre-1991 people are dumber in larger numbers, but the percentage is less.

The real big issue is the portrayal of said post-1991 generation in media. Beiber, Gomez, 1D etc from music, while tv shows (mostly US and Disney) such as Hannah Montanna do as much damage again. People also love to point at film with many modern films (and games, tv etc) as being simple and full of dumb jokes, but this is no more true than in the past. For every concert movie their is a "kids" film like Wall-E.

Then we get around to social media. This is the killer really. People online are dumb because most of the time they can be annonymous and such normal social rules do not apply. They will use "facts" in "debates" becuase such things can be quickly found on Google and then yelled to no real point. People might give off an image of a spoiled brat because of a lot of selfies on their FB page, which will also contain a lot of self obsessed ramblings. While hardly an endorsement of intelligence i admit, fuck sake it's facebook or twitter it's not meant to be smart.

As a keen Anthropologist specializing in Social Geographies there is a lot more to it than X Generation is dumber than Y Generation. Compared to pre-1900 standards a lot of 1901-1990 people would come off as dumb. For example i am fairly sure none of you speak Latin or could shoe a horse, whereas they could. Equally i reckon none of you pre-1991ers could sing all the songs by 1D. It may not be the same skills or knowledge but that doesn't mean it isn't knowledge.

While i admit their are clearly some dumb people in this generation don't let media stereotypes draw your conclusions. I went to school with someone who achieved 14 A* at GCSE and 4 A* at A-Level. I myself 100% an AS exam, and 97% the subject overall. And facts speak, ever year the overall level of pass grades in education rises, which has to count for something. And don't say exams are getting easier (admitedly most systems are flawed but then thats the pre-91 fault), try sitting a GCSE and A-Level paper. If you can score an A grade even that doesnt make them easy, it makes you smart.

And finally the 9.2% brain usage... Nope. You can call it an average but equally it means 49% of people use more, or that for 49% of the time more of it is used. And of course more will be used when it is needed, if i'm sitting around chatting i don't need to use more, but if i'm writing an exam i can use more, so it doesn't matter.
5000 people is a good size study but there will always be areas of contention over the people used and methods. 10% is a rough figure, but also saying 32% for X condition i equally not right. I've been working with kids who have Aspergers and various Autistic conditions and even if they use 32% they are still not smart as they cannot use it in the correct ways.
Better to apply all of 9.2% rather than none of 99%.