Jared Diamond makes a similar argument in Guns, Germs, and Steel for why the people of New Guinea are smarter than their western counterparts.
The important thing to note is the human population covers the globe. There are plenty of sifts in populations like Lactose tolerance becoming widespread over the last 7,500 years, but even that is a long way from becoming effectively universal.
That's generally a vary slow change consider most animals don't need dietary vitamin C even if there diets provide plenty of the stuff.
However, you used the term "a few generations" as in below 1,000 years and that's rediculusly fast for a species that takes as long as we do to reproduce and have so few children.
That said, you mentioned a beneficial trait becoming neutral, and that's making the assumption that greater intelligence is neutral. It could be that larger social organizations reward slightly less intelligent critters. That would do the trick rather quickly. I'm not making that claim, but am saying that discussions around evolution and traits that we have bias toward seeing as "good" are fraught.
There's also the further issue of intelligence being a very nebulous term that encompasses tons of cognitive traits, which makes any discussion of intelligence in evolutionary terms even more difficult.
It's also possible that some of what we call intelligence is epigenetic.
Just 500 years ago lots of people in North America where living nomadic hunter gather lifestyles. Even just 100 years ago you could find tribes in many parts of the world still living hunter gather lifestyles little changed over the last 15,000+ years and probably similar to how people lived 100,000+ thousand years ago.
As to epigenetic factors, the classic hunter gather lifestyle could be very healthy as long as population numbers stayed low. Farming actually lowered many heath indicators even as it allowed for massive increases in population sizes.
PS: Anyway, evolution is only 'fast' when there are significant benefits or harms. If a single mutation or environment change increases survival chances by say +/- 1% it takes exponentially longer to spread than a +/- 10% change.
"Farming actually lowered many heath indicators ..."
Indeed, it's uncontroversial that farming degrades the heath.
I do want to mention that larger social organizations were quite widespread in the Americas, despite the existence of hunter-gatherer tribes. The new world certainly wasn't an uncivilized wasteland when Europeans arrived.
Epigenetic factors can be triggered buy group size (domestication) as well. So those hunter-gatherer tribes might share the same genetic traits but express them differently based on their environment.
I don’t recall the study, it was either men in the US or Britton. But, income was positively correlated with number of children. Income and intelligence are also linked so that's evidence that right now intelligence is positively correlated with number of children. This may be strongly linked with prison time but again that links with intelligence.
The important thing to note is men often have children with more than one person and can have children vary late in life.
http://en.wikipedia.org/wiki/Human_height#History_of_human_h...
There is a fundamental mistake in assuming a "couple generation" linear trend will continue to infinity providing us with 30 foot tall men with IQs of 200+. We are probably at or around peak height and peak IQ right now for various rather obvious income inequality reasons, and we can expect both to decline in the future.
There is a coupling between womens pelvis size and newborn head size and medically assisted survival during childbirth which could provide some multigenerational effect to growth.
There have been a lot of arguments relying on generational inheritance of intelligence / IQ, however:
"As Flynn pointed out in his Ted Talk on the Flynn Effect, in 1900 only 3% of Americans performed "cognitively demanding" jobs - now the figure is 35%"
That would imply the percentage of humanity where cognition has no effect on their job, and presumably lifestyle, is at least 65% and has always historically been higher, as high as 97%, so an evolutionary argument for IQ growth seems extremely weak. It would be as if doctrine were peacock females select their mates based on tail feather size leading to large tail feathers, yet actual data shows 97% to 65% of actual living peacocks come from bald ancestors.
It's almost like how a goldfish will grow to the size of its bowl-- new humans will grow to and exceed the standards of the last generation, it's just human nature.
It's a bit trickier to apply the principle to creative pursuits than it is to physical ones, but I think it may help explain why students could appear cleverer over time without anything changing biologically.
To give one final example that doesn't involve sports. I am a trumpet player and am pretty familiar with the phenomenon that the best of the best in trumpet playing goes up as the decades go by. Rafael Mendez, Maynard Ferguson, etc. will forever be considered as the all time great virtuosoes, but the cold hard fact is that their technical abilities were only good in light of the time-- exceeded by hundreds if not thousands of the best trumpet players today. It's all about context.
Other humans!
You have to outsmart them and avoid being outsmarted by them, because they compete for the same resources and some of them will act in a predatory manner.
1. http://en.wikipedia.org/wiki/Fertility_and_intelligence 2. Kaufman, Scott Barry, et al. "The role of creativity and humor in human mate selection." Mating intelligence: Sex, relationships, and the mind’s reproductive system (2008): 227-262.
What about the multi-generational odds of survival and reproduction? What about surviving famines or predators? What about competition in sexual selection?
Idiocracy was an amusing movie, but it's underlying premise was never realistic.
You know, like when you're Amazon and you build a huge infrastructure to serve existing business, only to realize later you could do a lot more with it by simply re-purposing and re-configuring.
http://en.wikipedia.org/wiki/Heterosis
http://en.wikipedia.org/wiki/Heterozygote_advantage
I also must point out that "hard selection" (differential survival) is not the only operating mechanism in evolution. In complex organisms that reproduce sexually, it might not even be the dominant mechanism. You've got sexual selection and genetic drift, which in a species with low infant mortality and low mortality in general (a.k.a. us) are probably the dominant mechanisms. Beyond those you have things like the Baldwin effect, which get very interesting when coupled with epigenetics. Epigenetics is unquestionably real but as yet is not well understood. There could be all kinds of interesting things going on here.
We evolved to be the master hunters and nowadays our children aren't even aware meat comes from animals.
This same limited amount of intelligence we evolved with, is then free to do something else, like literature, art and music.