James Flynn famously proposed, some 3 decades ago, that measurable human intelligence has been increasing by about 3 IQ points per decade in developed Western societies. This came to be accepted and known in psychological circles as the "Flynn effect". We know, more or less, that genetic evolution doesn't work that quickly, and so are left with the hypothesis of memetic evolution. Flynn says that maybe we're no smarter than our forebears in terms of brain capacity, but more 'modern' in how we think. We've crossed some Malthusian threshold that lets us think more categorically where most of our ancestors thought more in terms of what was necessary to survive. Flynn offered an example of a question posed a hundred years ago and now: "What do a rabbit and a dog have in common?" A hundred years ago an acceptable answer might be, "Well, you use dogs to hunt rabbits". Today it might be, "Well, they're both 4-legged furry mammals." See the difference? I don't think that this is entirely attributable to how we engage the world (that we don't much hunt rabbits with dogs any more) but that we have changed the way we think, not just about what we think. That's categorical memetic evolution.
I have some concern with the validity of intelligence measurement. I was given the Stanford-Binet IQ test three times -- each conveniently about 10 years apart. First was when I entered public school and I got a certain score. Next was near the end of my scholastic career (I quit school at 16) and I scored 10 points lower. And then after dropping a lot of acid and living like a bear in the mountains, when I was applying to the US Army (based on standard Army tests they wanted me to be a spy, and so further testing was done) I scored 40 points higher than the test before. I don't think that I started out pretty smart and then got dumber with education and then smarter with isolation. I think that a long period of reflection just let me figure out how better to take tests.
And so three points per decade doesn't seem like much onto which to hang a hypothesis. Yes, we moderns do modern things better, and do primitive things less well. I often hear it said that modern culture is more complex, and that's probably true. But which is more complex; microwaving tater tots or preparing ground and growing potatoes? My ancestors couldn't have fathomed tweeting (nor can I), but they knew when to plant a certain crop so that harvest would mesh with when their neighbors harvested other crops at a time when it took everyone working together to make any part work. That was intelligence not necessarily measureable on any index of critical thinking; not because it wasn't critical thinking but because the accepted measure of it measured different values.
Intelligence is a multivariate quality ... or quantity, if you will. It is both innate and learned, nature and nurture, the product of both inbred potentiality and real-world exposure. After some fashions it may be called "wisdom," particularly among those who have sufficient experience to earn that sobriquet. It reflects the ability to observe, to apprehend what has been seen and appreciate that observation, to deduce from what has been observed and perhaps to apply the deductions to future occurrences and events. It may be best expressed in that difficult, near-impossible effort some make to take what they have learned and pass it on to others.
Real intelligence is both objectively and subjectively gained and not necessarily measured accurately by the efforts of Stanford-Binet, Wechsler, or other such metrics.
I think that theories of multiple intelligences are fairly well accepted. The kind of intelligence that an IQ test is trying to get at is a sort of "general" intelligence, perhaps more a measure of pattern recognition, mental speed and flexibility. You can train your brain to be better at it, and if you've taken IQ tests before, you'll be better able to predict what they want from you. So, yes, it's not exactly the most accurate measurement of anything.
There is some merit to intelligence testing as a sort of base for other cognitive studies. The Downing Effect is very interesting - people of below average IQs tend to overestimate their own intelligence, while people of above average IQs tend to underestimate their own intelligence. I guess because smarter people (at least in this limited sense) are more likely to know what they don't know.
Intelligence is knowing that you cannot put a square peg into a round hole. Better yet, intelligence is know how to put a square peg into a round hole.
Or ... how to ADAPT a square peg so that it will fit in a round hole.
Somewhere in all the variables that influence intelligence values play a role. For example, during tribal development, cooperation was a necessary attribute, with each one knowing his and her roles and learning what he and she needed to fulfill those expectations. Individuals learned to obey, to follow direction, to turn to others for instructions on how to thrive, to conform to social norms.
Somewhere along the time line, individuals pulled away from authority figures, gods, kings, land owners and popes. One by one, some thought authority created barriers to progress and new ideas sprang into being, even if it meant being burned at the stake or having one's nose cut off by religious zealots. Authority resided inside one's own capacity to think and reason.
Another shift in the time-line occurred as individuals began to realize that in order to advance, cooperative efforts that used many different ideas, techniques and values were necessary to progress. Looking to the stars, using telescopes that could reveal far distant galaxies, developing into space telescopes that were able to see beyond the atmosphere of the Earth opening up all new vistas. Old tribal beliefs, ancient attitudes, patriarchal systems, Biblical imperatives, outmoded authority traditions, give way to a new imperative to develop one's innate talents, join with others with a common goal, set aside tribal values and incorporate a systems way of thinking with many different ideas and personalities working for a common good.
Notions of obeying, submitting, acquiescing give way to personal development, interpersonal skills, finding values greater than tribal ones to an ideas of a network of co-passengers on planet Earth. The Earth must be factored into any decisions being made.
Indra's Net comes to mind, as do fractals. These systems theories value chaos as part of a process to establish order. Counterintuitive? I know!
Every generation, over the past two centuries has been witnessing increased complexities in life, mainly due to advancements in science and technology, and we have witnessed that level of intelligence rises in every generation to enable younger generations to cope up with the changed situation. Biological evolution of human race is almost over and intellectual evolution will have to follow the pace required to cope with the demands of the time.
According to Wikipedia the Flynn effect is mostly from improvements in the lower IQ scores, i.e. preventable causes of low IQ are being eliminated - improved schooling and literacy, improved nutrition etc.
I wonder if lead poisoning is still lowering the intelligence of a lot of children.