I have read about this subject a lot and came across the "Flynn Effect" several times. This is the phenomenon that the general population IQ has steadily increased by about 3 IQ pts per decade. Detractors of IQ such as Stephen Murdoch [[ASIN:0471699772 IQ: A Smart History of a Failed Idea]] argue that the Flynn Effect proves IQ measurements are meaningless. IQ proponents such as Charles Murray [[ASIN:0684824299 Bell Curve: Intelligence and Class Structure in American Life (A Free Press Paperbacks Book)]] and Arthur Jensen [[ASIN:0275961036 The g Factor: The Science of Mental Ability (Human Evolution, Behavior, and intelligence)]] vest little importance to the Flynn Effect. Yet, they have not managed to explain it away. I thought I'd study this strange phenomenon from the horse's mouth.

I was stunned. This book is brilliant. Flynn goes much beyond his predecessors in explaining what intelligence is and how it does change over time. The first thing Flynn did is disaggregate the IQ trends into their subcomponents. He observed that the improvement over time were very different depending on the domain. On arithmetic and vocabulary questions, IQs remained virtually flat for decades; Meanwhile, on `similarities' and `picture riddles' IQs went through the roof.

What's going on here? Flynn explains that our social context of everyday life has become more complex and scientific minded that have lead the population to think critically. In turn, this has contributed to higher scoring in similarities and complex picture riddles. As our society moved from an agricultural based one to an industrial one and ultimately information based one, our demand on critical thinking grew commensurately. The percentage of our labor force engaged in farming and factory work is declining while the one engaged in information based activities is growing rapidly. A half century ago a high school degree was a big deal. Now a college degree is often a prerequisite if not a graduate degree.

Flynn goes on capturing the dynamic interaction between the environment and cognitive capabilities over time. Flynn's "individual multiplier" stipulates that an individual influences the environment he operates in because the environment will respond to where the individual excels. Flynn's "social multiplier" describes the escalation of cognitive demands as we progressed from an agricultural to an information based society. The social multiplier is a giant mass wide positive feedback loop. Everyone's expectation of academic and professional achievement has risen over time.

Thus, Flynn concludes we have become far better at solving abstract problems because that is what society currently demands us to do. In his mind, this does not necessarily mean we are so much smarter than our ancestors (the physiological processing capability of our brains has not changed). It just means we have a different focus (abstract and post-scientific vs concrete and pre-scientific).

For Flynn, IQ can change over time depending of the choice you make. You decide to go on to college or grad school. You marry a smart one or a not so smart one. You decide to go in an intellectually intense profession or not. These decisions will affect your cognitive capabilities. That contrasts with Murray and Jensen who thought that IQ was pretty much fixed by the mid to late teens. The two positions are probably not so far apart. Cognitive capabilities can improve depending on favorable activities and exposure as Flynn suggests. But, those improvements are probably capped within a certain range as Murray and Jensen would indicate.

Flynn does not annihilate the foundations established by Murray, Jensen, and others. And, he certainly disagrees with Stephen Murdoch that IQ is meaningless. But, Flynn adds that you have to look at the dynamic time dimension associated with change in the social context (social multiplier) that have a strong impact on IQ. Let's face it, time affects everything. You can't compare GDP per capita, home prices, sports world records without putting them into an historical context. Flynn demonstrated the same is true for IQ.

In addition to this dynamic time dimension, Flynn strongly disagrees with Jensen's near obsessive attempt to establish a correlation between fast reaction time and IQ. Flynn states that depending on what reaction time measure you focus on, it can have either a positive or a negative or zero correlation with IQ. Despite these disagreements, Flynn pays much respect to Jensen's achievements including his G factor that captures a huge amount of information at any one static point in time.

Flynn adds that his field is much too focus on IQ. And, he invites his peers to come up with other measures that would capture critical acumen and wisdom. He has developed some rudimentary tests to measure those, but he acknowledges they need more work.

All the mentioned books are interesting and do contribute to fully understand how Flynn's book is revolutionary. Indeed, his theories are as much a leap as from Newton's law of gravity to Einstein's theory of relativity. It offers a totally different multi-dimensional intelligence framework.

This book may not be the final word on the Flynn Effect, but it makes enough progress in that direction that it is no longer reasonable to describe the Flynn Effect as a mystery. I'm surprised at how much Flynn has changed since the last essay of his I've read (a somewhat underwhelming chapter in The Rising Curve (edited by Ulric Neisser)).
Flynn presents evidence of very divergent trends in subsets of IQ tests, and describes a good hypothesis about how that divergence might be explained by increasing cultural pressure for abstract, scientific thought that could create increasing effort to develop certain kinds of cognitive skills that were less important in prior societies.
This helps explain the puzzle of why the Flynn Effect doesn't imply that 19th century society consisted primarily of retarded people - there has been relatively little change in how people handle concrete problems that constituted the main challenges to average people then. He presents an interesting example of how to observe cognitive differences between modern U.S. society and societies that are very isolated, showing big differences in how they handle some abstract questions.
He also explains why we see very different results for IQ differences over time from what we see when using tests such as twin studies to observe the IQ effects of changes in environment on IQ: the twin studies test unimportant things such as different parenting styles, but don't test major cultural changes that distinguish the 19th century from today.
None of this suggests that the concept of g is unimportant or refers to something unreal, but a strong focus on g has helped blind some people to the ideas that are needed to understand the Flynn Effect.
Flynn also reports that the rise in IQs is, at least by some measures, fairly uniform across the entire range of IQs (contrary to The Bell Curve's report that it appeared to affect mainly the low end of the IQ spectrum). This weakens one of the obvious criticisms of David Friedman's conjecture that modern obstetrics caused the Flynn Effect by reducing the birth related obstacles to large skulls (although if that were the main cause of the Flynn Effect, I'd expect the IQ increase to be largest at the high end of the IQ spectrum).
It also weakens the inference I drew from Robert Fogel's book on escape from hunger. Flynn does little to directly address Fogel's argument that the benefits of improved nutrition show up with longer delays than most people realize, but he does report some evidence that the Flynn Effect continues even when the height increases that Fogel relies on to measure the benefits of nutrition stop.
Flynn reports that the Flynn Effect has probably stopped in Scandinavia but hasn't shown signs of stopping in the U.S. His comments on the future of the IQ gains are unimpressive.
There are a few disappointing parts of the book near the end where he wanders into political issues where he has relatively little expertise, and his relatively ordinary opinions are no better than a typical academic discussion of politics. In spite of that, the book is fairly short and can be read quickly.
One interesting experiment that Flynn discusses tested whether students preferred one dollar now or two dollars next week. The results were twice as useful in predicting their grades as IQ tests. Flynn infers that this is a test of self control. I presume that is part of what it tests, but I wonder whether it also tests whether the students were able to realize that the testers' word could be trusted (due to better ability to analyze the relevant incentives? or due to a general willingness to trust strangers because of how the ways they met people selected for trustworthy people?).