In the mid-1980s, James Flynn made a groundbreaking discovery in human intelligence. The political scientist at the University of Otago in New Zealand found that over the last century, in every nation in the developing world where intelligence-test results are on record, IQ test scores had significantly risen from one generation to the next.

“Psychologists faced a paradox: either the people of today were far brighter than their parents or, at least in some circumstances, IQ tests were not good measures of intelligence,” writes Flynn.

Now, in a new book, Are We Getting Smarter? Rising IQ in the Twenty-First Century, Flynn unpacks his original finding, explaining the causes for this widespread increase in IQ scores, and reveals some new ones, regarding teenagers’ vocabularies and the mental decline of the extremely bright in old age. Ultimately, Flynn concludes that human beings are not smarter—just more modern.

Malcolm Gladwell explains why the “Flynn effect,” as the trend is now called, is so surprising. “If we work in the opposite direction, the typical teenager of today, with an IQ of 100, would have grandparents with average IQs of 82—seemingly below the threshold necessary to graduate from high school,” he wrote in a New Yorker article in 2007. “And, if we go back even farther, the Flynn effect puts the average IQs of the schoolchildren of 1900 at around 70, which is to suggest, bizarrely, that a century ago the United States was populated largely by people who today would be considered mentally retarded.”

In the last half-century, what have the IQ gains been in America?

The overall gain is about 3 points every 10 years, which would be 9 points in a generation. That is highly significant.

Now, on these tests [two that Flynn looks at are the Wechsler Intelligence Scale for Children, or WISC, and the Wechsler Adult Intelligence Scale, or WAIS], the gains vary by subtest. For example, there is a subtest called “similarities,” which asks questions like, what do dogs and rabbits have in common? Or what do truth and beauty have in common? On this subtest, the gains over those 50 years have been quite extraordinary, something like 25 points. The arithmetic subtest essentially tests arithmetical reasoning, and on that, the gains have been extremely small.

How do these gains compare to those in other nations?

If you look at the Wechsler gains abroad, they are pretty close to U.S. gains. There was a period of high historic gains in Scandinavia; these seem to have tailed off as the century waned. I thought that might be true of other countries as well. Maybe the engine that powers IQ gains was running out of fuel? But the latest data from South Korea, America, Germany and Britain show the gains still humming along at that same rate into the 21st century.

So, what has caused IQ scores to increase from one generation to another?

The ultimate cause is the Industrial Revolution. It affects our society in innumerable ways. The intermediate causes are things like smaller family size. If you have a better ratio of adults to children in the home, than an adult vocabulary predominates rather than a child vocabulary. Family size fell in the last century throughout the Western world. Formal schooling is terribly important; it helps you think in the way that IQ testers like. In 1910, schools were focused on kids memorizing things about the real world. Today, they are entirely about relationships. There is also the fact that so many more of us are pursuing cognitively demanding professions. Compared to even 1950, the number of people who are doing technical, managerial or professional jobs has risen enormously. The fact that our leisure has switched away from merely recovery from work towards cognitively taxing pleasures, like playing video games, has also been important.

What goes on in the person’s mind in the test room that allows them to do better on the test? One of the fundamental things is the switch from “utilitarian spectacles” to “scientific spectacles.” The fact that we wear scientific spectacles doesn’t mean that we actually know a lot about science. What I mean is, in 1900 in America, if you asked a child, what do dogs and rabbits have in common, they would say, “Well, you use dogs to hunt rabbits.” This is not the answer that the IQ tests want. They want you to classify. Today, a child would be likely to say, “They are both animals.” They picked up the habit of classification and use the vocabulary of science. They classify the world as a prerequisite to understanding it.

Do IQ gains mean we are more intelligent than our ancestors?

What is important is how our minds differ from those of people 100 years ago, not whether we label it “smarter” or “more intelligent.” I prefer to say our brains are more modern.

Our brains at autopsy are probably different. We have discovered that the brain is like a muscle. A weightlifter has very different muscles than a swimmer. Similarly, we exercise different portions of our brains in a way our ancestors didn’t. They might have had better memories than we do, so they would have a larger hippocampus [a part of the brain that forms, processes and stores memory]. But, we would have exercised certain areas in the prefrontal lobes more than they did. So, those things would be enlarged.

The other important factor is we have learned to use logic to attack the hypothetical. We have an ability to deal with a much wider range of problems than our ancestors would. For example, if you were a businessperson, you would be much more inventive. You would be more imaginative. We are better at executive functions, or at making business decisions. We are also better at moral reasoning.

In your research, you have found that there is a growing gap between the vocabularies of adults and their children. How big is this gap?

You look between 1953 and 2006 on the adult Wechsler IQ test, and its vocabulary subtest, and the gains have been 17.4 points. The gains for schoolchildren during a similar period have been only 4 points. That is a spreading difference of 13 IQ points. That’s huge.

What gives?

In 1950, something like 12 percent of Americans had experienced at least some tertiary, or post-high school, education; today it is up to 52 percent. More people go into cognitively and verbally demanding professions, like law, school teaching, counseling, psychology and journalism. This has had an effect on adult vocabulary.

The IQ gains of our children have been much more muted. You might say, well, the children haven’t been to university. But children are socialized by the adults that speak around them every day. The question is why are parents less capable of socializing their children into their own vocabulary than they were 50 years ago? I can only imagine that some cultural barrier has built up that insulates the speech of children from the speech of adults.

Could teenage subculture be this barrier?

The word “teenager” didn’t exist in 1950. I was a teenager in 1950, and like everyone else, I wanted to become an adult as quick as possible to get access to money, sex, privacy and a car. Today, teenagers have all of those things without becoming adults. They have enormous purchasing power, and they have developed their own subculture, which is often antagonistic towards their parents. They often have their own speech patterns from texting and slang. I suspect that at least for teenagers a cultural barrier has developed between parent and child. What has happened with younger children, I am still investigating.

In 1950, teenagers could not only understand their parents, but they could also mimic their speech. Today, teenagers can still understand their parents. Their passive vocabularies are good enough. But when it comes to the words they actively use, they are much less capable of adult speak. That is also true of what they would write on an essay.

You have also discovered a trend that you call the “bright tax.” What is this?

The wisdom always was that the brighter you were, the less your mental abilities declined in old age. I found that was an oversimplification. It is true of verbal intelligence. The brighter you are, the more you get a bonus for verbal skills. I call that a “bright bonus.” Your vocabulary declines at a much less steep rate in old age than an ordinary or below average person. But to my amazement I found that for analytic abilities it was just the reverse. There is a “bright tax.” The brighter you are, the quicker after the age of 65 you have a downward curve for your analytic abilities. For a bright person, you go downhill faster than an average person.

This raises an interesting question. Is it something to do with the aging brain, or does it have to do with environment? It could be that a good analytic brain is like a high performance sports car; it just requires more maintenance, and in old age, the body can’t give it. That would be a physiological explanation; the bright brain requires sustenance from the body, which as the body ages is no longer forthcoming. The environmental explanation would be that we use our analytic abilities mainly at work. That means that if a bright person is in a cognitively demanding profession, they are like an athlete; they build up a big exercise advantage over the average person, who has a humdrum job. Then, retirement would be a leveler. That is, if you give up work at 65, you are like an athlete who is retired from competition. You no longer have that exercise advantage of your analytic abilities that work affords. We don’t really know which of these things is true. It could be that they are both true to some degree.

I think this is a great fear for many retirees. What can someone do to stave off this decline?

Retire from your job, but read great literature. Read about the history of science. Try and keep up your problem solving skills. Every bit of evidence shows that the more you use your brain, the fitter it will stay.

The Organization for Economic Co-operation and Development (OECD) found that in countries like Sweden and Switzerland, where people did not retire early, the loss of working memory by the age of 65 was only half as great as in France, where people did retire early.

What do you predict will happen to IQ scores going forward?

One of the most interesting predictions is what will happen to the developing world. If they industrialize, in theory, they should have the explosive IQ gains in the coming century that we had in the last century.

In my book, I study six developing nations. Kenya is undergoing explosive IQ gains. Brazil and Turkey are undergoing quite profound gains. Nations like Saudi Arabia and the Sudan are not, but the Sudanese keep having civil wars and the Saudis are really just living off of oil revenue. They are not industrializing in any real sense. Dominica is the sixth case. There, they are making IQ gains, but their infrastructure is wiped out about every 10 years by hurricanes, earthquakes and tsunamis. I predict that Brazil, Turkey and Kenya will industrialize over the next century and begin to rival the Western world for IQ.