This article by Bryan Caplan went slightly viral: The Flynn Effect vs. the Greatest Minds in History
What do rising IQs really show? I remain undecided, but here’s an argument that strongly inclines me to pessimism. To wit: When I read the smartest thinkers from the 17th, 18th, and 19th centuries, they seem roughly as smart as the smartest thinkers from the 20th century. In fact, the same goes for the smartest Greeks from the 4th and 5th centuries B.C. What 20th-century thinkers credibly exceed the sheer intellectual firepower of Plato, Aristotle, Hume, Kant, Descartes, or Gauss? Note: I’m not naively comparing the best living thinkers to the totality of earlier thinkers. I’m comparing the best in the last century to the best in individual earlier centuries.
To answer his question, there are many examples of brilliant 20th-century minds; for example, top theoretical physicists and pure mathematicians such as Andrew Wiles, Ed Witten, Feynman, Sean Carroll, John Von Neumann, Einstein, Grigori Perelman, and so on.
I’m not so pessimistic though. Even if global mean and median IQ is falling slightly, total intellectual output is still rising.
As noted by the commenters, much of the low hanging intellectual fruit has been picked:
You haven’t considered the possibility that the top minds of today seem less impressive because there’s less low-hanging intellectual fruit about. Perhaps there are thousands of people today who could have discovered calculus, relativity or the theory of comparative advantage had they been born earlier. But today, they’re working in areas that are relatively less impressive or are a lot harder for one person to make a substantial impact in.
To build on the “low hanging fruit” argument, I would also consider the general complexity of today’s world. There is more information and previous results to build on, but building on it also requires an increasing amount of time and effort to understand what came before. I would expect fewer and fewer people over time to expend the necessary energy to get to the “frontier”.
Second, progress in math and science in recent centuries has been rapid. In the span of a few centuries, much of all modern physics–from electromagnetism to quantum theory–was discovered, that 2+ millennia of science didn’t accomplish. Same for math. It took two millennia just to get to calculus, and then in the span of 300 years all of modern math was conceived. I think this is evidence against IQ decline, at least on a multi-century time frame. Same for medicine and biology, too.
Were and ancient Greeks smarter? One possibly is that they weren’t exceptional, but that we perceive them so. The Model T is famous because it was one of the first mass produced cars, and also because of the iconography and veneration of Henry Ford, not because the Model T is a superior car or that Henry Ford is the best businessman who ever lived. Likewise, the ancient Greeks invented geometry and Western philosophy, although such geometric concepts obviously have been superseded by later works, but ancient Greek literature is still timeless. For what it’s worth, Anatoly Karlin puts an IQ of 90 for ancient Greeks.
In terms of complexity and detail, the intellectual contributions of America’s best and brightest–from theoretical physics, to programming, to abstract math–far surpasses the caliber of output centuries and millennia ago. To get an idea of how complicated modern physics and math is, this little symbol 
, a Rieman tensor, written out is a differential equation with over 250 terms, whereas Newton’s second law of motion is just a single differential equation. Andrew Wiles’ Fermat Last Theorem proof is 100+ pages of pure abstract mathematics.
But Dr. Caplan addresses this objection, arguing that contemporary mathematicians and physicists have a sort of advantage by building on prior knowledge, not because they are smarter:
2. Modern thinkers build on the shoulders of past giants, making it easy for them to exceed the knowledge and avoid the errors of earlier generations. This in turn fosters the illusion that moderns are genuinely smarter, rather than advantageously positioned in time.
However, the leap from euclidean ‘real’ math to abstract, non-euclidean/hyperbolic complex math was a spontaneous development that didn’t really have an antecedent that one could have readily built off of.
A mathematician or theoretical physicist today has to learn centuries of knowledge compacted in the span of 4-8 years, in addition to writing a dissertation that builds upon this huge body of knowledge, and they do. Not only that, but academic conditions are more competitive than ever in terms of getting into a good grad school and PHD program, due to all of the high-IQ foreign competition. People who argue that the world is dumbing-down or that people were smarter centuries or millennia ago, should spend some time on arXiv and they will see that maybe the world is not so dumb after all. If you use liberal arts instead of STEM as a yardstick for measuring the totality of human accomplishment and intelligence, yeah, the world may seem dumber; there is more subjectivity in appraising a piece of writing, music, or art, than science.
This means there is hope that civilization can be saved, and more importantly, there is enough brainpower to reverse the slide. But that is not an excuse to become complacent, and staving off potential IQ decline necessitates immigration control and eugenics to ensure that the genes that give rise to superior intellect are not diluted. Bryan Caplan, himself an open-borders advocate, opposes such policy even though it would help solve the very problem he speaks of.