That intelligence (g) in healthy people is nearly impossible to improve is clear from the failure of psychology to provide any such method. But why intelligence would be so constant is not as clear: many other cognitive abilities are improvable (like working memory), so why not intelligence?

Arthur Jensen noted the failure of interventions in the 1960s, and the failure remains complete now, half a century later: if you are a bright healthy young man or woman gifted with an IQ in the 130s, there is nothing you can do to increase your underlying intelligence by a standard deviation. New methods like dual n-back or nootropics are trumpeted in the media, and years later are discovered to increase motivation & not intelligence, or to have been overstated, or work only in damaged or deficient subpopulations, or to be statistical/methodological artifacts, or to be tantamount to training on IQ tests themselves which destroys their meaning (like memorizing vocabulary), or to be so anomalous as to verge on fraudulent (like the Pygmalion effect). The only question worth asking is which of these explanations is the real explanation this time.

For IQ in particular, people discussing human-enhancement (especially transhumanists) have proposed a pessimistic observation & evolutionary explanation, dubbed the “Algernon principle” or “Algernon’s law” or my preference, the “Algernon Argument”.

The famous SF story “Flowers for Algernon” postulates surgery which triples the IQ score of the retarded protagonist - but which comes with the devastating side-effects of the gain being both temporary and sometimes fatal; fictional evidence aside, it is curious that despite the incredible progress mankind has made in countless areas like building cars or going to the moon or fighting cancer or extincting smallpox or inventing computers or artificial intelligence, we lack any meaningful way to positively affect people’s intelligence beyond curing diseases & deficiencies. If we compare the smartest people in the world now like Terence Tao to the smartest people of more than half a century ago like John von Neumann, there seems to be little difference. Eliezer Yudkowsky expands the thought out in his essay “Algernon’s Law”, stating it as:

Any simple major enhancement to human intelligence is a net evolutionary disadvantage.

The lesson is that Mother Nature know best. Or alternately, TANSTAAFL: “there ain’t no such thing as a free lunch”.

Trade-offs are endemic in biology. Anything which isn’t carrying its own weight will be eliminated - organs which are no longer used will be stunted by evolution and within a lifetime, unused muscles & bones will start weakening or being scavenged for resources, as athletes and astronauts may find out the hard way and bodybuilders perpetually fight , while shrews cyclically shrink their brains & skulls by 15% to conserve resources in winter (Lázaro et al 2017). Often, if you use a drug or surgery to optimize something, you will discover penalties elsewhere. If you delay aging & length lifespan as is possible in many species, you might find that you have encouraged cancer or - still worse - decreased reproduction as evidenced by the dramatic deaths of salmon or brown antechinus ; if your immune system goes all-out against disease, you either deplete your energetic and chemical reserves or risk autoimmune disorders; similarly, we heal much slower than seems possible despite the clear advantage ; if you try to enhance attention with an amphetamine, you destroy creativity, or if the amphetamines reduce sleep, you damage memory consolidation or peripheral awareness ; or improving memory (which requires active effort to maintain ) also increases sensitivity to pain and interferes with other mental tasks (as increased WM does, slightly ); if a mouse invests in anti-aging cellular repairs, it may freeze to death , and so on. (What are we to make of inducing savant-like abilities by brute-force suppression of brain regions , or tDCS improving learning?) From this perspective, it’s not too surprising that human medicine may be largely wasted effort or harmful (although most - especially doctors - would strenuously deny this). “Hardly any man is clever enough to know all the evil he does.”

An analogy to complex systems is a superficial analysis at best. Many complex systems are routinely optimizable on some parameter of interest by orders of magnitudes, or at least factors. Economies grow exponentially, on the back of all sorts of improving performance curves which make us richer than emperors compared with our ancestors; the miracle of economic growth, built on thousands of distinct complex systems being optimized by humans, seems to go unnoticed and be so normal and taken for granted. If we were computers, an ordinary nerd with access to some liquid nitrogen could double our clock speed.

With intelligence, on the other hand, not only do we have no interventions to make one an order of magnitude smarter on some hypothetical measure of absolute intelligence (perhaps such a man would be to us as we are to dogs?) but we have no interventions which make one a few factors smarter (the smartest man to ever live?) nor do we even have any interventions which can move one more than a few percentage points up in the general population! We remain the same. It is as if scientists and doctors, after studying cars for centuries, shamefacedly had to admit that their thousands of experimental cars all still had their speed throttles stuck on 25-30kph - but the good news was that this new oil additive might make a few of the cars run 0.1kph faster!

This is not the usual state of affairs for even extremely complex systems. This raises the question of why all these cars are so uniformly stuck at a certain top speed and how they got to be so optimized; why are we like these fantastical cars, and not computer processors?