Thanks for the link! Glad to see people have discussed this.
The principle stated there, that any "genetically easy" modification to humans should be expected to cause a net reduction of fitness, seems useful and unimpeachable. With, of course, the caveat that we're not in the ancestral environment. Smart people can get all the calories, antibiotics, and c-sections they need.
But calling it the Algernon Principle implies that we should equate "physically easy" with "genetically easy." That seems unlikely to be true in general.
Pardon me while I have a strange interlude. We can show that the range of humanoids that you can build from proteins is astronomically greater than the range of humanoids you can build from genomes. Let N be the number of possible humanoid genotypes. For each n of those N genotypes, Prometheus can build a man who is somatically Walter Cronkite but whose germline DNA is derived from genotype n. Thus we have N distinct viable humanoids, all of whom look like Walter Cronkite. Since we know that not all viable humanoids look like Walter Cronkite, we know that there are more than N viable humanoids. Therefore, there are viable humanoid blueprints that are not accessible via mutation. Now imagine that I bothered to extend this proof in a bunch of combinatorial and exponential directions, and we get the astronomical part.
So, yes, we can deduce that any brain modification which evolution could easily do on its own is very unlikely to improve the subject's fitness. But we should not confuse this with the more general case, and the more general case is large. Which means that it's an unfair maligning of the experimenters in Algernon and Lensman to suggest that they should have known their efforts would end in disaster, any more than the Montgolfier brothers should have known that any hot air balloon must inevitably kill its passengers.
Edit: For an in-depth discussion of precisely this topic, see Nick Bostrom and Anders Sandberg's 2008 paper "The Wisdom of Nature: An Evolutionary Heuristic for Human Enhancement", available as a pdf here. This post was written before reading the paper.
There doesn't seem to be a thread discussing Eliezer's short-short story X17. While I enjoyed the story, and agreed with most of its points, I disagree with one assertion in it (and he's said it elsewhere, too, so I'm pretty sure he believes it). Edit: The story was written over a decade ago. Eliezer seems to have at least partially recanted since then.
Eliezer argues that there can't possibly be a simple surgical procedure that dramatically increases human intelligence. Any physical effect it could have, he says, would necessarily have arisen before as a mutation. Since intelligence is highly beneficial in any environment, the mutation would spread throughout our population. Thus, evolution must have already plucked all the low-hanging fruit.
But I can think of quite a few reasons why this would not be the case. Indeed, my belief is that such a surgery almost certainly exists (but it might take a superhuman intelligence to invent it). Here are the possibilities that come to mind.