by Daniel Keyes
Flowers for Algernon is an emotionally-charged parable about a young man named Charlie who undergoes an experimental procedure which boosts his intelligence level from retarded to genius. Alongside Charlie is Algernon, the mouse subject which preceded Charlie in trials of the procedure.
Keyes presents the story through a series of diary entries, a format which allows him to showcase the way Charlie's intelligence slowly improves after the experiment. From the position where Charlie was demonstrated to be dumber than Algernon, who solved mazes faster than him, Charlie accelerates up to normal human intelligence and beyond, eventually challenging and surpassing the intelligence of the scientists who performed the operation on him.
The first depressing thing in the book is how little benefit Charlie gets from his boosted intellect. The Charlie we met at the start of the book was the butt of many jokes and much mistreatment, and he hoped that being smarter would make him better at getting people to like him. You can see that this does not go well. While Charlie's intellect improves, he finds himself little better able to win friends or understand his own emotions, and as it accelerates him into the rank of genius he finds himself more and more estranged from those around him, who are unable to keep up. His own obliviousness to exactly how remarkable his talents have become manifests as paranoia -- he thinks that a lack of comprehension from those once his betters is a cruel joke -- and of course breeds resentment in the incredible intellects whose egos he bruises without realising.
The second depressing thing is that the procedure is not permanent. Signs of a reversal of intelligence appear, something that troubles both Charlie and Algernon, each frustrated to find themselves no longer capable of what they once were, terrified of the decline ahead. Charlie uses his last breath of genius to identify the effect which caused this reversal, and publish it, then seals himself away. Algernon dies unhappy, and is buried. Charlie desperately tries to cling on to his last shreds of intellect, the beauty of books and poetry he has read, but finds it harder and harder, until even the simplest of books frustrate him. He is worse than he ever was before, because he knows what he misses.
The analogy here is obvious, and terrifying. Whatever heights of intelligence a person will reach as they climb out of childhood, they will reach in their late 20s, after which an unremitting cognitive decline will set in. For a decade or so the effect will be mitigated by the increased body of knowledge they have amassed, but the fact remains that one's wits will dull and dull, and so far as we know there is nothing you can do about that.