Why Aren't People Smarter?
Evolutionarily, smarter should simply be better---or, at least, so one would think. Thus, we have a very interesting question: Why aren't people smarter than they are? Of course, no matter how smart people become this question could always be asked, and we would be forced to answer with something like the following: Not enough time has passed. So, perhaps we should state the question differently: Why is there so much variance in intelligence? Why is the distribution as wide as it is?
The answer, I am afraid, is that intelligence comes at a significant cost: It comes at the cost of increased risk of depression, mood disorders, and social ostracization. These costs are high enough that most people do better reproductively not being all that intelligent. Why would intelligence impose such a high cost? Sadly, much of it comes down to chemistry---and a few unfortunate facts about social dynamics.
Before turning to a discussion of the neurology of intelligence and emotion, we should point out a few obvious social costs to being intelligent. First, when you are smart, you have the ability to refute people---and this ability, if used, is likely to piss those people off. Make enough enemies this way, and you will have little time to devote to resource acquisition and mate finding.
Furthermore, the intelligent arrive at more realisitic appraisals of their true chances of success: And this undermines confidence, which is an important part of mate-finding. Also, seeing the downsides of life can cause depression which can destroy motivation---even the motivation to find a mate.
But even if some grand societal change were to eliminate these problems: The chemistry of the brain itself would remain an obstacle. The dopaminergic and cholinergic systems are in antagonism. This unfortunate fact limits natural selection's ability to select for intelligence. The dopaminergic system---which is responsible for the ability to get things accomplished---interferes with the cholinergic system, which (and this is an oversimplification) is responsible for intellectual functioning. However, if intelligence does not result in action, it cannot be selected for: It is in the realm of action where, after all, variable reproduction and survival take place.
The very structure of the brain seems to make the continued evolution of human intelligence a very likely quite slow affair indeed. It is only by the use of medications and genetic engineering that we will push through this boundary, most likely: And we must not fear using these tools. What we should fear is an end to human progress. And progress requires ever improving intelligence.