Anthropologist Peter Frost and population geneticist Henry Harpending authored a paper in 2015 arguing that the promiscuous use of the death penalty had a eugenic effect on the English population. Specifically, by removing at least 2 percent of the population each year, those prone to violence, the overall population became dramatically less violent over time.
It was and remains a controversial claim, mostly because of its implications, but also because it contradicts the prevailing orthodoxy. It suggests that people are not, in fact, amorphous blobs that can be shaped into anything. We are the product of our genetics, which is the result of the mating decisions of our ancestors. To accept that is to reject the ideological underpinnings of liberal democracy.
Others have argued that the Black Plague disproportionately removed the stupid from the European population. There is a correlation between IQ and health, so it stands to reason that plagues will hit the stupid harder than the smart. Remove enough stupid people from the population and the average IQ rises. This could explain why Europeans suddenly rocketed ahead of the rest of the world.
Of course, the same argument could be made on behalf of the death penalty. Generally, criminals are of below-average intelligence. If your policy is to remove criminals from the breeding stock via the hangman, you will inevitably be removing far more stupid criminals than smart ones. In other words, the death penalty probably helped raise the overall IQ of the population.
A population that is smart and peaceful will be more successful than a population that is stupid and violent. We only have to compare Finland with Somalia to see how this works in our own age. Somalia has an estimated average IQ of 67, while Finland comes in at 101. Even assuming measurement errors, the gap is massive, and it shows in the results on the ground.
In a high-trust society, you can take risks, like trying to invent a better plow or a better way to preserve food. You can also question official orthodoxy without fear of being killed, so new ideas can get room to breathe. European people made the modern world because they reward improvement. If you can produce a better way, even if it offends a rich person, you will be rewarded.
What all of this points to is that within every human population there is a defect rate and those defects have consequences. Some of the defects turn out to be harmless, like stupid people who can be employed in simple labor. Other defects can be quite harmful, like murderers and rapists. As evolutionary psychologist Ed Dutton points out, some of these defects result in spiteful mutants.
Spiteful mutants are the men in dresses demanding everyone pretend they are some third sex rather than a lunatic. These are the feminists who make war on the normal sexual relations of society. The people policing speech online and inflicting Diversity, Equity & Inclusion programs are spiteful mutants. These are the human defects slowly making life impossible in Western countries.