Some who are concerned about the quality of people's minds speak of hunter-gatherers in prehistoric times as maintaining a stable, somewhat uniform genetic quality. Those who in the 21st century are called "special needs" children most likely were not allowed to survive. Those born with the kind of chemical imbalance that produces dysfunctional behavior or psychopathy were killed or driven from the group and left to fend for themselves and die alone. None of these "dysfunctional persons" remained in the group long enough to make much of a contribution to that prehistoric gene pool.
Some add to this the notion of a male having been selected as chief because he had better genes and that he helped the gene pool by having greater access to females.
In early civilization, it is believed, infanticide was common and helped keep the "unfit" from polluting society's gene pool. And more intelligent people were more economically successful, giving them access to better nutrition and giving them more children who survived to sexual maturity and more of the political power that helped survival.
A British professor of psychology, Richard Lynn, in a work called Dysgenics: Genetic Deterioration in Modern Populations, published in 1996, claims that the years 1500 to 1800 were good for Europe’s gene pool. He points out that between 1620 and 1624 "the middle classes reported 4.4 children per woman compared to 2.1 for the working class."
Lynn asserts that natural selection in pre-industrial societies favored traits such as intelligence and character but that this natural selection has diminished. He describes advances in medicine as having added to the survivability of those with inferior genes.
Ideas about gene pool deterioration need not consider racial differences in "cognitive ability," but Lynn's study of developments in human genetics has included an attempt to do so – with use of the bell curve. His is an old argument against "race mixing." As to the quality of this work, Wikipedia reports:
Lynn's work on global racial differences in cognitive ability, mostly surveys, has been cited for misrepresenting the research of other scientists, and his work has been criticized for its associated measurement difficulties, distortion, and conclusions drawn from extremely poor and very limited samples.
Alongside the impossibility of measuring a general deterioration in intelligence across human history, there is what is called the Flynn Effect. This describes an increase in average intelligence measured in recent times – an increase found in what is categorized as the lower half of the population in intelligence. Some people attribute the Flynn Effect to environmental conditions: better nutrition, better health and educational opportunities. At any rate, the Flynn Effect says nothing about whether any gene pool has been deteriorating across recent centuries or since the rise of medicine and public health since the end of the Second World War.
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