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Human societies with very high mortality during childhood an early adulthood over many generations will be (relatively) low IQ societies - because reproductive success is mostly a matter of rapid development, early sexual maturity, and high fertility (number of childbirths) hence smaller and simpler brains.
Reproductive success is mostly a matter of fertility, sheer numbers of babies, to compensate for the high chance that each will be killed before completing reproduction.
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Societies with high mortality during childhood and also during early adult life may well favour rapid increase in general intelligence over the space of relatively few generations, insofar as higher intelligence reduces mortality rates.
Fertility will be (and needs to be) significantly above two babies per woman on average, but beyond this number, fertility is almost irrelevant because reproductive success comes from lowering mortality.
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Societies (such as the whole modern world) which have low mortality rates in childhood and early adulthood (low by human historical standards) will become (relatively) low IQ societies if they are secular - due to lower fertility among those with higher IQ -
- but this will not necessarily happen if/when societies/ groups (which must be coincident) are orthodox, traditionally religious (perhaps monotheistic).
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So there is one high IQ sweet spot for sure (high mortality through into early adult life with universal above-replacement fertility) and one possible sweet spot (low mortality but fertility differentially higher in higher IQ).
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4 comments:
Interesting, but it would be helpful if you could explain the difference between high and very high mortality, since according to your theory the consequences are very different for the average intelligence of the population. Also, do you have any examples to support your hypothesis?
For high IQ individuals to be higher fertility, irresponsible breeding, characteristic of low IQ individuals, has to have bad consequences.
In a number of societies, security was provided by males and through arrangements between males, thus women and children were provided with security and protection through their relationships with males, thus bastards and women who produced bastards tended to lack protection, resulting in enslavement or some intermediate less than free status, and/or high exposure to violence.
This resulted in frequent bad behavior towards weak people, which was the initial justification that progressives employed for a strong interventionist state, with themselves managing it.
@JAD - More or less - there could be various mechanisms.
To simplify (the way I see it): through most of human history, fertility was 'irrelevant' because almost 1. everybody (except those suffering from disease) had considerably above-replacement fertility, but 2. almost all children died without themselves reproducing.
Only those with favourable conditions had a reasonable chance of surviving.
In some societies (only some) higher IQ increased childhood survival (probably by a variety of mechanisms) and higher IQ was selected for.
High IQ individuals made various innovations which reduced mortality for everybody to the point that mortality was 'irrelevant' and most children born would survive to reproduce.
IQ would then become a net disadvantage to reproductive success and IQ would decline - unless there were (one of two main mechanisms): social mechanisms favouring fertility of higher IQ; or social mechanisms differentially acting-against lower IQ (by increasing child mortality - which is what you are talking about, I think).
But the main point of this post is that the high average and peak IQ which has been seen reaching its apex in some societies in the past few hundred years, is a 'sweet spot' - something that requires rather specific and likely-to-be-transient combinations of conditions.
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Note - I roughly-estimate that the average intelligence in England a bit more than a century ago, was probably at the level of the top 10-15 percent of modern English -
http://charltonteaching.blogspot.co.uk/2012/06/taking-on-board-that-victorians-were.html
Although I also expect that the standard deviation of intelligence (or at least of phenotypic, measurable intelligence) was greater in the past due to more malnutrition and disease.
jgress - I don't know the numbers. But if mortality falls in a highly 'random' fashion and on the very young, so that it becomes a 'lottery' then this favours large numbers of offspring (like frog spawn - zillions of fertilized eggs to produce two surviving frogs on average) - but if mortality is high but can be influenced by (eg) parental care, greater learning or planning etc - then it may tend to increase intelligence.
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