If IQ is distributed as normal and the population is given an IQ test at random then a ‘fail grade’ would be set at a certain percentage for each level of education.
Assume the average grade is 50% and the fail grade at level 4 is 40%.
However, knowledge (in theory) is not like IQ; knowledge is specific to certain areas of practice, certain trades and professions; as such, it has an association with organisations and cultures.
Therefore, as a population's knowledge in a specific area increases then the distribution will become positively skewed. The tail of the distribution will become extended as the average increases, e.g. to 60%, and the pass mark stays the same.
If so, less people will fail at 40%; but those who would of failed but now pass are those who have benefited (most) from the education or knowledge available within their organisation/culture. Those at the top would suffer a ceiling effect, as more people achieve the maximum limit of knowledge within that community.
The ones who would have failed and still fail are therefore ‘true’ low IQs in the area of knowledge concerned; whereas the ones who would have failed but now pass are therefore ‘false’ low IQs in the area of knowledge concerned.
If culture is a repository of knowledge for certain adaptive purposes then each culture will leave in its wake a trail of ‘true’ low IQs.
Furthermore, seeing as IQ is a multi-dimensional construct it should be possible to ascertain what culture adds in terms of knowledge/educational value by comparing the type of IQ deficits the different cultures leave behind. These can be considered as part of the spectrum of the ‘developmental disorders’.
In theory, if verbal language is the main repository of cultural knowledge in the West, the developmental disorders that involve the comprehension and production of verbal languages, viz. the dyslexias, will tend to be exposed the most; and this is certainly true, at least within the educational organisation/culture.
However, there are also some cases where cultural knowledge is transmitted across generations imperfectly, so that it is the environmental rather than the genetic component which causes the deficit in knowledge and, therefore, the construction of a ‘developmental disorder’.
These are still 'true' low IQs but low IQs that have a different cause; and potentially a reversible one.
Thus, although childhood trauma has been linked to some forms of damage in the brain, this may be a result of the environmental, rather than genetic, causes. This may or may not be reversible.
