It is possible that the dysgenic decline in average intelligence has been masked by underestimating the age-related decline in intelligence.
The numbers which follow are very approximate, rounded to fives and tens; and I cannot at present reveal all my sources.
The basic idea is that, using simple reaction times to estimate general intelligence:
1. Age related decline in intelligence in the average individual is roughly 2 IQ points per decade (through the working years, from say 20-70 years old); while
2. The population average dysgenic decline in intelligence has been roughly 1 IQ point per decade.
[See NOTE below]
Therefore, over a fifty year span - the dysgenic decline in the population is about 5 IQ points, while the age-related decline in each adult individual is about 10 IQ points.
Therefore, at any given time point, the average 20 year old is about as intelligent as the average 45 year old and roughly 5 IQ points more intelligent than the average 70 year old.
So, each new cohort of 20 year old sees themselves as being the same intelligence as the middle aged, and more intelligent than the elderly
- but by the time current twenty year olds have reached the age of seventy they will be significantly less intelligent than the current seventy year olds.
This also means that the effect of declining intelligence is time-lagged, only becoming apparent some decades after the decline has occurred - and becoming visible only when age-related decline has accumulated.
How come this was not obvious? One reason may be that rising average IQ test scores (i.e. the Flynn effect) has hidden it - because IQ test scores have been going up over recent decades, even as real, underlying intelligence has been going down. So the true age-related decline in intelligence did not show-up in longitudinal IQ testing - the real decline has been masked.
NOTE: This is put simply in order to get my point across. But I should add that the age related decline in intelligence is not linear at 2 IQ points a decade - that is probably about the average rate of decline from age 20-70 - but the rate starts out less than 2 points per decade and ends up higher - in other words, the rate of decline increases with increasing age.
Also, I see no reason why the dysgenic decline should be at a linear rate either - nor that the rate of dysgenic decline should be the same in all populations - indeed there are some populations (Sweden, Finland) where there probably was not a dysgenic decline of intelligence - at least, not a decline due to differential reproduction related to intelligence.