Today, as the title implies, I investigated the effect of the Flynn Effect upon cognitive decline with aging and made the interesting discovery that the Flynn Effect could account for almost all cognitive decline with aging up to the age of about 70.
This first chart shows the decline in IQ with aging for IQ levels of 150 and 100, prepared from the scoring tables for the Wechsler Adult Intelligence Scale-Revised (WAIS-R). It's noteworthy that the decline is about 20 points of IQ both for someone with an IQ of 150 and for someone with an IQ of 100. This is surprising since when mental ages are rising during childhood, they rise proportionately with IQ. The 12-year mental age of a 6-year-old child with an IQ of 200 will rise to a mental age of 24 when the child is 12, while the 6-year mental age of the average 6-year-old will only increase to 12 years of mental age when the average child turns 12. On the other hand, in later life, both their mental ages will drop the same number of years as they age.
This is consistent with the Flynn Effect. Since IQ tests are getting more difficult by 3 points of IQ per decade, people would naturally be expected to score lower and lower as the bar for an IQ of 100 or 150 is raised higher and higher. If these two 70-year-olds took the same IQ tests they took when they were 20, they would (according to this Flynn-Effect argument) get nearly the same score. The chart below shows what happens if we correct for the Flynn Effect. There's virtually no real age-related cognitive decline until the middle-60's. It also appears as though, beginning in the mid-60's, the rate of decline might be greater at the higher IQ level.
This also argues that the Flynn Effect is the same for someone with a 150 IQ as it is for someone with a 100 IQ--if this is, in fact, what's happening here.