I am wondering if someone could direct me to a longitudinal study examining IQ test scores over time. For example, testing a group of test subjects every 10 years from age 5 to age 65.
The reason I am looking for such a study is I am interested to know what proof there is or is not that IQ is stable over time. It seems to me that it is accepted that IQ should be stable over a lifetime, but I haven't found any study that demonstrates this.
If we assume that IQ is the result of genetics and early childhood environment, then it follows that it should be stable over a lifespan, but I am looking for a study that demonstrates this through repeated IQ measurement over the lifespan.
I'll just give a bit of background about what I understand about IQ below:
- IQ is obviously not an exact measure. It's reasonable that a person's measured IQ could vary by around +/- 6 IQ points from their true IQ, therefore it is still a stable measurements if all IQ measurements over a lifespan are within a 12 point range.
- Brain damage such as catastrophic injury or regular alcohol abuse can permanently lower IQ.
- During adolescence there has been shown to be greater than normal fluctuation in IQ. I would suppose that, because IQ is a measure against age peers, the wide differences in maturation for individuals in the adolescent age range account for the fluctuations in IQ. However I would be interested to know if there were studies that showed children's IQs to have permanently raised or lowered during adolesence.
- If an individual has difficulty with an IQ test not related to raw intellegence, for example, if they have ADHD and therefore difficulty concentrating, then treating their issue may allow them to score higher on an IQ test than prior to treatment. In these cases it would seem that the condition masked their intelligence, and treatment did not improve their intelligence but allowed it to be measured more accurately. The same could probably be said of an individual for whom cultural biases in an IQ test (eg with verbal knowledge) worked against them.