Why IQ test level fluctuates over your lifespan?

Recent research confirms that intelligence fluctuates quite a bit in adolescence.

What do you know about high IQ for people - average IQ score by age? 

In 1932, the entire population of Scottish 11 year-olds (87,498 children) took an IQ test level. Over 60 years later, psychologists Lan Deary and Lawrence Whalley tracked down about 500 of them and gave them the same test to take again. Here are the results:
Turns out, the correlation was strikingly high – 66, to be exact.

Those who were at the top of the pack at age 11 also tended to be at the top of the pack at age 80 and those who were at the bottom also tended to stay at the bottom. Equally as interesting the correlation was far from perfect. A few outliers could be found. One person had an IQ of over 100 at age 11, but scored just over 60 at age 80. There are many possible reasons for this outlier, including dementia. Other folks showed IQ increases as they aged. On average, people’s individual scores on the test taken again at age 80 was much higher than their scores had been at age 11, even though the rank ordering among people stayed roughly the same.
These results are illustrative of what psychologists find over and over again.

IQ tends to remain relatively stable over the lifespan. The keyword here is relative. IQ researchers are interested in explaining differences. Developmentally speaking, and individual’s intelligence is not fixed at birth.
Although rank ordering of scores tends to remain stable, scores within individuals fluctuate quite a bit. As the noted IQ researcher (Nicholas J. Mackintosh) notes in his superb book “IQ and Human Intelligence” “If aged 40 you obtain the same absolute score on the same test as the score you had obtained at age 10, something would have gone seriously wrong in your life. The long-term stability of IQ test level means only that you’re standing relative to others of your age stays much the same”.
<<< See more >>>  Are people with high IQ test score more successful?

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