The difference between intelligence and IQ is that intelligence is the real, underlying psychological function, whereas IQ is a score achieved in a test – a score which is intended to compare and measure intelligence but which is an indirect, only partly-precise and only partly-valid measure of intelligence. The IQ test is clearly a sound measure of intelligence – because IQ scores correlate with other measures of cognitive problem solving ability and thus brain functioning – but it is imperfect, meaning other factors than intelligence can impact the score. In much the same way, a bathroom scales measures weight – its results correlate with other measures of weight – but some scales are better than others and no scales is perfect.
Therefore, we can think of a qualitative, subjective understanding of the phenomenon of real intelligence as an irreducible entity – not understood in terms of other things nor only in terms of what it does, but in terms of itself as a real thing which we can detect and measure only indirectly. And we can then conceptualize IQ as the practical, simplified, publicly-shareable way of conceptualizing and investigating intelligence.
IQ can be, and usually is, researched in a ‘theory-free’ fashion, with operational definitions based on proxy description, measurement by comparison, and correlation – indeed intelligence is sometimes asserted to be nothing-more than a mathematical-derivation from IQ scores.
But we would emphasize that to understand intelligence requires understanding that sometimes a person may be of high intelligence and not have a similarly high IQ score (in other words, their IQ score is under-estimating their intelligence) – and that this may be the case no matter how validly, how often and how carefully the IQ is measured and calculated. And another person may have high IQ scores, measured in the best ways and by the best methods, yet not be of similarly high intelligence (in other words, their IQ score is over-estimating their intelligence).
Highly intelligent people who do not score as highly on IQ tests are easy to understand – because anything which reduces test performance could lead to this outcome: illness, pain, impaired consciousness and impaired concentration from sleepiness, drugs, drug-withdrawal, mental illness ... there are multiple causes, and some are chronic (long-lasting, perhaps life-long).
And people with high IQ scores who are not of similarly high intelligence to their scores are familiar to anyone who has attended a highly-selective college or educational programme or who are members of intellectually ‘elite’ professions; since they typically make-up a large proportion of participants. The ‘Flynn Effect,’ named after its discoverer New Zealand psychologist James Flynn, refers to the phenomenon of rising average IQ scores over the twentieth century in Western countries. The fact that this has taken place in a context of declining average real-intelligence means that the Flynn Effect can indeed be understood as evidence that IQ tests measure issues other than just intelligence, meaning they are imperfect.
(Plus, even the most reliable IQ test only has a reliability of about 0.9 when retesting the same person.)
One possible explanation for the Flynn Effect, proposed by Flynn himself, is that modern society – due to higher levels of education in the general population – makes us think in a more scientific way and this ability is partially a reflection of intelligence and partially of a separate ability that does not rely on intelligence. As such, IQ tests can be used to compare intelligence within a current population but they cannot be used as easily to make comparisons over time because they are examinations and people will tend to get better at them by practicing them and thinking in the way that permits optimum performance in them as society becomes more educated. So, up to a point, IQ scores may increase over time, despite the fact that intelligence is decreasing.
After (probably) six or eight generations of rising average IQ scores and falling real-general intelligence; there has been a progressive breakdown in the strength of correlation between intelligence measured in terms of IQ scores, and intelligence understood as a real underlying, brain functional phenomenon. Indeed, it seems likely that many or most people among modern high IQ scorers do not have similarly high real-intelligence. This would be expected to apply especially at highly-educationally-selective institutions where Endogenous personalities are substantially selected-out by the decades-long trend for an increasingly-high minimum-threshold of conscientiousness imposed by educational qualifications.
The correlation between IQ score and ‘g’ was probably much higher in the past (a century plus ago) than it is now – meaning that the distinction between IQ score and real, underlying intelligence is more important now than it used to be.