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Certain groups perform worse on IQ tests than other. How do psychologists ensure that those differences aren't purely related to differing levels of education between those groups?
How do we know that IQ is a real phenomenon, and not just a purely social factor based on level of intelligence?
Short answer
Any intelligence test will be designed for a certain target population. That target population's score is set at an index of 100 for the standard IQ test. The same test applied to people from a different cultural background, or other socioeconomic status may yield numbers that are unreliable, as they my not understand the question (albeit knowing the answer), or they may lack the vocabulary, or even interest altogether.
Background
IQ is highly dependent on education; For instance, an illiterate person cannot perform an IQ test at all. And indeed, education can generally affect IQ scores (e.g., Dear & Johnson, 2010). Further, IQ tests are Western constructs and do not apply in many other cultures. So IQ tests are anything but objective. They only apply validly to compare people with a certain educational background. Bush people in the outbacks of Africa may not use numbers at all, but instead may rely on 'one', 'two', perhaps' three' and 'many'. They may be highly intelligent, we simply cannot assign a number to it, simply because they don't care about numbers, geometry, classifications, logic, and other standard components of IQ tests. The APA says rightfully
Researchers of cultural differences in intelligence face a major challenge, however: balancing the desire to compare people from various cultures according to a standard measure with the need to assess people in the light of their own values and concepts.
Reference
- Dear & Johnson, Int J Epidemiol (2010); 39(1): 1362–9
I'm a research psychologist. I don't agree with some of the assertions by @AliceD above. A lot of the disagreement on this topic is about what we mean when we say intelligence, so it would be helpful to define what you mean by IQ. To a psychologist, IQ refers to a specific measure, not the general construct of intelligence. Certainly the use of measure matters a lot:
Discussion of what intelligence means across cultures: https://www.apa.org/monitor/feb03/intelligence.aspx
The original IQ tests such as the Stanford-Binet were more culturally dependent, similar to the infamous SAT question about a 'regatta', which is a term that certain income and ethnic groups learn about more frequently.
However, for decades cognitive scientists measure intelligence with diverse methods, many without words at all (so there's less room for some types of bias). Performance on these tasks all correlate positively in populations, and are taken to indicate a stable underlying trait: g for general intelligence. Education improves g, but not very much! About 1 IQ point per year of education, or ~1/15th a standard deviation per year.
The biggest effects on g are genetic mutations, lead exposure, malnutrition, stuff like that. All g outcomes are complex interactions between genes and environment.
Accessible, affordable, brief book of the research that addresses many misunderstandings: https://www.amazon.co.uk/Intelligence-That-Matters-Stuart-Ritchie/dp/1444791877
About the original question, if one measures education and g separately, one can see whether variation in one is due to variation in the other. That's how psychologists assess independence. These are reliably linked, but only weakly.
