An excellent article on the British Psychological Society’s Research Digest blog considers why it is so difficult to establish cause and effect in studies of links between intelligence and education. One difficulty is that it is not ethical to remove one randomly-selected group of children from education to test what happens to their intelligence levels in later life. Another is that genetic differences between children may have as much to do with later differences in their IQ scores as the education they receive. Again, it seems unlikely that psychologists will consider it ethical (or practical) to clone children in order to remove this genetic influence from research on education and intelligence.
Such is the ingenuity of psychologists, however, that studies have tackled both these difficulties without unethical methods being resorted to. So for example, identical twins have pretty much 100% genetic similarity, so any differences in reading ability between twins should be due to differing environmental influences.
The difficulty then comes in establishing what those differing environmental influences are. The researchers writing the article could identify an effect, but could only speculate on the possible causes – and all the possible alternative explanations for their findings. But they hope that by using twin-studies in this way, they are ‘edg[ing] further up the causal ladder, away from the basic correlational study’.