Intelligence has always been tricky to quantify, not least because it seems to involve most of the brain and so is almost certainly not one "thing". Even so, scores across different kinds of IQ tests have long shown that people who do particularly well—or badly—on one seem to do similarly on all. This can be crunched into a single general intelligence factor, or “g”, which correlates pretty well with academic success, income, health and lifespan. So more intelligence is clearly a good thing, but where does it come from? A large part of the answer seems to be genetics. In 1990, the first twin studies showed that the IQ scores of identical twins raised apart are more similar to each other than those of non-identical twins raised together. Since then a few genes have been linked to IQ, but all of them scent to have a tiny effect and there are probably thousands of genes involved.That doesnt mean the environment plays no part, at least in childhood. While the brain is developing, everything from diet to education and stimulation plays a huge part in developing the brain structures needed for intelligent thought. Children with a bad diet never fulfill their genetic potential.The word "those" in paragraph 2 refers to .............