Although it has long been known that genetic errors such as Down's syndrome and phenylketonuria often impact negatively on intelligence as well, the extent to which normal genes play a role in determining human intelligence is much less clear. To determine what, if any, role genetics plays in human intelligence, the effects of nature and nurture upon intelligence must be separated. In practice, this is quite difficult. Not only are genetics and environment simultaneous influences on every person's life, but environmental influences also determine whether specific genes switch on or off. Because of such epigenetic modification, even monozygotic (identical) twins, who share the same genes because they came originally from the same fertilised egg, will have differences in gene expression.
Separating out the effects of genetics and environment in short-generation lab animals is much simpler. In 1940, Tryron decided to breed laboratory rats based on their maze-running ability. After seven generations, the offspring of rats selected for high maze-running ability performed much better in problem-solving skills than the offspring of rats selected for low maze-running ability. Yet even here, the picture is not so clear. In his 1942 follow-up study, the difference vanished entirely within a single generation when the environment of the low-performing rats was enriched by giving them more objects to explore and more social interaction.
Tryron's studies strongly suggest that in rats at least, intelligence is determined by both genetics and environment. Translating these findings to human beings is much more problematic. For one thing, obviously it is unethical to breed human beings as one would rats, so the simplest study mechanism for analysing genetic patterns is unavailable. Instead, the approach used by many researchers in this field is to study multigenerational trends in families which already exist.
In analysing this data, we must proceed with caution. One of the simplest forms of analysis is correlation of data, whereby we can learn how likely it is that another individual who shares one trait also shares another. Correlation statistics must be considered with care because they tell us nothing about cause and effect, only about probabilities. A correlation of 1.0 means that every individual who exhibited one trait also exhibited the other; while a correlation of 0 means the exact opposite. If genetics plays a role in human intelligence which is not entirely drowned out by environment, we might expect correlation of IQ among relatives to correspond with the number of alleles they are likely to share.
In 1982, Bouchard and McGue examined the IQ scores of family members as correlated across 111 different studies. They found that the mean correlation of IQ scores was highest between monozygotic twins, at 0.86, and lowest between cousins, at 0.15. This suggests that there might be something genetic at work here, but that there are also other factors involved. The researchers also identified a further complicating factor: correlations based on degree of kinship varied with age. Common sense might suggest that the effect of genetics would probably decrease as the individual gained more experience. In fact, the opposite seems to be the case: the older the individual, the greater the genetic influence upon intelligence.
Yet these studies do not actually tell us that the intelligence (as measured by IQ tests) correlations with kinship are due to genetics. The trend exists: but does it exist because the child inherited genes for high intelligence or because gifted parents are more likely to provide a more enriched environment for their children?
Many genetic studies have attempted to identify even a single locus in the genome which can be directly linked to intelligence. Most such studies choose to examine one or another of the genes known to be linked with brain development. The two most promising studies to date have found a small rise in IQ linked with the CHRM2 gene, and another small rise when the 'C' version of the FADS2 gene is combined with breastfeeding. However, the 2004 study which has used the most rigid criteria to date in genetically comparing a high IQ group with an average IQ group failed to find any single linked gene.
When combined with correlational data, these studies suggest that human intelligence might be in part genetically determined, but that intelligence is almost certainly a polygenic trait involving hundreds if not thousands of genes.
It gets even more complicated. Biological families are always looked at first when studying any type of hereditary patterns because they share the greatest number of genes within a given population, but this relatively small genetic variance is not the only variable within a family. Even biological siblings do not experience the same environment growing up. This could be a matter of birth order, gender, changes in school policy or school between one sibling and the next, changes in residence ... the list goes on and on. Even same-sex twins may not be treated completely alike. Thus biological family studies are valuable in determining hereditary patterns for biological conditions such as colour blindness, but the number of confounding variables in any given family make it nearly impossible to separate the effects of environment and genetics in determining human intelligence when using this measure alone.
Even if by some incredible coincidence we could find a family where siblings were treated absolutely identically in every part of their life, we would still have to add in the effects of the in utero endocrine environment. Even as simple a genetic trait as XX or XY can be almost completely overruled by hormonal effects during gestation. A child can be born female and grow up thinking itself female, only to discover later that its 23rd chromosome is actually XY. The only visible difference such a child might have from XX females is infertility. Most people don't realise just how common these kinds of endocrine disruptions are until they happen to come into the public spotlight, as has happened in Olympic controversies over athletes who had thought themselves female all their lives until gene typing revealed otherwise.
Some of these confounding factors can be eliminated by studying only monozygotic twins. Although even this subject pool cannot control entirely for variation in genetic expression, it comes much closer than other types of subject pools. Mutation is always a factor, although it plays much less of an abrupt role in polygenetic traits than in single gene traits such as the haemophilia which ravaged Queen Victoria's chidren and grandchildren. As many as one third of all monozygotic twins have separate placentas, which means that their in utero environment could be quite different from each other. This is likely the reason that some monozygotic twins have different sexual phenotypes despite their identical genetics. The role of epigenetic modification in genetic expression has already been mentioned. Younger twins have fewer epigenetic differences than older twins, and twins who have lived apart have more epigenetic differences than those who spend their lives together. Yet for some reason we don't yet understand, intelligence and personality actually seem to become more alike as twins age.
Thus another way to determine what role genetics have in determining human intelligence is through studies of monozygotic twins who have been separated and raised in different environments, usually as a result of adoption. These results are then compared with dizygotic (fraternal) twins who have been similarly separated. Alternately, results may also be compared with monozygotic twins who have been raised in the same home environment.
Results of such twin studies have been all over the board, which we now understand is in part because of Bouchard and McGue's discovery of age as a confounding genetic factor. Just about all monozygotic twin studies do show that genetics definitely do play a role in intelligence, but the degree swings wildly. As a result, estimates of the overall effect of genetics in determining intelligence range from as low as 0.4 to as high as 0.9.
Based on all the current research, we can now say with some certainty that genetics do determine human intelligence, but that we don't yet understand how. Intelligence is also a polygenic trait, probably involving more genes by far than any other human polygenic trait. Because much of this genetic inheritance is also epigenetic, the genetic expression of intelligence is inextricably linked with environment.