Is Intelligence Inherited

The term "g" is used by researchers to describe "General intelligence". It represents a measure of overall cognitive ability across a diverse variety of tests. It's not the same as IQ but it does tend to correlate. Everyone agrees that "g" tends to run in families. Lots of studies have shown it and most people can see it when they look at their own families, friends and acquaintances. But is this down to genetics or to environmental influences?

Is There a Gene For Intelligence?

Although there have been some suggestions, no single gene has yet been conclusively linked to intelligence. Rather it appears to be a case of complex interactions on many levels between many different genes – something known as polygenic inheritance.

Twin Studies

Identical twins have exactly the same genes, while non-identical, or fraternal twins share about half their genes, as do all siblings. Another feature of twins that makes them an ideal choice for studies is that they tend to be raised in pretty much the same environment as each other. If a particular feature is the same in identical twins, but not in fraternal twins, then chances are it's mainly genes that are controlling that feature.

So what do the twin studies show? Well, first degree relatives tend to have "g" correlation of about 0.4 -0.5. (Perfect correlation is 1; correlation of 0 means that the 2 things in question are totally unrelated). Identical twins have a correlation of 0.85, while for non –identical twins it's about 0.6.

So according to the twin studies it seems that genes play a very important role, but are not the only factor, since if they were, the correlation between identical twins would be 1.

Adoption Studies

Identical twins reared apart are almost as similar in "g" scores as those reared together. Adopted children and their adoptive parents have a "g" correlation of zero, while adopted children and their biological parents tend to have the same correlations as any parent –child pair. So although genes don't seem to be the only thing affecting intelligence, their effects do seem to be constant and apparently not overridden by environment. Disappointingly, parenting doesn't seem to have much effect.

Does Heritability of Intelligence Alter Over a Lifetime?

Remarkably, it appears that it does. "g" heritability climbs gradually from 20% in babies to 40% in children, peaking at 60% in adults. Why this should be is still a matter of speculation. It's been suggested that as our cognitive abilities become more complex, new genes may come into play that were not needed when brain function were less sophisticated. Or individuals may be drawn towards environments that fit with their genetic makeup, as time goes by and genetic effects that started out small in childhood build up together during adulthood.

Inherited Features Inside the Brain

Post mortem brain study has long revealed that the brain consists of grey matter and white matter. The grey matter is made up of the central cell bodies of the brain cells. The white matter consists of long filamentous protrusions from the cell bodies that carry messages between them. The grey matter has historically been linked to intelligence, though without much evidence. Modern studies, including techniques such as magnetic resonance imaging (MRI) of the brain have confirmed that the grey matter really is the bit that's critical for intellect.

The volume of grey matter is seen as a measure of the density of the brain cells, and reflects cognitive performance. So the more grey matter you've got, the brainier you are. Remarkably, grey matter volume is strongly determined by genes, and found to be most similar in people who are genetically alike. The frontal cortex (critical for most conscious thought) and the language related areas also tend to be very similar in family members.

Researchers have used the data from brain scanning studies to create colour coded images to demonstrate which parts of the brain are influenced mainly by genetic make-up and which are more adaptable to environmental cues, like stress and learning.

Future Directions

Brain regions where genetic influences are paramount may be particularly vulnerable to diseases that run in families, like dementia and some types of psychosis. New genetic brain mapping approaches have been suggested as a means of screening family members of patients for early signs of brain changes that may herald diseases like Alzheimer's or schizophrenia. When risk genes are known, these techniques may also help scientists to understand how they relate to inherited brain disorders.

On a darker note, there are those that worry that the new research findings that so clearly relate intelligence to inheritance may lead to thorny ethical issues and to racism disguised as "eugenics" – the idea that the human race can be controlled by selective breeding. Indeed, one of the founders of the Eugenics Society in the UK was Francis Galton, a pioneer of early intelligence testing. It is to be hoped that the current scientists and psychologists working in the field have a greater ability to see the full picture and to understand its complexity. As with so many other aspects of cognitive neuroscience, our knowledge at present is only observational, and is nowhere close to any type of practical application.