The Heritability of Intelligence

There are good reasons to think that some significant part of intelligence is inherited genetically, and therefore determined at conception regardless of any environmental factors operating afterwards (see the Galton Institute Newsletter, March and September 1995). If so, this will have important evolutionary consequences, since it is only the genetically heritable component which will evolve in response to ordinary natural selection.

But a great deal of what is regarded as intelligence ("cognitive ability") affecting human intellectual performance derives from cultural inheritance, passed on by teaching and learning from one generation to the next. This also responds to selection, as better practices and ideas replace what went before, but since it is now generally agreed that Lamarckian inheritance of characters acquired during an individual’s lifetime does not occur, this will have no effect upon the genetical inheritance of the species.

There is little reason to think that there has been any significant increase in the genetically heritable component of human intelligence over the past few thousand years, but material culture certainly has enormously increased, where genetical inheritance plays no part. Human cultural evolution occurs much more rapidly than does biological evolution, and it differentiates faster too. The very large differences to be seen today between the material cultures developed by different peoples, ranging from hunter-gatherers only just emerging from the Stone Age to modern industrial societies, have largely been due to different rates of cultural evolution. Possibly there may be differences in intelligence as well, but that is far from sure and the effect would anyway be quite a small one.

It is also important to remember that where genetically heritable characters in a population show variance, mean values can change quite quickly over a few generations in response to selection. For example, there is some evidence that the mean IQ scores of people of African origin in the USA may be as much as 10 points below the white population. That is a lot, although it will still mean that the brightest of the blacks will score higher than the majority of whites. And there is every reason to think that much of the difference may be environmentally determined, since the living conditions of the blacks are usually much less favourable than those of the whites. It could however be that there was some residual difference in genetically heritable intelligence, represented by perhaps a few points in the mean IQ score. If that were so at the present time (and it would be denied by many), this could very well change over a few generations, and even be reversed, if the selective situation were to become different.

In all populations of living creatures, including humans, evolution by natural selection will occur if, and only if, there is variance for genetically heritable characters and individuals carrying the genes for a particular character leave on average the most descendants in to future generations. And that will be so whether or not the genes selected for will in the long run be the best for the species concerned. Evolution is always progressive, but it can progress down as well as up, even to extinction.

There are however two different forms of biological evolution. The first is produced by selection operating on genes already present in the population and which show variance in their effects. This can lead to rapid changes of the sort which are to be seen in animals and plants under domestication, and which also could well be responsible for short-term changes in the mean intelligence of different human populations. But the second results from the origin of genes and mutations which really are new and have not appeared before. This leads to the truly progressive evolution of new types, which will themselves of course be subject to selection and will only survive if they represent an improvement on what went before. An example would be the threefold increase in human brain volume over the past few million years, responsible for the enormous increase in genetically heritable intelligence, together with the evolution of human cultural inheritance although that is not itself inherited genetically.

The evolution of material culture and of the cultural component of intelligence which accompanies it, unique to humans, is quite different from that of the genetically heritable component, and the two can occur independently of one another. We are at present in an explosive phase of cultural evolution, and it is hard to see when it will stop. But there is no reason to think that there has been any appreciable increase in the genetically heritable component over the past few thousand years: few would suppose that Periclean Athens was much behind modern Europe in general intelligence, but it was probably nearer to its palaeolithic predecessors than to ourselves where the material conditions of life are concerned.

All animal species, if they do not evolve in to something else, eventually become extinct. There is no reason to think that this will not apply with us, and there have been a number of suggestions as to how that might happen.

We now have, or soon will have, the power to blow ourselves all to kingdom come, either deliberately by atomic warfare, or inadvertently through some nuclear disaster. But that probably wouldn’t lead to complete extinction. Even if 99.9% of the population were to be destroyed there would still be about a million left, which is probably more than the human species has numbered over practically the whole of its history. This should therefore be more than enough for it to start again, even after any such an unimaginable disaster.

Should world population continue with its present exponential growth it is bound eventually to be limited by starvation, if by nothing else. Ten thousand million will certainly be reached during the lifetime of people now living and these could probably be supported and fed, and even perhaps twice as many. But a hundred thousand million might be a different matter, and it wouldn’t stop there. Effective ways of controlling population numbers are of course now available, and it is to be hoped that they will be used before other Malthusian checks start to operate. That may not be enough however, if only because those who are unable or unwilling to control their own reproduction will outbreed those who do. Even so it is unlikely that the whole population would quietly starve to death. This is because the stronger and more enterprising people would take for themselves whatever they needed, by force if necessary and the Devil take the rest. That may be a very unpleasant prospect, but it wouldn’t lead to the extinction of the whole human species.

There is however another possibility, all the more insidious since it might not be recognized until too late. This is that the genetically heritable component of intelligence might start to decline. That would inevitably result, for precisely the same selective reasons as it has risen in the past, if and when those whose genetically heritable intelligence was below the average began to leave the most descendants in to future generations. And since cultural inheritance evolves independently of the genetically heritable component of intelligence, this could quite well happen while the material conditions of life continued with their present improvement. The end result could be that our successors, whose mean level of intelligence was well below what it is now, might find themselves unable to manage a highly complex material inheritance produced by ancestors more intelligent than themselves. And that looks like a recipe for disaster, especially if desirable physical characteristics had degenerated too, leaving a population incapable of coping with any simpler way of life. So, après nous le déluge: but fortunately none of us will be around to see it!

C B Goodhart