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05/30/2001 Archived Entry: "Genetic determinism"

Continuing on from the last post, we have a major unanswered question - considering that intelligence does have a genetic component to it, does this mean that our genes ultimately determine what our intelligence will be as adults? In other words, is intelligence subject to genetic determinism?

Well, first we have to understand the issues. Clearly individuals are grown based on the 'recipes' of our genes and the way in which our genes differ produce different individuals. Take height for example. We all know that there is definitely a genetic component to height; if both your parents are tall, you have a much better chance of being tall than if both your parents were short, so you could say that there are 'tall' genes and 'short' genes.

You might then assume that people with tall genes might always be taller than people with short genes. Well, you'd be assuming wrong.

Looking at the graph, let's ignore everything apart from the bottom line for the 'short' genes. What the graph is showing you is that as your diet when you were a child contains more proteins (important biochemical molecules), your height as an adult would end up taller. This also applies to people with 'tall' genes. In other words, irrespective of what your genes are, the better your diet as a child, the taller you will be.

Of course, there is a genetic component that comes into play. At any specific level of protein in the childhood diet, the short gened people will always be shorter than the tall gened people. The thing is, even a short gened person with a really good diet could end up taller than a tall gened person with a poor diet.

Before I move on, some nomenclature. The V(g) refers to the genetic variation of height in the population represented in the graph - it's the variation that exists between the shortest person and the tallest person when the environment (i.e. diet) that they are subjected to is identical. V(t) refers to the total genetic variation in height spanning all environments.

As I mentioned in my last post, heredity is V(g) divided by V(t) - it's the proportion of variation which can be attributed by genetic, not environmental, factors within a population for a specific trait.

So for height you could say that it is genetically determined, to a certain extent. Does intelligence work in the same way?

Well, a while back a group of scientists decided to find out. What they did was to take a population of rats and test how long it took each of them to solve a maze. They then took the fastest maze solvers (the 'most intelligent') and bred them together, and did the same for the slowest maze solvers. Over a period of time, they kept this up, breeding the fastest maze solvers over and over again until they came up with a really good rat maze solver. Same for the rats who were bad at maze solving. And when they were finished, they came up with these results.

On the vertical axis, we have maze solving ability, so the results that are higher on the graph represent rats that can solve the mazes quickly. On the horizontal axis we have the level of 'interestingness' of the environment that the rats were subject to when they were growing up.

The results they collected showed that at a 'normal' level of environement (i.e. they had a few toys to play with when they were growing up), one set of rats consistently outperformed the other rats at maze solving ability, and of course these were the rats which were bred for their maze solving ability.

Now the researchers were quite excited by this because they had rightly thought that they had managed to find a gene or set of genes that contributed to intelligence (or at least maze solving ability) in rats. Considering that the two sets of rats were subject to the same environment for their entire lives, it couldn't have been anything else - they only differed in their genes.

The researchers continued to gather results for this, expecting that they would follow the same sort of pattern for height. Except they didn't...

Instead of the parallel lines that we saw with height which indicated that no matter how good a short gened individuals diet was, they'd never be taller than an individual with tall genes who had the same diet, the results for maze solving ability were significantly different.

See, when the rats were subjected to a 'low', unstimulating and boring environment when they were growing up (i.e. they lived in a bare box), it didn't matter whether they had 'good' genes or not, both sets of rats performed rubbishly at maze solving ability. And when both sets of rats had 'high', stimulating and interesting environments (i.e. loads of toys and places to explore), they both were extremely good at maze solving.

This confounds the idea of genetic determinism in intelligence to quite a large amount, and it also prevents us from working out a decent value for heredity. Sure, we can figure out V(t) by measuring the difference in maze solving ability between the fastest and slowest across the entire range of environments, but the difference between the fastest and slowest at any specific level of environment changes depending on the environment. The shape of the graph isn't a parallelogram, it's an ellipsoid and as such, we can't really work on heredity.

(In other words, my 50% value for heredity of intelligence in my last post wasn't actually correct.)

This is heartening news because, if extrapolated to 'general' intelligence in humans, it means that we shouldn't worry about whether you have good genes for intelligence, we should instead worry about giving everyone the highest possible level of education and a stimulating environment when they grow up so that we can all be good at solving mazes... uh... I mean, we can all be really intelligent.