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Nature versus Nurture

In this article we will be exploring the age-old nature or nurture debate: which is more relevant; your genes or how you are bought up. This particular article will be concerned with inteligence.

Evolution and Genetics

Firstly, what is evolution? It is a process where the creatures that are best adapted to survive will live longer and therefore be able to reproduce more and have a greater number of offspring. Many of these offspring will have this characteristic and over the course of many generations all in that species will have this characteristic.

It can be defined briefly as: a gradual change in a species over time because of a natural selection of the best adapted to an environment.

Now we move on to a simple look at genetics. As you know, we have genes which give us different characteristics and these genes are inherited from our parents. The actual genetic code is known as the genotype. However, you get one gene from each parent for everything, but obviously only one of these can be expressed; so how the genes actaully manifest themselves is called the phenotype.

Twin Studies

A twin study is one of the method used. There are two types of twin: dizygotic (DZ) who are regular fraternal twins; and then monozygotic (MZ) which are identical twins and so have exactly the same DNA as each other, i.e. the genotypes are identical. The table below summarizes what the various findings would mean.

twin studies

In particular it is useful to look at MZ twins (since there genetics are the same) who have been raised together or apart. This was done by Shields (1962), and found the correlation between inteligences of MZ growing up together was 0.83 and growing up apart was 0.51. This would appear to show the environment is more important, however there are some methodological issues related to this study.

A correlation shows how two things are related, but don't give a definite reason. Also, it is only recently that we can prove two twins are identical, it could be that DZ twins that looked very similar were included because they thought they were identical. And finally there is the issue of what was counted as 'seperated' in the study, some twins were included who were raised but different family members but lived near to each other, regually saw each other and went to the same school.

Adoption Studies

Adoption studies involve looking at someone who has been adopted and comparing their intelligence with their biological and adoptive parents. If the IQ was more similar to their biological parents who have DNA in common, then we could conclude intelligence was as a result of nature. However, if the IQ was more similar to the adoptive parents who have the upbringing in common, it would be nurture.

Now to evaluate these studies:
The matching of parents. The authorities tend to find adoptive parents who are similar to the biological so the transition is easier for the child. For example, matching up parental occupations.
Small samples; it is difficult to find people willing to come forward who also know their biological parents, this makes the results less represntative.

Selective Breeding

Selective breeding studies involve breeding intelligent parents to see if the offspring are intelligence. Needless to say that for ethical reasons such studies cannot be carried out on humans. But in 1940, Tyron conducted a selective breeding study on rats.

AimUse selective breeding in rats to see how genetics influences intelligence.
MethodA large number of rats were trained to run a maze. It was then recorded how many errors each made.

The best rats at the task were called maze bright and put together and the rats poor at the maze were maze dull and they were also put together to breed.
ResultsThe maze bright rats improved their performance over the generations of breeding; however the maze dull rats in fact got worse.
ConclusionIntelligence is a heritable characteristic in rats.
EvaluationThe positives of this type of study are that rats have a rapid life cycle so many generations will be produced in a relatively short time and that the vast majority of DNA in rats and humans are the same.

However the big weakness is that it doesn't very well relate to humans, since maze solving does not equate to human intelligence and that breeding within a family (as the rats did) is known to very often cause genetic problems that can severely impair intelligence.

Very sorry to dissappoint you, but the conclusion is that both nature and nurture play an important role, however perhaps that the environment is the deciding factor.

The idea used to describe it is the Rubber Band Hypothesis. It says that the potential (length of the band) is decided by genetics. However the environment stretches this band, so that someone with a low potential could be stretched beyond someone with a high because of environmental differences. However if someone with a high and someone with a lower genetic potential had the same environment, that person with the higher genetic potential would be able to 'stretch' their intelligence to a higher level.

the rubber band hypothesis of intelligence