Whether we are mostly determined by genes or environment, by nature or nurture, is a long-standing and hotly contested issue. But extensive studies on twins, raised together or apart, have illuminated this topic.
Much of this work was conducted at the Minnesota Centre for Twin and Adoption Research, under the direction of Thomas J. Bouchard Jnr.
About once in every 300 births, a fertilised egg splits in two, producing identical twins, each of whom has the same genetic make-up.
On the other hand, fraternal twins come from separate eggs each fertilised by different sperm, and are no closer genetically than any two siblings.
In some cases, soon after birth, twins are given up separately for adoption and raised apart. Psychological studies of such twins as adults have provided powerful insights into the relative influence of genetics and environment in shaping personality and capacity.
The most valuable are those studies made of identical twins raised apart. Because both twins are genetic clones of each other, any differences detected between them as adults must be ascribed to factors other than genetics. The other main factor is, of course, the environment, including developmental noise.
The studies have shown that, on average, the personality traits of identical twins have a 50 per cent correlation. The personality traits of fraternal twins have a 25 per cent correlation.
Siblings who are not twins have an 11 per cent correlation, and the personality traits of strangers have a correlation near zero.
The 50 per cent similarity between identical twins raised apart must be explained by genetics, whereas the 50 per cent dissimilarity must be due to environmental influences.
A significant amount of the variation between identical twins seems to be due to developmental noise, i.e., random and uncontrollable little differences in biological development.
Remarkably, the studies observe that identical twins who have been raised apart (in circumstances not grossly dissimilar) are just as similar as adults as identical twins raised in the same family.
When two genetic patterns are identical they can exert equally powerful influences in a range of environments, so long as the range is not too broad. This finding contrasts strongly with an influential opinion in psychological and educational circles that the environment is pre-eminently important in child development.
Although the genes are important, their power has limits. Even identical twins can be much less similar to each other than might be supposed. For example, take the case of two identical twins reared apart and reunited as adults to find one is gay and the other heterosexual.
But the most dramatic illustration is the story of Eng and Chang Bunker, born in Siam (now Thailand) in 1811. Eng and Chang gave the name of their birthplace to the general phenomenon of conjoined or Siamese twins. They were identical twins joined together at the abdomen by a band of flesh.
Eng and Chang married a pair of sisters and fathered about 10 children each. Their wives lived in separate houses, and Eng and Chang spent half the week in each house. It is not possible to imagine any pair of identical twins who could be more close than Eng and Chang.
Nevertheless, their personalities were quite different. One became morose and alcoholic while the other remained cheerful and teetotal.
The human embryo receives half its genes from the father and half from the mother at conception. Some characteristics in the born individual are determined by a single gene, for example blood type, and other characteristics result from the effects of several genes.
Characteristics that are determined by several genes can either be additive or non-additive genetic traits.
Stature is an example of an additive trait. Consider a tall father and a short mother. If the child receives all the father's genes for tallness but few of the mothers genes for smallness, she will be tall.
If the child receives half the father's genes for tallness and half the mother's genes for smallness, she will be medium sized.
Non-additive genetic traits on the other hand are not passed on proportionally. Only a unique combination of genes produces a certain effect. A slightly different combination will produce a different effect.
Beauty is an example of a non-additive physical genetic trait. There are non-additive genetic personality traits also, and the twin studies have produced fascinating insights into heritability here.
Traits that are more than twice as common in identical twins than in fraternal twins are likely to be non-additive. The studies have shown that identical twins reared apart are more than twice as similar in religious inclinations, political leanings, and in enthusiasm for arts and crafts than fraternal twins reared apart.
What about the heritability of intelligence? This is a controversial subject. There are several reasons for this.
First of all it would be generally considered that intelligence is one of our highest human capacities.
It is natural therefore that people would like to have a degree of flexibility as regards the development of this personal capacity, and would like to feel that a less-than-generous initial capacity for intelligence could be improved with diligent effort.
Another reason why the notion of heritable intelligence is challenged is the negative historical legacy of eugenics and racism.
Some psychologists have claimed that intelligence cannot be defined objectively, is an arbitrary category, and is not heritable. But there is a growing consensus among psychologists, sociologists, and behavioural geneticists that IQ is heritable. In fact, IQ is the single most heritable trait in the twin studies programme. Various twin studies have consistently found that intelligence is 60 to 70 per cent heritable.
Identical twins raised apart are more similar than fraternal twins raised together in almost every physical and psychological trait. This seems to show clearly that genes are at least as important as the environment in shaping us. One school of psychological and educational thought has maintained that the environment is more important than genetics in shaping intelligence.
In that case it would be expected that unrelated adults reared together would have similar IQ scores. But the studies show they are no more similar in IQ than strangers reared apart.
The studies show that genes play a powerful role in determining behaviour but that the environment is also important. Genes are shown to make a person more likely to respond to the environment in certain ways.
When a person is supplied with a supportive and varied environment, he will pick out and interact with aspects of the environment that best match his genetic endowment. For example, a person who is sociable, strong and well co-ordinated is likely to become interested in a team sport.
Biological determinists hold that the genes more or less determine everything. Environmental determinists believe the environment determines most behaviour. The twin studies would refute both schools of thought.
They neither exalt the role of genes nor deny the effects of the environment. We are embedded in the biological tapestry of life, but many aspects of the texture of our own tapestry are modulated by the environment in which we hang.
William Reville is a senior lecturer in biochemistry at UCC
Studies have shown that personality traits of identical twins have a 50 per cent correlation