Nature versus nurture
The nature versus nurture debate is about the causes of differences between people.
Like all living things, people have inherited innate qualities. There are also events or experiences which happen during life. 'Nature' describes the effect of a person's genes, whereas 'nurture' describes whatever happens during life.
In the language of population genetics, the heritability of a feature is the extent to which it is inherited genetically. That includes traits of behaviour and character. Though the public debate is all about humans, the principles apply to any living thing, plants as well as animals.
The phrase 'nature versus nurture' was suggested by the Victorian polymath Francis Galton. He was influenced by Darwin's On the Origin of Species. He investigated the influence of heredity and environment on social advancement.
It was always known that people inherited some features, but were modified during life. The terms had been contrasted, for example, by Shakespeare (in The Tempest: 4.1). Even before Shakespeare, the English schoolmaster Richard Mulcaster wrote in 1582:
- "Whereto nature makes him toward, but that nurture sets him forward".
Galton did not oppose nature to nurture as two alternatives. The phrase 'nature vs nurture' has been rightly criticized for its over-simplification. Almost all writers have realised that both play a part in our make-up. One who, at first sight, seemed to think humans got their 'mind' from nurture (the tabula rasa or blank slate theory) was philosopher John Locke. He, however, was only concerned with how we acquire knowledge from sense data.
Both nature and nurture play interacting roles in development, and many modern psychologists and anthropologists consider the contrast naive. They see it as an outdated state of knowledge.
Identical twins are natural clones. Because they carry the same genes, they may be used to investigate how much heredity contributes to individual people. Studies with twins have been quite interesting. If we make a list of characteristic traits, we find that they vary greatly in how much they owe to heredity. For example:
- Blood groups: entirely inherited. Eye colour: almost entirely inherited.
- Weight, height: partly inherited, partly environmental. Intelligence: more inherited than not, if IQ tests are used as a measure.
- Which language you speak: entirely environmental.
The way the studies are done is like so:
- Take a group of identical twins and a group of fraternal twins, and a group of siblings from the population.
- Measure them for various traits.
- Perform a statistical analysis (such as analysis of variance), which will tell you to what extent the trait is inherited. Traits which are partly inherited will be significantly more similar in identical twins.
Studies like this may be carried further, by comparing identical twins brought up together with identical twins brought up in different circumstances. That gives a handle on how much circumstances can alter the outcomes of genetically identical people.
The person who first did twin studies was Francis Galton, Darwin's half-cousin, who was a founder of statistics. His method was to trace twins through their life-history, making many kinds of measurement. Unfortunately, though he knew about mono and dizygotic twins, he did not appreciate the real genetic difference. Twin studies of the modern kind did not appear until the 1920s.
This kind of research works well when the features can be measured simply. It works less well when the measurement is itself controversial. This was the case with I.Q. measurement, where the method of measurement was not well agreed between researchers.
Estimates of the heritability of IQEdit
Studies have found the heritability of IQ to be between 0.7 and 0.8 in adults and 0.45 in childhood in the United States. It may seem reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. However, that the opposite occurs is well documented. Heritability measures in infancy are as low as 0.2, around 0.4 in middle childhood, and as high as 0.8 in adulthood. Everyday experience would suggest that as people grow older, they get to know themselves better. They choose, if they can, jobs and activities which make the most of their abilities. The effect of this would be to show a closer fit between their genetics and their environment.
A 1994 review in Behavior Genetics based on identical/fraternal twin studies found that heritability is as high as 0.80 in general cognitive ability but it also varies based on the trait, with 0.60 for verbal tests, 0.50 for spatial and speed-of-processing tests, and only 0.40 for memory tests.
In 2006, The New York Times Magazine said about three quarters for heritability (0.75) was found in most studies. A 2004 analysis of reports in Current Directions in Psychological Science gave an overall estimate of around 0.85 for 18-year-olds and older.
- Galton, Francis 1875. Proceedings of the Royal Institution of Great Britain, volume 7. Of Men of Science, their nature and their nurture.  Also, later, Galton, Francis 1895. English men of science: their nature and nurture. Macmillan, London and Appleton, New York.
- PROSPERO: A devil, a born devil, on whose nature Nurture can never stick; on whom my pains, Humanely taken, all, all lost, quite lost.
- Mulcaster, Richard 1582. Elementaries.
- Dusheck, Jennie 2002. The interpretation of genes. Natural History, October 2002.
- Carlson N.R. et al. 2005. Psychology: the science of behaviour. 3rd ed, Pearson. ISBN 0-205-45769-X
- Ridley, Matt 2003. Nature via Nurture: genes, experience, & what makes us human. Harper Collins. ISBN 0-00-200663-4
- Westen D. 2002. Psychology: brain, behavior & culture. Wiley. ISBN 0-471-38754-1
- Almost: during the development of a foetus errors in cell division result in some genetic changes. Rui Li et al 2013. Somatic point mutations occurring early in development: a monozygotic twin study. Journal of Medical Genetics. 
- Advanced methods are not described here; just a simplified account of how the problem can be approached.
- A good account is given in Bodmer W. & McKie R. 1994. The book of man: the quest to discover our genetic heritage. Abacus, London. p188–197 ISBN 0-349-10620-7
- Bulmer M. 2000. Francis Galton, pioneer of heredity and biometry. Johns Hopkins, Baltimore MD. p67
- Galton, Francis (1876). "The history of twins, as a criterion of the relative powers of nature and nurture". The Journal of the Anthropological Institute of Great Britain and Ireland. 5: 391–406. doi:10.2307/2840900.
- Plomin R. et al. 1994. Variability and stability in cognitive abilities are largely genetic later in life. Behavior Genetics 24 (3): 207–215. doi:10.1007/BF01067188
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- Bouchard, Thomas J. (2004). "Genetic influence on human psychological traits: a survey". Current Directions in Psychological Science. 13 (4): 148–51. doi:10.1111/j.0963-7214.2004.00295.x.
- Plomin R. et al 2001. Behavioral genetics. 4th ed, Worth Publishers.
- David L. Kirp (2006). "After the bell curve". New York Times Magazine. Retrieved August 6, 2006.
- Duyme M. 1999. How can we boost IQs of "dull children"? A late adoption study. Proceedings of the National Academy of Sciences 96 (15): 8790–8794. doi:10.1073/pnas.96.15.8790