The etiologies of most human traits are impossible to tease apart. Although genetic factors and rearing conditions probably can explain variation in many human attributes, there seemed to be no ready way to distinguish hereditary from environmental influences on quantitative traits such as various behavioral phenotypes.
hereditary factors; environmental rearing environment
The etiologies of most human traits are impossible to tease apart. Although genetic factors and rearing conditions probably can explain variation in many human attributes, there seemed to be no ready way to distinguish hereditary from environmental influences on quantitative traits such as various behavioral phenotypes.
In the late 1860s and early 1870s, the Victorian polymath Francis Galton (a cousin of Charles Darwin) drew attention to the use of twin studies for elucidating the relative importance of hereditary factors and environmental rearing conditions in features such as human intelligence. Although the modern concept and cellular basis of monozygotic twinning had not yet been established, Galton’s basic idea was that because many human twins are presumably genetically identical, any observed differences between such individuals must be due to something (namely, rearing conditions) other than their genes. Galton was a remarkable individual who is also credited with pioneering the statistical concept of correlation, and founding such diverse fields as meteorology (scientific weather forecasting), forensics (using human fingerprints), eugenics (which envisions the genetic improvement of humanity), and psychometrics (the science of measuring human mental faculties). His scientific legacies are thus varied and profound.
Throughout much of the ensuing 20th century and beyond, three types of twin protocols tracing back to Galton have been employed in scientific investigations of genetic versus cultural influences on human behaviors. The least critical approach analyzes trait correlations in monozygotic twins reared apart (MZA twins), the rationale being that any resemblance between MZA twins suggests (but does not prove) genetic influence. The second approach compares trait correlations for monozygotic twins versus dizygotic (non-identical or fraternal) twins, under the rationale that any higher correlations observed in identical twins register genetic influences. The third approach compares monozygotic twin-sets reared together versus those reared apart, the rationale being that any greater trait differences between separated twins must register environmental effects. Each of these approaches has conceptual as well as logistical limitations, so none yields definitive conclusions about the relative impacts of “hardwired” versus environmental influences on particular phenotypic features.
Twin studies helped to initiate and propagate the longstanding “nature versus nurture” controversy in evolutionary genetics, and thus were of considerable historical importance to the field. However, the modern paradigm is that both genes and the environment, as well as interactions between the two, are all important contributors to phenotypic variation in most quantitative traits (in humans as well as in other creatures).
1. Galton F. Hereditary Genius London, UK: Macmillan; 1869.
2. Wilson EO. On Human Nature Cambridge, MA: Harvard University Press; 1978.
3. Lewontin RC, Rose SW, Kamin LJ. Not in Our Genes New York, NY: Random House; 1984.
4. Bouchard TJ, Lykken DT, McGue M, Segal NL, Tellegen A. Sources of human psychological differences: the Minnesota study of twins reared apart. Science. 1990;250:223–228.
5. Avise JC. The Genetic Gods: Evolution and Belief in Human Affairs Cambridge, MA: Harvard University Press; 1998.
6. Burbridge D. Francis Galton on twins, heredity and social class. Br J Hist Sci. 2001;34:323–340.