Twin studies have become an important tool for understanding the roles of genes and environment in human traits, behaviors and diseases. By comparing monozygotic (identical) and dizygotic (fraternal) twins, researchers can estimate the relative contributions of genetic and environmental factors. The first documented twin study was published in 1875 by Francis Galton, who was interested in the heritability of human intelligence. However, the origins of twin research can be traced back much further.
Early Documented Cases of Twin Studies
Though Galton published the first systematic twin study, there are accounts of twins being studied for research purposes as early as the 11th century. Here are some key early examples:
- 11th century – Arab physician Avicenna observed differences between identical and fraternal twins and concluded that environment also played a role in development.
- 16th century – Italian physiologist Girolamo Cardano studied twins and noted that identical twins shared more similarities than fraternal twins.
- 17th century – German anatomist Johann Sperl studied two stillborn twins and concluded they were identical due to sharing the same placenta and membranes.
While these early accounts documented observations of twins, Galton’s 1875 study was the first to systematically analyze twins to draw conclusions about the inheritance of human traits.
Francis Galton’s 1875 Twin Study
In 1875, British scientist Francis Galton published the paper “The history of twins, as a criterion of the relative powers of nature and nurture” in the journal Fraser’s Magazine. This pioneering study was the first rigorous scientific examination of twins to determine the roles of heredity and environment in shaping human characteristics.
Galton examined 80 pairs of twins, targeting those who had been separately raised from birth. His main findings were:
- Identical twins had a much higher correlation in measured traits than fraternal twins, suggesting genetics were very influential.
- Even identical twins raised apart showed striking similarities, indicating heredity’s importance over environment.
- For traits like height and forearm length, genetics appeared to account for 89% and 96% of variance, respectively.
Based on these observations, Galton concluded that human abilities and traits were largely inherited rather than shaped by environment. His study served as a catalyst for widespread interest in twin research to explore heritability.
Galton’s Methods and Data
Galton gathered his data by sending out detailed questionnaires to families and schools requesting information on twin similarities and differences. He received completed surveys for 80 pairs of twins, categorized as:
- 39 pairs living together
- 7 pairs living apart
- 34 presumably identical pairs
- 46 presumably fraternal pairs
The questionnaires asked about various metrics including:
- Height
- Eye color
- Complexion
- Visual acuity
- Temperament
- Moral feelings
- Intellectual capabilities
Galton primarily relied on analysis of correlations in identical versus fraternal pairs to draw conclusions about heritability. While limited by today’s standards, Galton’s methods were pioneering for his time.
Impact and Criticisms
Galton’s twin study had a profound influence on scientific thinking about heredity and spurred great interest in twin research over the next few decades. However, his conclusions were also controversial and critiqued by some. Key impacts and criticisms included:
- Catalyzed a surge of twin studies in the late 19th and early 20th centuries.
- Influenced debate between Mendelians and biometricians over importance of discrete units of inheritance.
- Criticized for overstating heritability and underplaying effect of environment.
- Small sample size and reliance on subjective questionnaires.
- Did not distinguish identical and fraternal twins conclusively.
While flawed in some respects, Galton’s work firmly established twin studies as a tool to analyze nature vs. nurture and shaped the foundation of behavioral genetics.
The Surge of Twin Research (1875-1924)
In the 50 years following Galton’s publication, there was an explosion of twin studies seeking to quantify the inheritance of mental and physical traits. Some key twin studies during this pioneering era include:
Siemens (1924)
In 1924, German psychiatrist Hermann Siemens published a influential twin study comparing 80 pairs of identical and fraternal twins on tests of intelligence and personality. Siemens found a much higher correlation in identical twins and estimated a heritability of 89-95% for intellectual ability. His meticulous methodology helped improve on weaknesses in Galton’s original study.
Merriman (1924)
In 1924, American psychologist C. Merriman examined 78 pairs of twins to analyze the roles of heredity and environment in personality traits such as initiative, social presence, and cheerfulness. His results suggested personality was strongly inherited.
Newman, Freeman & Holzinger (1937)
In a landmark 1937 study, American psychologists H.H. Newman, F.N. Freeman and K.J. Holzinger analyzed IQ correlations in 19 pairs of identical twins and 57 fraternal pairs. They concluded that IQ was highly heritable, with genetics accounting for approximately 80% of variance.
Burt (1966)
British psychologist Cyril Burt conducted highly influential twin studies in the 1920s-1960s examining traits like intelligence and personality. His research claimed that IQ was strongly determined by genetics, but was highly controversial even in his time.
This surge of twin studies produced a wealth of data supporting Galton’s original thesis that human traits were predominantly inherited. However, the studies of this era also had limitations such as small samples and questionable assumptions about twin zygosity. More rigorous twin research developed in the mid 20th century.
The Modern Era of Twin Studies
In the 1950s, scientific understanding of human genetics advanced rapidly with the discovery of DNA’s structure and the chromosomal basis for inheritance. This catalyzed more sophisticated and rigorous twin studies with improved methodologies. Major developments included:
- Larger sample sizes enabled higher statistical power.
- Objective blood marker analysis allowed reliable identification of identical and fraternal pairs.
- Increasingly precise measurement of phenotypes.
- Sophisticated modeling of gene-environment interplay.
Some key twin studies of the modern era include:
Shields (1962)
In 1962, American psychologist J. Shields studied 44 pairs of identical twins and 31 fraternal pairs using blood typing to conclusively assign zygosity. He found a 0.85 correlation for IQ in identicals versus 0.60 in fraternals, supporting high heritability.
Bouchard (1990)
In 1990, American psychologist Thomas Bouchard published his landmark “Minnesota Twin Study,” examining over 100 traits in identical and fraternal twins raised apart. He found all traits were substantially heritable, with genetics strongly influencing even social attitudes.
Plomin (2013)
In 2013, British behavioral geneticist Robert Plomin reviewed decades of twin research in his book “G is for Genes.” He argued that genetics were foundational for all major psychological traits, accounting for 35-50% of variance in outcomes.
van Dongen (2012)
A 2012 meta-analysis by Dutch researcher van Dongen aggregated 7,905 twin pairs across 39 studies of occupational interests. Genetics accounted for 49-54% of variance in vocational interests.
Thanks to these and many other rigorous modern twin studies, we now have a detailed understanding of the strong influence genetics exert on human differences in traits ranging from intelligence to personality to interests and attitudes. Twin research continues advancing our knowledge of human inheritance.
Conclusion
In summary, the origins of twin studies in research can be traced back centuries, but Galton’s pioneering 1875 study marks the genesis of modern twin research. His landmark paper catalyzed a surge of twin studies from 1875-1945 that confirmed the predominant role of inheritance in shaping human traits. In the 1950s, advances in genetics enabled more sophisticated twin studies with improved methodologies, larger samples and reliable zygosity testing. These rigorous modern studies have firmly established the substantial heritability of nearly all psychological and behavioral human traits, while also elucidating the complex interplay between genes and environment. After over 140 years, twin research remains an invaluable tool for unraveling the nature of human differences.