Twin Research Misperceptions
Dr. Nancy L. Segal
California State University, Fullerton
Dr. Irving I. Gottesman Dr. Eric Turkheimer
University of Minnesota, Minneapolis University of Virginia, Charlottesville
Dr. Nicholas G. Martin Dr. Margaret Gatz
Queensland Institute of Medical Research University of Southern California, Los Angeles
Slate Magazine has recently published a series of twin research articles, some of which are very interesting and informative. However, the one by Brain Palmer entitled "Double Inanity" (http://www.slate.com/id/2301906/) presents only misleading and incorrect information. Colleagues and I responded with a letter to Slate detailing our objections. I believe this dialogue will be of interest to PT readers and have reported it here.
August 27, 2011
Slate Magazine's Brian Palmer has published one of the most inaccurate, misleading and uninformative essays on twin research to come along in a while ("The Methodological Confusion of Twin Studies," August 24, 2011). His work manifests both sloppy journalism and poorly informed scientific opinion. Findings from twin research have made important contributions to our understanding of human development and disease. Twin research has attracted both enthusiastic supporters and harsh critics. However, the various challenges to the validity of twin research designs and their results have been carefully addressed by the scientists responsible for these studies. Despite these efforts, misinterpretations still abound, as evidenced by Palmer's article. He simply repeats the words of twin research's harshest critics, while failing to consult primary sources or rebuttals from the original scientists.
As scientists who have been involved in twin research for decades we will address some of the unfair criticisms that Palmer has put forth as fact. More detailed analyses of these issues appear in a forthcoming book, Born Together-Reared Apart: The Landmark Minnesota Twin Study (Harvard University Press, Spring 2012), a comprehensive overview of the Minnesota Study of Twins Reared Apart, as well as the journal Twin Research and Human Genetics (published since 1952 with various titles), the Institute of Medicine's web page chronicling studies of veteran twins (going back to 1958), the textbook Behavioral Genetics (2008) and the Handbook of Behavior Genetics (2010) to name a few.
• Genes as Destiny. Palmer's opening remark, "One of the main messages of science over the last couple of decades is that genes are destiny"" is a grotesque caricature of genetic research in general and twin studies in particular. Who exactly is supposed to have said this? Indeed, genetic factors have been shown to contribute to individual differences in virtually all measured traits including general intelligence, personality, social attitudes and major mental illnesses. However, the degree of genetic influence varies from trait to trait, the mechanisms of genetic influence are highly complex and dependent on environmental input, and genes alone are never determinative of anything, except perhaps for rare single gene disorders like Huntington's Disease.
• Populations vs. Individuals. Palmer tells us that "half of your altruism [and] one-quarter of your financial decisions" are explained by your genes. This statement represents the most basic error that can be made about inference from twin studies. Genetic and environmental explanations of behavior apply only to differences among individuals. It is meaningful to say that 80% of differences in height among individuals in the modern world are associated with genes, the statement that 80% of a single person's height is explained by genes is completely meaningless. To take a more controversial example, genetic influence on individual differences in general intelligence has been estimated to be about 50% in adulthood, but if an individual person has been severely deprived of nutrition or educational experiences then the environment will count for much more than genetics.
• Genetic Identity of Identical Twins. Identical (monozygotic or MZ) twins result from the early division of a single fertilized egg (zygote). Identical twins are not strictly identical genetically given copy number variations or co-twin difference in DNA segments; Palmer points these out, and they are widely acknowledged in the field. At the same time, Palmer ignores other sources of differences between identical twins, such as unequal prenatal blood supply (twin-to-twin transfusion syndrome), differential prenatal infections, differential X-inactivation between identical female twins, and epigenetic (gene-regulating) events that are also widely recognized by twin researchers. Despite these biological differences, MZ twins (whether reared apart or together) are more alike than any other pair of individuals. Moreover, to the extent that MZ twins are not genetically identical, the true importance of genes is actually underestimated by the MZ-DZ comparison.
• Equal Environments Assumption (EEA). A fundamental assumption of twin research is that environmental factors relevant to traits under study are the same for both types of twins. A common criticism, cited by Palmer is that identical twins receive what he calls "special treatment" from parents, teachers, and others, explaining their greater similarities relative to non-identical twins. Palmer fails to mention that numerous twin studies have examined this challenge and have found it to be wanting in most cases. The landmark 1976 twin study by Loehlin and Nichols showed that identical twins who were dressed alike, or who had the same teachers were not more behaviorally alike than those who did not. Others have found that identical twins whose parents mistakenly thought them to be fraternal twins were not less alike behaviorally than those who were correctly classified as identical. Further, adoption studies, which make a very different set of assumptions, have confirmed the basic findings from twin strategies. And recently, new statistical genetic techniques making use of molecular genetic data in large samples of unrelated individuals (i.e., no twins), have estimated genetic variance for height and IQ consistent with traditional estimates of heritability from twin studies (Yang et al., 2010, Davies et al, (2011).
• Father of the Twin Method. True, the biology of twinning had not been established when Sir Francis Galton conducted the first twin study in 1875. However, he correctly surmised that there were two types of twins, recognizing the value of comparing twins who were "closely alike [in childhood]" with those who were "exceedingly unlike." He stated: "It is, that their history affords means of distinguishing between the effects of tendencies received at birth, and of those that were imposed by the circumstances of their after lives; in other words, between the effects of nature and nurture." He did not draw his conclusion that "nature prevails enormously over nurture" based (as Palmer tells us) on the "incredible similarities he found between twins in 80 questionnaires." Galton obtained information from 80 twin pairs, 35 of which showed close similarity and 20 of which showed much less similarity from an early age. And his studies that shaped his thinking about hereditary contributions to eminence were based not on twins, as Palmer tell us, but on fathers and sons. In any case, Galton's concern with the contest between nature and nurture has not occupied modern scientists for a long time now. Palmer may still be fighting this battle, but we are not. The field recognized long ago that both genes and environment are required for the development of any trait in any organism, and has advanced to deeper questions about how genes and environments combine to create individuals and the differences among us.
It is the hope of those signing this letter (twin researchers representing diverse psychological and medical disciplines), that anyone who reads Palmer's essay will read this one as well. Understanding of a topic as complicated as genetic effects on human behavior must be built on careful study of original materials, not on repetition of the one-sided comments of a handful of critics. The danger lies not in twin studies and what they might tell us about behavior, but rather in feckless editorializing that is not grounded in a thorough understanding of the scientific issues.
Davies et al., (2011). Molecular Psychiatry, 9 August 2011; doi:10.1038/mp.2011.85/
Galton, F. (1875). Journal of the Anthropological Institute 5, 391-406.
Kim (2010). (Ed.) Handbook of Behavior Genetics. NY: Springer
Martin et al. (1997). Nature Genetics 17, 387-392.
McClearn et al., (1997). Science, 276, 1560-1563.
Plomin et al. (2008). Behavioral Genetics. NY: Worth.
Yang et al. (2010), Nature Genetics, 42, 565-571.
My recent book SOMEONE ELSE'S TWIN: THE TRUE STORY OF BABIES SWITCHED AT BIRTH is now available. I hope readers have a chance to take a look. I welcome any comments or questions.