However, there are several limitations to twin studies that must be considered in assessing the findings, including the assumption that the environment of each twin pair is the same. In summary, twin studies support the role of both genetic and environmental influences on alcohol dependence.
What are the limitations of the classical twin studies?
Despite the facts that the classical twin studies are still being guided by assumptions made back in the 1920s and that the inherent limitation lies in the study design itself, the results suggested by earlier twin studies have often been confirmed by molecular genetic studies later.
What are the advantages and disadvantages of twin studies?
Advantages of twin studies Twin studies allow disentanglement of the shared genetic and environmental factors for the trait of interest. Researchers can estimate the proportion of variance in a trait attributable to genetic variation versus the proportion that is due to shared environment or unshared environment.
What is the beauty of twin studies?
The beauty of twin studies resides in the potential of studying the unobserved by the logic of a design. The aim of this article is to outline central theoretical foundations and possible limitations, and to review selected key findings. We describe the inherent fundamentals of the classic and extended twin designs.
What is the origin of twin studies?
Palmer correctly identifies Francis Galton as the first person to use twin studies as a methodology to seek the heritable components of human behavior. Galton also coined the term “eugenics,” giving twin studies an intellectual and political origin that few modern scientists care to acknowledge.
Why are twin and adoption studies problematic?
Evaluation of Genetic Explanations Twin studies and family studies are seen as problematic since they don't rule out the impact of the environment. The results could be due to social learning since the twin sets and families concerned are all occupying the same environment.
Are twin studies inconclusive?
Abstract. A well-conducted twin study has the potential to evaluate the relative contribution of genetic and environmental factors to a given disease. Many studies are inconclusive because of problems with methodology.
What are the main conclusions of the Minnesota twin studies?
The research team found that identical twins who are reared apart had the same chance of being similar as twins who were raised together. Bouchard and his colleagues concluded that genetic factors have a large influence on behavioral habits demonstrating the influence of the genetics on development.
Are twin studies valid?
Twins are indeed representative of the general population for a wide range of traits and diseases, and available evidence strongly suggests that twin studies do not violate the equal environments assumption.
What is the main problem associated with conducting twin studies?
What is the main problem associated with conducting twin studies? Though it allows us to look at the genetic side, the environmental influences are almost impossible to control.
Do twin studies prove nature or nurture?
The studies of reared-apart twins have shown that in general, half the differences in personality and religiosity are genetically determined, but for a trait like I.Q., about 75 percent of the variation, on average, is genetic, with only 25 percent influenced by the environment.
Why are twin studies reliable?
Advantages of twin studies Twin studies allow disentanglement of the shared genetic and environmental factors for the trait of interest. Researchers can estimate the proportion of variance in a trait attributable to genetic variation versus the proportion that is due to shared environment or unshared environment.
Was the twin study ethical?
“The study was ethically defensible by the standards of its time,” Hoffman and Oppenheim write, though they acknowledge that such a defence will not be satisfactory to the families who live with the study's legacy.
When did the twin study end?
The twins study they were involved with was never completed. The practice of separating twins at birth ended in the state of New York in 1980, shortly after Neubauer's study ceased. Neubauer reportedly had Yale University lock away and seal the study records until 2066.
Why do twin studies lack population validity?
Twin studies often have insufficient statistical power to detect small but non-negligible proportions of shared environmental variance [11], and previous studies may have been limited in their ability to detect shared environmental sources of covariance between CA and substance use problems [8,9].
Are twin studies quantitative?
One of the major methods used in quantitative genetics to estimate genetic and environmental influences is the twin method. This design allows researchers to investigate the causes or influences that affect phenotypes (i.e., their aetiology).
Which is a common criticism of twin studies quizlet?
Which is a common criticism of twin studies? Similarity in environmental experiences, not genetic similarity, may be the more important contributor to the similarity seen in twins. may underestimate the full range of environments experienced by a child.
What makes a study inconclusive?
Inadequate Design Quality: studies do not provide enough information or have significant limitation in the study design (for example, small sample size, threats to internal validity, high attrition rates, etc.)
What is an inconclusive study?
If research or evidence is inconclusive, it has not proved anything. Research has so far proved inconclusive. Synonyms: uncertain, vague, ambiguous, open More Synonyms of inconclusive.
What is true about twin studies?
Twins provide a valuable source of information for health and psychological research, as their unique relationship allows researchers to pull apart and examine genetic and environmental influences. Twin study findings have been influential in detecting and treating various diseases and psychological disorders.
Can there be a hidden twin on an ultrasound?
Ultrasound can tell us a lot about a pregnancy, but it's not always perfect. This is particularly true in the early months of pregnancy. Though it is rare, it is possible to have a "hidden twin" that is not visible during early ultrasound screenings.
Why are identical twins considered monozygotic?
As monozygotic (identical) twins develop from a single egg fertilized by a single sperm, which splits after the egg starts to develop , they are expected to share all of their genes, whereas dizygotic (fraternal) twins share only about 50% of them, which is the same as nontwin siblings.(8) Thus, if any excess similarity is seen between the identical twins when a researcher compares the similarity between sets of identical twins to the similarity between sets of fraternal twins for a trait or condition, then most probably the reason behind this similarity is due to genes rather than environment.
How can twins be analyzed?
The large pool of data related to twins gathered can be analyzed in various ways with the help of new, innovative as well as complex statistical softwares. Twin studies intend to measure the heritability of a trait, which can be determined by concordance rates.
What is the debate about nature versus nurture?
Means of distinguishing between the effects of tendencies received due to genes at birth and those imposed by the different environments they were exposed to during their lives after birth have always been the subject of interest to researchers. The objection to statistical evidence in proof of the inheritance of peculiar traits has always been blamed upon similar environmental conditions playing as a confounder.(1) Twin studies provide a strong basis for exploring the importance of any potential risk factors on a trait or condition by controlling the genetic variations. It has been one of the favorite research tools of behavioral geneticists and psychologists since long, mainly utilized to estimate the heritability of traits and to quantify the effect of a person's shared environment (family) and unique environment (the individual events that shape a life) on a trait.(2)
How many twins are there in Sweden?
There are 1,37414 twins still alive and living in Sweden.(15,16)
What are the assumptions of fraternal twins?
Some assumptions are also made in twin studies; one of them is the assumption of random mating, which assumes that people are as likely to choose partners who are different from themselves as they are to choose partners who are similar for a particular trait. If, instead, people tend to choose mates like themselves, then fraternal twins could share a greater percentage of their genes than expected. In the case of nonrandom mating, fraternal twins would have more genetically influenced traits in common than expected because the genes they receive from their mothers and fathers would be similar to each other. Similarly, the assumption of equal environments is also made, which assumes that fraternal and identical twins raised in the same homes experience similar environments. It is assumed that genes and the environment typically make only separate and distinct contributions to a trait. In general, it is also assumed that only one type of genetic mechanism—usually additive—operates for a particular trait. However, traits can be inherited through different genetic mechanisms. Additive genetic mechanisms mix together the effects of each allele. For example, if genes for curly hair were additive, a curly-haired father and a straight-haired mother might have a child who has wavy hair.(8)
How many twins are there in Denmark?
The Danish Twin Registry is the oldest national twin register in the world, initiated in 1954, and contains information about more than 88,000 twin pairs born in Denmark since 1870, in addition to triplets and quadruplets.(13)
Why are twin studies important?
Therefore, twin studies will continue to inform mankind about the relative importance of genes and the environment on traits in ways that no other type of research ever can.
Is it true that twins are identical?
Twin studies rest on two fundamental assumptions: 1) Monozygotic twins are genetically identical, and 2) the world treats monozygotic and dizygotic twins equivalently (the so-called "equal environments assumption"). The first is demonstrably and absolutely untrue, while the second has never been proven.
Do monozygotic twins share traits?
So if the pairs of monozygotic twins tend to share a trait more often than the pairs of dizygotic twins —be it the likelihood they will vote, a tendency toward altruism, or a strategy for managing their financial portfolios—the difference can be chalked up to genetics.
Who was the first person to use twin studies?
Palmer correctly identifies Francis Galton as the first person to use twin studies as a methodology to seek the heritable components of human behavior.
Who coined the term "eugenics"?
Galton also coined the term “eugenics,” giving twin studies an intellectual and political origin that few modern scientists care to acknowledge. While Galton ’s initial approach to twin studies was surely rudimentary compared to today’s standards, Palmer draws attention to the assumptions embedded in modern twin studies that continue ...
Who critiques the proliferation of scientific findings based on twin studies that claim to isolate the genetic underpinnings of?
Hidden in this bunch is one by Brian Palmer that critiques the proliferation of scientific findings based on twin studies that claim to isolate the genetic underpinnings of human behavior. Palmer notes that in twin studies,
Why are series of twin pairs selected?
Series of twin pairs selected because one (or both) is ill are prone to biassed ascertainment, and great care has to be taken to avoid this. Such bias is absent if the primary source is a twin registry established at birth. In general, series of twin pairs have no advantage over studies on sibs in assessing the size of genetic contribution ...
Who wrote Genetic Factors in the aetiology of Multiple Sclerosis?
Compston A (1986) Genetic factors in the aetiology of multiple sclerosis. In: McDonald WI, Silberberg DH (eds) Multiple sclerosis. Butterworths, London, pp 55–73
What is a GWAS?
Genome-wide association studies (GWASs) seek to understand the relationship between complex phenotype (s) (e.g., height) and up to millions of single-nucleotide polymorphisms (SNPs). Early analyses of GWASs are commonly believed to have "missed" much of the additive genetic variance estimated from correlations between relatives. A more recent method, genome-wide complex trait analysis (GCTA), obtains much higher estimates of heritability using a model of random SNP effects correlated between genotypically similar individuals. GCTA has now been applied to many phenotypes from schizophrenia to scholastic achievement. However, recent studies question GCTA's estimates of heritability. Here, we show that GCTA applied to current SNP data cannot produce reliable or stable estimates of heritability. We show first that GCTA depends sensitively on all singular values of a high-dimensional genetic relatedness matrix (GRM). When the assumptions in GCTA are satisfied exactly, we show that the heritability estimates produced by GCTA will be biased and the standard errors will likely be inaccurate. When the population is stratified, we find that GRMs typically have highly skewed singular values, and we prove that the many small singular values cannot be estimated reliably. Hence, GWAS data are necessarily overfit by GCTA which, as a result, produces high estimates of heritability. We also show that GCTA's heritability estimates are sensitive to the chosen sample and to measurement errors in the phenotype. We illustrate our results using the Framingham dataset. Our analysis suggests that results obtained using GCTA, and the results' qualitative interpretations, should be interpreted with great caution.
How do longitudinal genetic models explain the relationship between genetic and environmental factors?
Conventional longitudinal behavioral genetic models estimate the relative contribution of genetic and environmental factors to stability and change of traits and behaviors. Longitudinal models rarely explain the processes that generate observed differences between genetically and socially related individuals. We propose that exchanges between individuals and their environments (i.e., phenotype–environment effects) can explain the emergence of observed differences over time. Phenotype–environment models, however, would require violation of the independence assumption of standard behavioral genetic models; that is, uncorrelated genetic and environmental factors. We review how specification of phenotype–environment effects contributes to understanding observed changes in genetic variability over time and longitudinal correlations among nonshared environmental factors. We then provide an example using 30 days of positive and negative affect scores from an all-female sample of twins. Results demonstrate that the phenotype–environment effects explain how heritability estimates fluctuate as well as how nonshared environmental factors persist over time. We discuss possible mechanisms underlying change in gene–environment correlation over time, the advantages and challenges of including gene–environment correlation in longitudinal twin models, and recommendations for future research.
How many variants are there in the human genome?
Using genome-wide data from 253,288 individuals, we identified 697 variants at genome-wide significance that together explained one-fifth of the heritability for adult height. By testing different numbers of variants in independent studies, we show that the most strongly associated similar to 2,000, similar to 3,700 and similar to 9,500 SNPs explained similar to 21%, similar to 24% and similar to 29% of phenotypic variance. Furthermore, all common variants together captured 60% of heritability. The 697 variants clustered in 423 loci were enriched for genes, pathways and tissue types known to be involved in growth and together implicated genes and pathways not highlighted in earlier efforts, such as signaling by fibroblast growth factors, WNT/beta-catenin and chondroitin sulfate-related genes. We identified several genes and pathways not previously connected with human skeletal growth, including mTOR, osteoglycin and binding of hyaluronic acid. Our results indicate a genetic architecture for human height that is characterized by a very large but finite number (thousands) of causal variants.
How does cognitive ability affect education?
Cognitive ability and educational success predict positive outcomes across the lifespan, from higher earnings to better health and longevity. The shared positive outcomes associated with cognitive ability and education are emblematic of the strong interconnections between them. Part of the observed associations between cognitive ability and education, as well as their links with wealth, morbidity and mortality, are rooted in genetic variation. The current review evaluates the contribution of decades of behavioural genetic research to our knowledge and understanding of the biological and environmental basis of the association between cognitive ability and education. The evidence reviewed points to a strong genetic basis in their association, observed from middle childhood to old age, which is amplified by environmental experiences. In addition, the strong stability and heritability of educational success are not driven entirely by cognitive ability. This highlights the contribution of other educationally relevant noncognitive characteristics. Considering both cognitive and noncognitive skills as well as their biological and environmental underpinnings will be fundamental in moving towards a comprehensive, evidence-based model of education.
How are language problems related to internalizing problems?
Studies have identified concurrent, longitudinal, and bidirectional associations between language difficulties and internalizing problems. This is commonly explained by social exclusion or withdrawal from peers, but underlying mechanisms are not well understood. This study uses sibling data to investigate if the comorbidity between language difficulties and internalizing problems is best explained by familial factors shared by siblings, such as genes or family environment, or nonfamilial factors specific to each child, such as peer environment. Data include 5,568 siblings at 5 years and 3,654 siblings at 8 years participating in the Norwegian Mother, Father and Child Cohort Study (MoBa). We constructed a latent factor model at 5 and 8 years, including a family comorbidity factor capturing correlations between language and internalizing problems that were equally strong between as within siblings. Results showed that the correlation between one sibling’s internalizing problems and the other sibling’s language problems was mostly accounted for by a family comorbidity factor. The best-fitting longitudinal model included stability of the family comorbidity factor and stability of language and internalizing problems within each sibling and no cross-sibling or cross-trait longitudinal associations. This suggests that the association between language and internalizing problems may be best explained by family factors.
What is the term for the premature fusion of one or more cranial sutures?
Craniosynostosis is one of the pathologic craniofacial disorders and is defined as the premature fusion of one or more cranial (calvarial) sutures. Cranial sutures are fibrous joints consisting of non-ossified mesenchymal cells that play an important role in the development of healthy craniofacial skeletons. Early fusion of these sutures results in incomplete brain development that may lead to complications of several severe medical conditions including seizures, brain damage, mental delay, complex deformities, strabismus, visual and breathing problems. As a congenital disease, craniosynostosis has a heterogeneous origin that can be affected by genetic and epigenetic alterations, teratogens and environmental factors and make the syndrome highly complex. To date, approximately 200 syndromes are correlated to the craniosynostosis. In addition to being part of a syndrome, craniosynostosis can be seen as nonsyndromic formed without any additional anomalies. More than 50 nuclear genes that are relating to the craniosynostosis have been identified. Besides genetic factors, epigenetic factors like microRNAs and mechanical forces also play important roles in suture fusion. Due to craniosynostosis being a multifactorial disorder, evaluating the craniosynostosis syndrome requires and depends on all the information obtained from clinical findings, genetical analysis, epigenetic or environmental factors or gene modulators. In this review, we will focus on embryologic and genetic studies, as well as epigenetic including environmental studies. We will discuss published studies and correlate the findings with unknown aspects of craniofacial disorders.
How many twin studies have been published?
During the last fifty years more than 2700 twin studies have been published, examining the etiology of a high number of traits. Twin studies enable investigation of both genetic and environmental effects, and thereby also examination of causal factors involved in human traits and disorders. The beauty of twin studies resides in the potential of studying the unobserved by the logic of a design. The aim of this article is to outline central theoretical foundations and possible limitations, and to review selected key findings. We describe the inherent fundamentals of the classic and extended twin designs. The logic of the main analytic approaches is outlined, and the principles of univariate biometric modelling described. Next, we review different multivariate models, including the Cholesky, correlated factors, common factor, common pathway and phenotypic causality models. Additionally, the cotwin-control approach, representing a natural experimental design, and mimicking a counterfactual situation, is outlined. Central assumptions, threats and limitations of the twin design are discussed. In particular, we address the issue of missing heritability, non-random mating, the equal environment assumption and gene-environment correlations. Finally, we review some selected findings from the field of behavior genetics and twin studies.
What is the correlation between EEG and twins?
Twin studies of EEG power spectra, in general, and alpha power, in particular, show a striking similar ity (with correlation coefficients > 0.8) of EEG parameters in monozygotic (MZ) twins. Of the different EEG frequency bands, alpha band power (8–13 Hz) shows the highest heritability. Correlations are as strong as the corresponding test–retest correlations within the same individual. Moreover, correlations for a sample of MZ twins reared apart are virtually at the same level as for MZ twins reared together. EEG heritability is illustrated in Fig. 2.2.12 taken from the 2014 review by Andrei Anokhin from Washington University School of Medicine.
How do twin studies inform the genetic mode of transmission of a disease?
Twin studies may also inform the genetic mode of transmission of a disease by inspection of the degree of adherence of the difference in risk between monozygotic and dizygotic twins to the Mendelian ratio of 50%.
What are the twin studies of alcohol dependence?
Overall, twin studies do reveal that the concordance of alcohol dependence is higher in MZ twins than DZ twins, suggesting a role for genetics in alcohol dependence. On average, these studies suggest that between 40 and 60% of variance in drinking can be explained by genetic influences. However, there are several limitations to twin studies that must be considered in assessing the findings, including the assumption that the environment of each twin pair is the same.
How can twin studies provide information on genetic and environmental sources of sex differences in a disease?
Twin studies can also provide information on the genetic and environmental sources of sex differences in a disease by investigating sex-specific concordance rates. Environmental exposures may also be identified by comparing differential exposures among discordant monozygotic twins.
How do twin studies measure resemblance?
Twin studies assess the resemblance of MZ and DZ twins, measured by concordance rates (the degree to which twins share the same discrete trait) or correlation coefficients (the degree to which twins resemble each other for continuously measured phenotypes).
How does the twin model increase statistical power?
Two studies have augmented statistical power by incorporating more than one occasion of measurement into the twin model (Bulik et al. 1998, Wade et al. in press ). This approach controls for unreliability of diagnosis, increases power to detect both a 2 and c 2, and therefore provides the most reliable information regarding the nature and magnitude of genetic and environmental contributions to BN. In short, these two studies reveal a markedly greater contribution of additive genetic effects to the liability to BN (59 and 83 percent, respectively), a negligible contribution of shared environment (0 percent in both studies), and a moderate contribution of unique environmental effects (41 and 17 percent). Although the parameter estimates for c 2 were 0, the confidence intervals did not completely rule out a contribution of shared environment. The results from these two studies confirm the central role of genes in the observed familiality of BN.
What are the findings of twin and adoption studies?
The findings from twin and adoption studies indicate the importance of a genetic factor in the intergenerational transference of addictive behavior, yet like family studies, these imply that other mediating processes are functioning to initiate the transmission of such problems.
