The debate of nature versus nurture is known since antiquity. The close resemblance of twins has been the subject of many works of fiction as well. 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. 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.
The similarity between twins has been a source of curiosity since time immemorial. The idea of using twins to study the heritability of traits can be traced back to the British researcher Sir Francis Galton. Based on the similarities he found between twins from 80 questionnaires, Galton proudly announced his conclusion to the world that nature soundly beats nurture, though his sample was too small and consisted of all upper-class individals, without any control group.
The first reported classical twin study was a study performed by Walter Jablonski in , investigating the contribution of heredity to refraction in human eyes. Jablonski examined the eyes of 52 twin pairs and by comparing the size of within-pair differences between identical and nonidentical twins was able to infer the heritability of a trait. Even later, in , Thomas J.
Bouchard, Jr. Segal at the University of Minnesota conducted one of the most famous research studies on genetic influence in humans. They studied identical twins separated since birth and raised by different families adoption studies , and so assumed that similarities, if found any, must be those that are heavily influenced by a person's genetic heritage.
The study was invoked by the sensational news reports of two identical twins reunited after a lifetime apart. James Lewis and James Springer were separated 4 weeks after birth and each infant was taken in by a different adoptive family.
When they were reunited at the age of 39, an extraordinary collection of coincidences emerged. Both had second marriages with women named Betty. Both had police training and worked part-time with law enforcement agencies. Both had childhood pets named Toy. They had identical drinking and smoking patterns, and both chewed their fingernails to the nub. Furthermore, identical twins reared apart were eerily similar to identical twins reared together in various measures of personality, personal mannerisms, expressive social behavior, and occupational and leisure-time interests.
However, they did not find outstanding similarities between identical twins on measures such as standardized personality tests. Still, Bouchard's findings can be interpreted as strong support for genetic influences on personality.
Bouchard's data set was unique and probably a one-time event in history because modern adoption agencies no longer break up sets of identical twins. The modern-day classical twin study design relies on studying twins raised in the same family environments, which provides control not only for genetic background but also for shared environment in early life.
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.
There can be variations in the classical model, which may sometimes provide an added advantage, for example if twins are followed up over longer duration of time in longitudinal manner to assess the development of adult-onset traits and conditions.
This slight deviation will allow for a more complete and accurate assessment of environmental factors over time. Similarly, on combining with molecular genetics, information about the presence or absence of specific genetic variants to determine the impact on the trait of interest can be explored. The advances in molecular genetics have substantiated hypotheses generated by the traditional twin research design by pinpointing the effects of a particular gene.
Depending on the objectives of the study, one may need only monozygotic or dizygotic twins, or a combination of the two. A twin registry is a database of information about both identical twins and fraternal twins, which is often maintained on a country-wide level or by an academic institution, such as a university or other research institution.
Registration of some twin registries are mandatory by law, for example Norway, where all births of twins since have been registered by the Norwegian government. The twin registry in some countries have also made extensive outreach efforts, for example examining hospital birth records and then making multiple follow-up efforts such as in-person visits to find the twins and have them agree to be registered.
The Danish Twin Registry is the oldest national twin register in the world, initiated in , and contains information about more than 88, twin pairs born in Denmark since , in addition to triplets and quadruplets. The Danish Twin Registry is used as a source for studies on genetic influence on normal variation in clinical parameters associated with clinical studies of specific diseases, the metabolic syndrome and cardiovascular diseases, and aging and age-related health problems.
In all cohorts the ascertainment has been population-based and independent of the traits studied, although different procedures of ascertainment have been employed. The Swedish Twin Registry STR , managed by the Karolinska Institute, is the largest population-based twin registry in the world containing approximately 1,70, twins in 85, pairs born There are 1, 37 twins still alive and living in Sweden. It is a unique resource for clinical, epidemiological, and genetic studies.
Information has been mainly collected for demographic, medical, and lifestyle characteristics, with special attention to general health, cardiovascular and respiratory disease, legal drug use, and dietary and psychosocial conditions. It is currently in the final phase of a complete telephone interview screening of all twins born in or earlier regardless of gender composition or vital status of the pair.
The famous Minnesota Twin Registry is a registry of all twins born in Minnesota from to and from to ; it was started in Both studies include over individuals comprising twins, siblings, and parents. The MTFS began in , when it enrolled 1, pairs of identical and same-sex fraternal twins and their families from the upper Midwest. Twins were identified through public birth records and invited to participate with their parents in a full-day intake assessment.
SIBS is a study of adoptive as well as biological sibling and their parents. It is one of the largest studies of adolescents and their families ever conducted.
It is comprised of a volunteer cohort of 14, twins 7, pairs and sets of triplets, and a population-based cohort of 19, 9, pairs twins and 89 sets of triplets. Though there are many small-scale twin studies published in various journals related to metabolic syndromes, 19 , 20 , 21 cardiovascular diseases, 22 respiratory diseases, 23 cerebrovascular diseases, 24 epilepsy, 25 , 26 dermatology, 27 , 28 ophthalmology, 29 , 30 psychology 31 chromosomal disorders, 32 and dentistry, 33 , 34 , 35 , 36 among others, there exists no twin registry in India to documenting the details of twins borne.
In addition, there is no provision of any law for mandatory twin registration. There are many practical problems associated with registering twins borne, one of the important concerns being the large number of home deliveries. In a country where recording the birth weight of every newborn is not yet possible, mandatory twin registration may prove a distant dream for the already overburdened health-work force.
Still, outreach activities can be planned to register the twins. Apex medical institutes and tertiary care centers can take the initiative to maintain and analyze data regarding twins in their areas to find out various genetic as well as environmental confounders in various diseases. 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. Concordance rate CR for a disease or trait among identical and fraternal twin pairs is actually a statistical measure of probability: If one twin has a specific trait or condition, what is the probability that the other twin has or will develop that same trait or disease?
When MZ concordances are greater than DZ concordances, genetic influences are indicated. Quantitative genetic analyses and heritability estimation, including comparisons of concordances or intraclass correlations and structural equation modelling, can also be used to investigate the relative importance of genetic and environmental influences on a particular trait or condition.
Linear structural equations and fit models over all types of twins can be used to describe the causes of variation in a phenotype. Structural equation modelling of data can provide further refinement in the results. The total variance in the trait can be partitioned into genetic variance, common environmental variance including shared familial environmental variance, and unique environmental variance. In order to estimate the parameters of interest, the equation for the twins is written and the parameters studied.
Heritability, the relative importance of genetic influences for variation in a trait, is defined as genetic variance divided by the total phenotypic variance. Twin research has proliferated in recent years. This is largely because the power of the twin method for understanding the origin and development of human traits has become increasingly appreciated by investigators representing diverse fields. Twinning rates have also increased dramatically since , especially the rate of fraternal twinning as a consequence of fertility treatments.
There have been stunning advances in quantitative mathematical methodology that continue to increase the value of twin studies. Lastly, there have been enormous developments in the molecular genetics and genomics fields with respect to associating genes posing increased risks for specific behaviors and disease. Twins will continue to play a prominent role in these endeavors.
The sources presented in this article represent a wide range of areas and topics within twin research. General overviews of the field, both historical and current, are provided, as well as a listing of special collections in twin research, that is, books and journals focusing on a particular topic or theme and web addresses. The largest section includes topics reflecting the widening range of psychological, biological, and medical traits that have been examined via twin research methods.
The section on twin-based perspectives provides sources treating unusual twin-related topics. Twin research has had a successful yet controversial past, a trend that has continued through the present. Despite the wealth of information that has been derived from twin studies, various methodological and interpretive aspects continue to be questioned. The historical roots of twin studies, its acceptance into the mainstream of psychological and medical research, and its challenges are documented in a number of books, articles, and essays.
The resources in this section span a wide range of twin-related topics. The five books are appropriate for experienced investigators and new scientists, as well as general audiences searching for information about the many ways twins are used in scientific studies. Johnson, et al. The selections here include general overviews of the biological and psychological aspects of twinship Scheinfeld , the nature-nurture debates Wright , overviews of unusual topics in the study of twins Segal , and cultural issues Stewart , Piontelli An older, but still informative account of the biology and psychology of twinning is also provided Bryan Classical twin studies and beyond.
Nature Reviews Genetics — DOI: Describes and documents the potential of large twin registries to study complex human traits. Discusses various twin research designs e. Includes lists of twin registers in and outside European countries. Bryan, Elizabeth. The nature and nurture of twins. A comprehensive examination of biological and psychological aspects of twinning by a British physician.
Includes helpful information on twin types, twinning rates, and related topics. Also includes some specific topics not covered elsewhere, such as twin loss and twins with special needs. Gottesman, and Thomas J. Bouchard Jr. Beyond heritability: Twin studies in behavioral research.
Current Directions in Psychological Science — The scientists conducted their experiment at the University of Minnesota, in Minneapolis, Minnesota. The researchers physiologically and psychologically assessed monozygotic twins or triplets who were reared apart, comparing the similarity of those twins to twins who were reared together.
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. The research team, led by Bouchard, included David T. Segal, and Auke Tellegen.
Bouchard, Lykken, McGue, Segal, and Tellegen studied different aspects of psychology and behavioral genetics. Many of those scientists had previously conducted twin studies. Often, twins are a subjet of interest to scientists because identical or monozygotic twins share all of their genes , which allows for the control of genetic differences that is otherwise difficult to achieve with non-identical individuals.
Bouchard and his colleagues conducted their study to determine the origin of individual differences in ability, personality, interest, and social attitudes. As the researchers mention in their article, they studied monozygotic twins who were separated early in life and raised apart, because they provided a way to separate the influence of environmental and genetic factors on human characteristics. Monozygotic or identical twins occur when a single egg is fertilized and forms one zygote that later separates into two embryos.
In contrast, dizygotic twins occur when multiple fertilized eggs are implanted in the uterus at the same time and develop into separate embryos. According to the Genetic Science Learning Center of the University of Utah in Salt Lake City, Utah, the strength of twin studies arises from the fact that monozygotic twins share all of their genes , while dizygotic twins on average share about 50 percent of them. Because of that fact, the authors conclude that individual differences between monozygotic twins who are reared apart should only be caused by the influence of the environment they live in.
The occurrence of twins being reared apart is rare, but provides an effective way to compare the genetic and environmental influences on human characteristics. The authors break the paper into eight distinct sections. In the introduction, the authors start by describing the assessments the participants performed during the study and give a summary of their procedure, results and interpretations.
The authors also detail their methods, data, and interpretations in different sections. In the first section, the authors describe the formula they use to correlate the effects of genetic and environmental factors on twins. In the following section, the authors discuss the details and results of multiple tests and assessments that the twins performed during the study.
In the third section, the authors explore the effects of similarities in the rearing environment on the IQ results of monozygotic co-twins reared apart. In the fourth section, the authors explore the correlation between monozygotic co-twins reared apart on their IQ results. In the fifth section, the authors discuss various similarities between monozygotic twins.
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