Twin studies5/30/2023 ![]() ![]() 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.( 6 7) Still, Bouchard's findings can be interpreted as strong support for genetic influences on personality. However, they did not find outstanding similarities between identical twins on measures such as standardized personality tests. 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. Their firstborn sons were named James Alan Lewis and James Allan Springer.( 5) Bouchard and Segal reported that about 70% of the variance in intelligence quotient (IQ) found in their particular sample of identical twins was found to be associated with genetic variation. They had identical drinking and smoking patterns, and both chewed their fingernails to the nub. ![]() Both had police training and worked part-time with law enforcement agencies. Both had second marriages with women named Betty. Both of the “Jim twins” had married and divorced women named Linda. When they were reunited at the age of 39, an extraordinary collection of coincidences emerged. James Lewis and James Springer were separated 4 weeks after birth and each infant was taken in by a different adoptive family. The study was invoked by the sensational news reports of two identical twins reunited after a lifetime apart. 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. Segal) at the University of Minnesota conducted one of the most famous research studies on genetic influence in humans. ![]() and his colleagues (including esteemed twin researcher Nancy L. 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.( 4)Įven later, in 1990, Thomas J. The first reported classical twin study was a study performed by Walter Jablonski in 1922, investigating the contribution of heredity to refraction in human eyes. After nearly five decades, in the 1920s researchers “perfected' Galton's methods by comparing identical and fraternal twins and inferring heritability from the differences between the two.( 3) 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. His pioneering work The History of Twins in 1875 inspired much debate by suggesting that England's “chief men of genius” were the product more of good breeding (nature) than of good rearing (nurture). The idea of using twins to study the heritability of traits can be traced back to the British researcher Sir Francis Galton. The similarity between twins has been a source of curiosity since time immemorial. 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) Evolution of Twin Studies 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. 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 close resemblance of twins has been the subject of many works of fiction as well. The debate of nature versus nurture is known since antiquity. ![]()
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