Twins And Genetics Essay Research Paper Behavioral

Twins And Genetics Essay, Research Paper

Behavioral genetics is a field of research that investigates the relative effects of heredity and environment on behavior and ability (Plomin, 1997). Two of the primary methods used by behavioral geneticists are the twin study method, first used by Galton (1975) in his studies of heredity, and the adoption method.

In the twin study method, researchers studies identical twins (monozygotic twins) and fraternal twins (dizygotic twins) to determine how much they resemble each other on a variety of characteristics. Identical twins have exactly the same genes because a single sperm cell of the father fertilizers a single egg of the mother, forming a cell that then splits and forms two human beings-“carbon copies.” But fraternal twins are no more alike genetically than any two siblings born in the same parents. In the case of fraternal twins, two separate sperm cells fertilize two separate eggs that happen to be released at the same time during ovulation.

Twins, who are raised together, whether identical or fraternal, have similar environments. If identical twins raised together are found to be more alike than fraternal twins on a certain trait, then that trait is assumed to be more influenced by heredity. But if identical twins and fraternal twins from similar environments do not differ on a trait, then that trait is assumed to be influenced more by environment.

In the adopting method, behavioral geneticists study children adopted shortly after birth. By comparing their abilities and personality traits to those o their adoptive family members with whom they live and those of their biological parents whom they may have met, researchers can disentangle the effect of heredity and environment (Plomin et al., 1988).

Adoptive research has assembled the Minnesota Twin Registry, which in 1998 included over 20,000 twin pairs (Bouchard, 1998).

Probably the best way to assess the relative contributions of heredity and environment is to study identical twins that have been separated at birth and raised apart. Although it seems amazing, researchers have found that identical twins that are brought up in the same family are no more alike as adults that are identical twins who are reared apart. When separated twins are found to have strikingly similar traits, it is assumed that heredity has been a major contributor to those traits heredity, and the adoption method.

One of the most extensive investigation of twins raised in separate homes is the Minnesota Study of Twins reared apart, which over the past 20 years has studied hundreds of twin pairs who were separated early in life (Bouchard, 1994; Finkel et al., 1995). This study, like others of its kind, has consistently found such striking psychological and behavioral similarities between monozygotic twins that the important role of genes in personality development can no longer be denied.

Typical is the case of Oskar Stohr and Jack Yufe, identical twins born of a Jewish father and Christian mother in Trinidad in the 1930s. Soon after their birth, Oskar was taken to Nazi Germany by his mother to be raised as a Catholic in a household consisting mostly of women. Jack was raised as a Jew by his father, spending his childhood in the Caribbean and some of his adolescence in Israel.

On the face of it, it would be difficult to imagine two more disparate cultural backgrounds. And when the twins were reunited in middle age, they certainly had their differences. Oskar was married and a devoted union member; Jack was divorced and the owner of a store in southern California. But when the brothers met for the first time in Minnesota,

Similarities started cropping up as soon as Oskar arrived at the airport. Both were wearing wire-rimmed glasses and mustaches, both sported two-pocket shirts with epaulet. They share idiosyncrasies galore: they like spicy foods and sweet liqueurs, are absentminded, have a habit of falling asleep in front of the television, think it’s funny to sneeze in a crowd of strangers, flush the toilet before using it, store rubber bands on their wrists, read magazines from back to front, dip buttered toast in their coffee. Oskar is domineering toward women and yells at his wife, which Jack did before he was separated. [Holden, 1980]

Their scores on several psychological tests were very similar, and they struck the investigator as remarkably similar in temperament and tempo.

Other pairs of monozygotic twins in the study likewise startled the observers with their similarities, not only in appearance and in test scores but also in mannerisms and dress. One pair of twins, raised in rather serious homes, giggled at almost everything. In fact, when they were interviewed, it was hard to gather information because every comment triggered peals of laughter. Another set of female twins, separated since infancy, arrived in Minnesota each wearing seven rings (on the same fingers) and three bracelets, a coincidence that likely was partly genetic. Their genes endowed both women with beautiful hands. But, it is quite possible that they both have genetic interest in attractive objects and in self-decoration. Some people like the feel of rings, bracelets, and such against their skin, while others do not-for reasons that could be related to the sense of touch, itself genetically influenced.

Case after case in this study has produced similar findings of surprising “coincidences,” suggesting that genes affect a much greater number of characteristics than was previously suspected by most psychologists, including the leader of the Minnesota study, Tomas Bouchard.

Bouchard now concludes that genetic variation is significant for “almost every behavioral trait so far investigated, from reaction time t religiosity” (Bouchard et al., 1994).

Many researchers are astonished at the similarities they find in monozygotic twins raised separately (Lykken et al., 1992). Their findings make is wonder anew about the sources of our own individuality.

Probably the best way to assess the relative contributions of heredity and environment to personality is to study identical twins that have been separated at birth and reared apart. When identical twins that were reared apart have strikingly similar traits, as in the case of Oskar and Jack, it is assumed that heredity has been a major contributor.

In the Minnesota twin study, Tellegen and others (1988) found that the identical twins are quite similar on several personality factors regardless of whether they are raised together or apart. Heritability refers to the degree to which a characteristic is estimated to be influenced by hereditary. After studying heritability of trait in 573 adult twin pairs, Rushton and colleagues (1986) found that nurturance, empathy, and assertiveness are substantially influenced by heredity. Even altruism upbringing, were actually more heavily influenced by heredity. A meta-analysis by Miles and Carey (1997) revealed that the heritability of aggressiveness might be as high as .50 (or 50%).

Twin studies have also revealed a genetic influence on social attitudes such as traditionalism-whether a person endorses traditional moral values and follows rules and authority (Finkel & McGue, 1997). The risk of divorce appears to have a heritability of .55 for women and .59 for men. This is not to say that there is a gene for divorce; rather, the risk derives from a genetic influence on certain personality factors (Jocklin et al., 1996). Genes also

influence personality factors that correlate with psychological disorders as measured by the Minnesota Multiphasic Personality Inventory (DiLalla et al., 1996). There even seems to be

a genetic influence on people’s sense of well-being (Lykken & Tellegen, 1996), their interests (Lykken et al., 1993), how they tend to view their environment (Chipuer et al., 1993), and how they perceive life events, partially controllable ones (Plomin & Rende, 1991).

Evidence from behavioral genetics suggests that the average heritability of the Big Five personality factors is about .41 to .42 somewhat less than the earlier twin study estimates of around .50 (Bouchard, 1994). It shows the heritability estimates for the Big Five from the Minnesota studies of twins reared apart.

An interesting study that I can across was the study that Michael Lyons and his colleagues (1995) conducted. This study was regarding antisocial behavior among members of the Vietnam Era Twin Registry. The individuals in the study were about 8, 000 twins who served in the military from 1965 to 1975. The investigators found that among monozygotic twins there was a greater degree of resemblance for antisocial traits than among dizygotic twins. The difference was greater for adult antisocial behavior than for juvenile antisocial behavior. The researchers concluded that the family environment was a stronger influence than genetic factors on juvenile antisocial traits, and that antisocial behavior adulthood is more strongly influenced by genetic factors. In other words, after the individual grew up and left his family origin, early environmental influences mattered less and less.

I found this research very fascinating due to the fact that I am an identical twin. I definitely agree that even if Anna (my twin) was separated from me, our genetic makeup is just too powerful that we would definitely be pretty much the same people we are today.

Twins and Genetics


November 1st, 2000



Bouchard, Tomas(1998). Sources of human psychological differences: The Minnesota study of twins reared apart. Science, 250, 223-228.

Bouchard, Tomas (1994). Genes, environment, and personality. Science, 164, 329-334.

Finkel, D. (1995). Heritability of cognitive abilities in adult twins: Comparison of Minnesota and Swedish data. Behavior Genetics, 25, 421-432.

Galton, F. (1975). The history of twins as a criterion of the relative powers of nature and nurture. Journal of the Royal Anthropological institute, 5, 391-406. [8]

Holden, Constance. (1980). Identical twins reared apart. Science, 207, 1324-1328.

Lykken, D.T (1992). Emergenesis: genetics traits that may not run in famioies. American Psychologist, 47, 1496- 1502.

Plomin, Robert. (1997). Molecular genetics and psychology. Current Directions in Psychology Science, 4, 114-117.

Plomin, Robert. (1988). Enviorment aznd genes: Determinants of behavior. American Psychology, 44, 105-111. [8,13]


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