pondelok 21. januára 2013

Twin Studies


                         Minnesota Study of Twins Reared Apart


What is this Twin Study about:?

 Aim: In 1979, T.J. Bouchard began to study twins who were separated at birth and raised in different families. The Minnesota Twin Study of Adult Development began in 1986 to identify what causes individual differences in aging. Study of identical (MZ) and fraternal (DZ) twins allows for estimation of how genes and environment affect the aging process.

Example: One example of the amazing similarity of twins reared apart is the so-called “Jim twins”. These twins were adopted at the age of four weeks. Both of the adopting couples, unknown to each other, named their son James. Upon reunion of the twins when they were 39 years old, Jim and Jim have learned that:
  • Both twins are married to women named Betty and divorced from women named Linda.
  • One has named his first son James Alan while the other named his first son James Allan.
  • Both twins have an adopted brother whose name is Larry.
  • Both named their pet dog "Toy."
  • Both had some law-enforcement training and had been a part-time deputy sheriff in Ohio.
  • Each did poorly in spelling and well in math.
  • Each did carpentry, mechanical drawing, and block lettering.
  • Each vacation in Florida in the same three-block-long beach area.
  • Both twins began suffering from tension headaches at eighteen, gained ten pounds at the same time, and are six feet tall and 180 pounds.

What were the findings?                                                             

1.Genetic factors appear to influence personality, mental, and activity-level changes as adults become older.
 2. Maintaining an active lifestyle will contribute to more successful aging
 3. Continuing to engage in intellectual activities will help adults retain cognitive functioning as they age;
 4. Keeping an active social life will contribute to stronger feelings of happiness and well being.


 Participants of Study?

Participants of Study were twins born in Minnesota from 1936 to 1955 to be used for psychological research. Recently, it has added twins born between 1961 and 1964. It primarily conducts personality and interests tests with its 8,000+ twin pairs and family members via mail.


An identical twin reared away from his or her co-twin seems to have about an equal chance of being similar to the co-twin in terms of personality, interests, and attitudes as one who has been reared with his or her co-twin.  The differences between twins reared apart must be due totally to the environment, and given that these twins are just as similar as twins reared together, we can conclude that the environment, rather than making twins alike, makes them different.

Ethical Issues?

Scientists make genetics too important. For example they believe when the twins have many similarities in their behavior it's because of genetics. But it can be effect of something else such as coincidence. Twins are not a random sample of the population, and they differ in their developmental environment. 
Twins differ very little from non-twin siblings. Measured studies on the personality and intelligence of twins suggest that they have scores on these traits very similar to those of non-twins. Twin studies are thus in part motivated by an attempt to take advantage of the random assortment of genes between members of a family to help understand these correlations.
While the twin study tells us only how genes and families affect behavior within the observed range of environments, and with the caveat that often genes and environments will covary, this is argued to be a considerable advance over the alternative, which is no knowledge of the different roles of genes and environment whatsoever.Twin studies are therefore often used as a method of controlling at least one part of this observed variance

                                Adoption Study

Aim: To determine the contribution of environmental and genetic factors to the poor performance of black children on IQ tests as compared to white children. The initial study was published in 1976 by Sandra Scarr and Richard A. Weinberg.
The Minnesota Transracial Adoption Study tried to answer whether the gap is primarily caused by genetic factors or whether it is primarily caused by environmental and cultural factors.

Example: Children who are socially identified as black may still be subject to racial discrimination despite being raised by white parents. Yet, it was previously known that adoption into upper-middle class white families has a positive influence on the IQ and school performance of white children.
The study showed significant differences in adoption patterns of mixed-race black/white and black adopted children.

Participants Of Study?

Scarr and Weinberg studied black, white, Asian, indigenous American, and mixed-race black/white children adopted by upper-middle-class white families in Minnesota.

 Findings Of Study?

 The average IQ of the adopting parents was more than one standard deviation above the population mean of 100. The biological children of these parents were also tested. The sample of adopted children was selected by eligible parents contacting the researchers for participating following a newsletter call. The geographical origin of the adopted children was not uniform. All except one white adopted child was adopted in-state. Black and interracial children came from twelve states and Asian and indigenous American children came from Minnesota as well as from Korea, Vietnam, Canada and Ecuador.


 On measures of cognitive ability (IQ tests) and school performance, black children in the U.S. perform worse than white children. The gap in average performance between the two groups of children is approximately one standard deviation, which is equivalent to about 15 IQ points or 4 grade levels at high school graduation. The average IQ score of black children in the U.S. is approximately 85, compared to the average score of white children of 100. 

Ethical Issues?

There have been conflicting findings concerning whether adopted adolescents have more psychological and behavior problems than non-adoptees. Brodzinsky, Schecter, Braff, & Singer study in 1984, Brodzinsky, 1990; Brodzinsky, 1993 studies found mean differences between adoptees and non-adoptees.
These studies suggest that as a group, adopted adolescents are more likely to be maladjusted than their non-adopted counterparts.
The Minnesota Transracial Adoption Study provided the opportunity to explore differences in psychosocial adjustment among transracial adoptees, Caucasian adoptees, and their non-adopted siblings during late adolescence and early adulthood, a potentially vulnerable point in their development. The use of a sample not drawn from clinical populations resulted in a more representative group than those in most investigations of the adjustment of adopted adolescents.
 In general, the birth offspring had the fewest reports of significant adjustment problems, and the adopted adolescents were reported as more likely to have experienced poor adjustment. The findings of the current study are consistent with the literature showing that adoptees, including adolescents, are at greater risk than non-adoptees for externalizing behavior problems, learning disabilities, school adjustment problems, and delinquent behavior. Shireman in 1988 had found that parents reported more serious academic and behavior problems among Black transracial adoptees. However, this difference was due primarily to a larger number of reports of learning disabilities for these adoptees.

Sources: http://www.personal.kent.edu/~mvandul/Weinbergetal2005.pdf

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