1. Why Would We Study Twins?
Using correlational research to establish a genetic argument for the origin of human behavior

2. Correlation
Correlations show the relationship between two variables. There is no manipulation of an IV, so cause and effect are not established.
A correlation of +1.0 means as x increases, y increases means that x increases, y decreases.
In twin research, we expect to find a higher correlational than when comparing a child to someone outside of the family.

3. What is inherited?
Which of the following do you think might be inherited? Why?
Intelligence
Depression
Risk-taking behavior
Homosexuality
Problem solving skills
Agoraphobia

4. Techniques
Comparing Monozygotic twins (MZ) with Dizygotic twins (DZ). Since MZ twins have identical DNA, there should be a higher concordance rate.
Comparing MZ twins reared together vs. MZ twins reared apart. Why might this be better than the first technique?

5. ADOPTION STUDIES
Adoption studies tend to compare the adopted child with the biological mother and the adoptive mother to determine which has the higher concordance rate for a given behavior.
Why might adoption studies at times be preferable to twin studies?

6. Prospective Studies
Prospective studies identify traits that are considered to be inherited. Those who are considered “genetically vulnerable” are then followed over many years to see if they actually manifest the behaviour. This is used, for example, in the study of schizophrenia.
There is an ethical concern in such research that it may cause undue stress and lead to iatrogenic effects in those who are labeled as vulnerable.

7. STRENGTHS OF TWIN STUDIES
Twin studies have produced a great deal of data in support of biological roots of disorders; this has helped psychologists to stress prevention for those who are vulnerable to such disorders.
There is a high cross-cultural reliability of concordance levels.

8. Limitations OF TWIN STUDIES
MZ twins are rarely separated at birth and raised in a totally different environment, yet this is really necessary to substantiate claims.
MZ twins reared together share many of the same experiences.
DZ twins reared together may not share the same experiences due to levels of attractiveness or temperament.

9. STRENGTHS OF Adoption STUDIES
Adoption studies allow researchers to isolate variables. If a biological mother has no contact since birth and still matches for a specific trait - for example, the adopted child’s IQ is better than the adoptive mother and in concordance with the birth mother - this is strong evidence that genes play a significant role in the development of this trait.

10. Limitations OF Adoption STUDIES
Selective placement is a problem. Babies tend to placed with families similar in background to the natural parents.
Adopted children – as well as twins - are not representative of all children.
The act of being given up for adoption may affect the child’s behavior.