Behavioral Genetics Class Week 2: Twin Studies

This is the summary of Week Two of my Behavioral Genetics class.  This week, the instructor discussed the types of experiments used to investigate genetic and environmental contributions to individual differences:

Twin Studies
- The main type of experiment used is twin studies.
- Twin studies work well because they provide two excellent naturally occuring test groups.  Monozygotic twins ("MZ") share 100% of their DNA and also have a (roughly / arguably) 100% similar upbringing.  Dizygotic twins ("DZ") share 50% of their DNA (like siblings) and have a 100% similar upbringing.  Comparing and contrasting these two groups can help show whether traits are more influenced by genetics or environment.
- Occasionally, MZ twins are reared apart, so in this case they share 100% of their DNA and (again, roughly / arguably) 0% of their upbringing, so all physical and psychological similarities between twins reared apart are due to genetics.
- The findings from twin studies are:
  - MZ twins are almost always more similar than DZ twins, even when reared-apart.
  - For many (but not all!) traits, reared-apart twins are not much less similar than reared-together twins.  (The most notable exceptional trait is IQ, where environment seems to play a bigger role.)
  - MZ twins are never perfectly similar (so, while the first two points show that genetics plays a big role, this shows that environment also plays a role).

Adoption Studies
- A second type of study used is adoption studies.  Psychologists compare adopted siblings (not genetically related, but raised in the same home).  Also, psychologists compare adopted children to their birth parents and also to their adoptive parents.  The findings:
- In the adopted sibling studies:  There is very little similarity in personality (ie emotionality, happiness, etc.) between adopted siblings.  The correlation is not zero, but less than the correlation with twins or biological siblings.
- In the parent studies:  Adopted individuals tend to resemble both sets of parents, but they resemble their birth parents more than their adoptive parents (for psychological traits).