- To find out whether any similarities among relatives are due to the environment
or heredity, the two effects are separated and studied
1) Biologically related relatives who do not live in the same household
2) Adoptive relatives who are not biologically related
- Pairs of relatives are used in the study; two brothers, a mother a daughter,
opposite sexed twins.
- They fill out a personality questionnaire, or observer reports are done.
- The results are studied all together and compared. The variance is determined and
the researcher checks to see if variance is due to WITHIN FAMILY variation or
BETWEEN FAMILY variation. This is done through a technique called ANOVA
• Intraclass correlation coefficient: to figure out how similar relatives are,
researchers calculate the proportion of variance due to between-family
- Researchers often use twins to study the nature vs. Nurture debate
- When they are comparing the variance in a trait between two non identical twins,
non twin siblings, or mother and child, then they double the correlation found by
2, because they only share 50% of their genes. This doubling is not done with
identical twins because they share 100% of their genes.
- Even after the doubling, the correlation because non identical twins is usually less
than that of identical twins
• Additive genetic effect: genes working together contribute to personality
traits. Each gene’s effect will either lower or higher the level of a trait. The
combined effect of the genes can be estimated by adding together their
• Non-additive/multiplicative genetic effect: for example let’s say that genes a
and b are very rare, so if a person has BOTH genes, then their level of a
certain trait will be very high, but if they only have a or b, then the level will
not be even remotely high. Both genes need to be present for a increasing
effect to take place.
- Comparing similarity of difference kinds of relatives: study the variations in traits
between identical and fraternal twins raised together.
- Ex: correlation between identical twins was 0.7 and for fraternal twins it was 0.4.
Therefore 70-40=30. Multiply 30 x 2 because of the 50% genetic difference.
Therefore there is 60% genetic influence on a trait. REVIEW PAGE 120.
- There are two types of environments that people living in the same household
• Shared environment: everything that happens in the house that everyone
• Non shared or unique: these are difference experiences that are unique to the
individual. Their separate set of friends, the unique way they are treated by
their parents, etc.
- When the researcher wants to measure the variance in traits as an affect of the
shared environment, they will use pairs of relatives who are not biologically