Biological Approach: Epigenetics & Depression - Twin studies & Caspi

Question 1

What does epigenetics argue?

Answer 1

• in order for a behavior to occur, genes must be “expressed”
• genetic expression is a complex chemical reaction to environmental or physiological changes that allow a gene to “do its job”
• an individual may have a gene that could lead to a behavior, but if the gene is never expressed, then this behavior will not occur

Question 2

What are MZ twins?

Answer 2

• Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell

Question 3

What are DZ twins?

Answer 3

• Dizygotic (DZ) twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle

Question 4

What is meant by “concordance rate?”

Answer 4

• the percentage of pairs of twins or other blood relatives who exhibit a particular trait or disorder
• the concordance rate of MZ twins is never 100% because they share the same genes but may not have been exposed to the same environmental stressors, thus same genes may not be expressed
• there is a lower concordance rate in DZ twins

Question 5

Kendler et al (2006) - aim

Answer 5

• to find out whether depression might be inherited through observing a large sample of both MZ and DZ twins

Question 6

Kendler et al (2006) - procedure

Answer 6

• psychologists argue that there is a genetic component to the behavior if concordance rate is higher in MZ than in DZ twins
• although one may have a certain genetic makeup (genotype), not all of the genes may be expressed
• He predicted that the MZ twins would have a greater concordance rate for depression than DZ twins

Question 7

Kendler et al (2006) - findings

Answer 7

• researchers found that MZ twins had a significantly higher concordance rate than DZ twins
• even though the MZ twins shared the same genotype, their concordance rate was not 100%

Question 8

Kendler et al (2006) - conclusion

Answer 8

• the difference in rates indicates that depression may be inherited
• the study supports the theory of gene expression

Question 9

Kendler et al (2006) - evaluation

Answer 9

strengths:
- the study appears to confirm previous research, strengthening the reliability of the findings
- the very large sample size taken from a single population helps to mediate some of the concerns below

limitations:
- study is correlational, so no cause-and-effect relationship can be determined - no particular genes were isolated and tested in the study
- the interviewers did not officially diagnose the twins and they accepted the diagnoses made by clinicians when it was reported by the interviewee; as we know, clinical diagnoses are not highly reliable, and making such a diagnosis by telephone may be considered questionable validity

Question 10

Caspi et al (2003) - aim

Answer 10

• to determine whether there is evidence for a gene-environment interaction (G x E) for a mutation of the serotonin transporter gene - 5-HTT
• The serotonin transporter is involved in the reuptake of serotonin in brain synapses

Question 11

Caspi et al (2003) - procedure

Answer 11

• a sample of young adults was assessed for mental health on an every-other-year basis until they were 21
• they were divided into 3 groups based on their 5-HTT alleles:
• Group 1 had two short alleles
• Group 2 had one short and one long allele
• Group 3 had two long alleles
• the mutation of the 5-HTT gene has shorter alleles
• participants were asked to fill in a “Stressful life events” questionnaire which asked them about the frequency of different events - including financial, employment, health, and relationship stressors
• they were also assessed for depression

Question 12

Caspi et al (2003) - findings

Answer 12

• people who had inherited one or more short versions of the allele demonstrated more symptoms of depression and suicidal ideation in response to stressful life events
• the effect was strongest for those with 3 or more stressful life events

Question 13

Caspi et al (2003) - conclusion

Answer 13

• the 5-HTT gene could affect vulnerability to depression following stress

Question 14

Caspi et al (2003) - evaluation

Answer 14

strengths:
- theory acknowledges the interaction between both biological and environmental factors in depression - a more holistic approach, not reductionist

limitations:
- study is correlational, so no cause-and-effect relationship can be determined
- study makes the assumption that serotonin causes depression
- information about life events was self-reported - it may be the salience of the negative life events which plays a role in depression - that is, those that recalled them more easily may have a tendency towards depression
- those who are more resilient, may not recall negative life events as easily
- later studies have not been able to show similar results - it appears that the study has low reliability
- some participants who did not carry the gene mutation became depressed; therefore, we cannot say that gene expression alone can cause depression