Biological Approach: Epigenetics & Depression - Twin studies & Caspi Flashcards
What does epigenetics argue?
- 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
What are MZ twins?
- Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell
What are DZ twins?
- 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
What is meant by “concordance rate?”
- 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
Kendler et al (2006) - aim
- to find out whether depression might be inherited through observing a large sample of both MZ and DZ twins
Kendler et al (2006) - procedure
- 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
Kendler et al (2006) - findings
- 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%
Kendler et al (2006) - conclusion
- the difference in rates indicates that depression may be inherited
- the study supports the theory of gene expression
Kendler et al (2006) - evaluation
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
Caspi et al (2003) - aim
- 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
Caspi et al (2003) - procedure
- 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
Caspi et al (2003) - findings
- 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
Caspi et al (2003) - conclusion
- the 5-HTT gene could affect vulnerability to depression following stress
Caspi et al (2003) - evaluation
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