Paper 1 - Biological Approach - Genes and Behavior (depression)

Major depressive disorder (depression)

Mental disorder characterized by low mood and low energy levels.

Genes

The unit of hereditary, a portion of DNA which codes for a specific function or trait.

-> we inherit 50% of our genes from each parent.

-> these genes don't code for complex behaviors (e.g. intelligence) but may be the building blocks of these behaviors.

Genotype

the set of genes responsible for a particular trait

Phenotype

refers to the observable characteristics (e.g. height and hair color)

Gene expression

the process by which specific genes are activated to produce a required protein.

Epigenetics

Epigenetics argues that for a complex behavior to occur, gene expression must occur, which is a complex chemical reaction to environmental or physiological changes that allow a gene to 'do it's job'.

Diathesis-stress model

Used to explain the origin of behavior.

Argues that depression may be the result of a genetic predeposition and environmental stress.

Predicts that when an individual w/ certain genes, is exposed to a stressful environment, they are more likely to develop depression than someone who does not have those genes.

Link between genes and behavior

There is no single cause and effect relationship between genes and behaviors.

5HTT gene

The 5HTT gene plays a role in the serotonin pathways scientists believe are involved in controlling mood, emotions, aggression, sleep and anxiety.

It recycles serotonin from the synaptic cleft to the presynaptic neuron, terminating its effects and enabling its reuse.

People who inherit two short versions of the 5HTT gene are more likely to develop major depression after a stressful life event.

Capsi et al (2003) aim

To investigate whether a functional change in the 5HTT gene is linked to a higher or lower risk of depression in an individual.

Capsi method

Opportunity sample of 850 26 year old ppts who were part of another longitudinal study. They were split into 3 groups, depending on their 5HTT transporter gene (regulates serotonin).

- Group 1: 2 short alleles

- Group 2: 1 short and 1 long allele.

- Group 3: 2 long alleles

The study assessed stressful life events between the 21st and 26th birthday.

Past-year depression was assessed using the Diagnostic Interview Schedule.

A correlation was tested for between stressful life events and depression, between the length of alleles and depression.

Capsi results

Ppts with the two short alleles on the 5HTT gene reported more symptoms of depression in response to stressful life events than either of the two groups.

Capsi conclusion

The presence of 2 short alleles of the 5HTT gene increases vulnerability to depression when exposed to stressful life events, suggesting a gene-environment interaction in the development of depression.

Capsi evaluation

+ it's longitudinal design. Capsi et al followed ppts from birth to adulthood, collecting data on genetic factors and life events. This allows the researchers to assess the correlational relationship between depression and genes.

- While the stressful life events were standardized (employment, health etc), whether or not a ppt experienced a certain event as highly stressful is personal. This is because stress is subjective and results in low construct validity.

Kendler et al aim

to determine the role of genetics in major depression disorder

Kendler method

sample of 15000 twin pairs listed in the national Swedish twin registry were used.

carried out interviews assessing lifetime major depression using modified DSM-IV criteria.

they also asked questions about the twins' shared environment when they were living together, and their individual specific environment, which included any adult personal life events that could make a member of the twin pair more susceptible to depression.

Kendler results

the results indicated the concordance rates for major depression were significantly higher in MZ twins than DZ twins.

the estimated heritability of depression was 38%

Kendler conclusion

this study shows depression is somewhat heritable, but that environment also plays a role, as the concordance rates and heritability was less than 100%

Kendler evaluation

- MZ and DZ twins may not grow up in equally similar environments. Twin studies are based on the equal environment assumption, which assumes that both MZ and DZ twins share roughly the same environment. However, this may not always be the case. If the environment is more alike for MZ twins than DZ twins, then we can't be sure genes are solely responsible for the higher MZ concordance rate.

+ Twin studies offer high genetic control. Twins, especially MZ pairs, offers an almost unparalleled control over genetic variables. This makes it easier to hone in on the effects of environmental differences on behavior.

Critical thinking/evaluation of genes and behavior

+ Research into genes and behavior has real-life implications. By mapping the human genome and epigenome, we have increased our understanding of the importance of environmental influences of behavior, through the regulation of gene expression. This could have applications in the treatment and prevention of psychiatric disorders.

- Genetic arguments are correlations. No studies yet have manipulated the IV of genes to see its effect on behaviors like depression. This means no cause-and-effect relationship can be established between genes and behavior.

Cognitive approach and depression

Argues depression is caused by irrational thinking

-> negative triad: tendency for depressed individuals to have negative thoughts about themselves, the world and the future.

-> negative schemas: individuals perceive themselves and their experiences in a negative light.