Biological Explanations of Major Depressive Disorder (MDD) (SL IB Psychology)

• All biological explanations of MDD take the ‘nature’ side of the nature/nurture debate which assumes that human behaviour is the result of innate, inherited traits
• A genetic explanation of MDD may be found by investigating rates of heritability within families i.e. the extent to which genes may be responsible for someone developing MDD
• Historically, twin studies and adoption studies have been used to determine the likelihood of one person developing a condition such as MDD if their twin or their biological parent also has the condition

• Concordance rates are used to measure the strength of ‘agreement’ between pairs e.g. McGuffin et al. (1996) found high concordance rates for MDD between monozygotic (MZ) twins compared to dizygotic (DZ) twins (see the revision note on Two Key Studies of Genetic Similarity on this site)

• Polymorphism in the 5-HTT gene may be another genetic explanation for MDD (Fratelli et al. 2020) as it is thought that variations or adaptations in this gene may affect the likelihood of someone becoming depressed (this explanation also crosses over into the neurochemical explanation - see below)

Evaluation of the genetic explanation of MDD

Strengths

• Twin and adoption studies tend to use large samples, generating robust quantitative data which increases the reliability of the findings
• Concordance rates provide clear data which is easy to compare and analyse, highlighting patterns and tendencies e.g. higher concordance rates of depression between MZ twins

Weaknesses

• If the genetic explanation was completely valid then concordance between MZ twins should be 100% but this has not been found in any research study
• The influence of the environment cannot be ruled out as even MZ twins will not experience life in exactly identical ways e.g. they may be treated slightly differently by their parents; they may have different friends or develop different interests i.e. identical genotype; different phenotypes

The genetic explanation suggests that if one of them develops MDD then the other one is likely to develop the disorder as well.

• All biological explanations of MDD take the ‘nature’ side of the nature/nurture debate which assumes that human behaviour is the result of innate, inherited traits
• A neurochemical explanation of MDD may be found by investigating neurotransmitters and their effect on behaviour
• Polymorphism in the 5-HTT gene also provides a neurochemical explanation for MDD as genetic variation in this gene has been linked to the monoamine hypothesis

• Monoamines are chemicals containing amino acids which transport serotonin and other neurotransmitters around the brain 
• Serotonin is implicated in mood disorders such as MDD i.e. low or irregular levels of serotonin have been linked to depressive symptoms
• Evidence to support the monoamine hypothesis can be found in the action of Selective Serotonin Reuptake Inhibitors (SSRIs) which work to prevent the reabsorption of serotonin into the neuron, ensuring that these extra serotonin molecules cross the synaptic gap to the post-synaptic neuron
• SSRIs are widely prescribed for depressive disorders highlighting a clear link between their use and the monoamine hypothesis i.e. serotonin may play a role in the onset and experience of MDD

Evaluation of the neurochemical explanation of MDD

Strengths

• There is clear research support for the monoamine hypothesis (Barchus & Altemus, 1999) based largely on the role of SSRIs in treating depression
• If the monoamine hypothesis is valid then this is good news as drug treatment in the form of SSRIs is cheap and widely available for clinicians to prescribe

Weaknesses

• The monoamine hypothesis is still being debated as a valid explanation for MDD: some increasing levels of GABA can help to alleviate symptoms of depression (Goldberg et al. 2014)
• Antidepressants such as SRRIs do not work for everyone; research has found that only one third of patients responded positively to SSRI treatment (Trivedi et al. 2006)

Which studies investigate a biological explanation of MDD?

• McGuffin et al. (1996): genetic explanation - concordance rates for MDD are higher for MZ twins than for DZ twins

• Caspi et al. (2003): neurochemical explanation - a genetic variant of the 5-HTT serotonin gene combined with stressful life events may be implicated in MDD 

Both McGuffin et al. (1996) and Caspi et al. (2003) are available as Two Key Studies of Biological Explanations of MDD – just navigate the Explanations for Disorders section to find them.