Genetic Inheritance & Major Depressive Disorder (DP IB Psychology): Revision Note

Genetic inheritance & MDD

• Major depressive disorder (MDD) is characterised by the following diagnostic criteria:

  • Feeling sad, hopeless, worthless; being more irritable than usual

  • Experiencing less pleasure in formally enjoyable activities

  • Changes in appetite with associated weight loss/gain 

  • Insomnia or sleeping too much

  • Being indecisive and lacking motivation

  • Suicidal ideation, i.e., thoughts of ending one's life

• 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

• Twin studies and adoption studies have been used to determine the likelihood of one person developing a condition such as MDD

• Concordance rates are used to measure the strength of ‘agreement’ between pairs, e.g., if one twin has MDD the concordance rate reflects the likelihood that the other twin also has MDD

• Twin studies are useful ways to study genetic inheritance, as monozygotic twins have identical DNA so high concordance between them may indicate a genetic basis for the behaviour/condition

Research support for genetic inheritance & MDD

McGuffin et al. (1996)

Aim:

• To investigate concordance rates for MDD in MZ and DZ twins

Participants: 

• 177 twin probands who were registered between 1948 and 1986 with the Maudsley Hospital in London as suffering from MDD

• The same-sex twin siblings of each proband were used as comparison participants 

Procedure: 

• Clinicians assessed twins’ emotional states using tests and interviews

• Blind researchers conducted the assessments (unaware if twins were MZ or DZ)

• Researchers were also blind to whether twins had MDD

• Additional data came from twins’ medical records

Results: 

• MZ twins showed a 46% concordance rate for MDD compared to 20% in DZ twins

• When one MZ twin experienced a short period of depression, their co-twin often showed a similar episode

Evaluation of genetic inheritance & 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

Limitations

• 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 identical ways

  • E.g., they may be treated differently by parents, have different friends, or develop different interests — showing that the same genotype can lead to different phenotypes

Link to biological reductionism

•  The genetic explanation of MDD is reductionist, as it overlooks the complex, multi-layered variables that may influence whether one or both twins develop MDD

• This reflects parsimony – choosing the simplest theory to fit the data (i.e., attributing MDD to genes only)

Link to concepts

Perspectives

• Depression is unlikely to stem from purely biological factors

• MDD may also result from environmental and social influences, such as:

  • Learning depressive behaviours from parents or family members

  • Adverse living conditions (e.g., poverty, abuse)

  • Substance misuse or addiction