The Biological Theory of Addiction (AQA GCSE Psychology): Revision Note

The influence of nature: hereditary factors & genetic vulnerability

• Biological explanations of addiction take the ‘nature’ side of the nature/nurture debate which assumes that human behaviour is the result of inherited traits

• A genetic explanation of addiction assumes that the genes a person inherits are responsible for their addiction (hereditary)

  • Variations or adaptations in some genes may affect the likelihood of someone becoming addicted to a substance i.e. there is no ‘one gene’ that codes for addiction

  • If someone inherits a combination of genes that make them more susceptible to addiction then - given certain conditions - addiction may follow

  • A genetic vulnerability increases a person’s risk of developing addiction as explained in the diathesis-stress model

• Twin studies have been used to determine the likelihood of one person developing an addiction to a substance if their twin is also addicted 

• As monozygotic (MZ) twins have identical DNA they are often used for research into genetic vulnerability

  • MZ twins are compared to dizygotic twins (DZ) who share 50% of their DNA to measure the extent to which particular traits/behaviours may be inherited

  • If there is a high concordance rate for the specific behaviour (e.g. addiction) between the MZ twins then this is used as evidence for a biological basis to behaviour

Kaij’s twin study of alcohol abuse

• Kaij (1960) conducted the first systematic study to investigate whether alcoholism was the result of genes or environment (i.e. nature or nurture) in twins

• Aim: Is there a genetic vulnerability for alcoholism (alcohol addiction) i.e. is alcoholism inherited?

• Sample: 

  • 310 male twins from Skåne, Sweden were identified where at least one twin was registered with the Swedish temperance board for alcohol problems

  • This gave a total of 384 individual twin participants once co-twins were included — the numbers don't match because in some pairs both twins were registered, in others only one was

  • 48 pairs were MZ; 126 pairs were DZ

• Procedure: 

  • Kaij interviewed the registered twins, and in some cases their close relatives, to establish drinking habits and whether each pair was MZ or DZ

    • There was no DNA testing available in the 1960s, so zygosity relied on this self-report

• Results: 61% of MZ twins shared an addiction to alcohol compared to 39% for DZ twins

• Conclusion:

  • This supports a genetic vulnerability to alcoholism rather than pure heredity

  • If alcoholism were entirely genetic, we'd expect 100% concordance in MZ twins, not 61%

  • Separately, alcoholic twins were overrepresented for social problems, suggesting environmental factors also contribute

Evaluation of Kaij’s twin study of alcohol abuse

Strengths

• Kaij's findings have been supported by later, better-controlled research: Kendler et al. (1997) used a larger sample (2,516 individual twins from the Swedish temperance register) and again found co-twins were more likely to also be alcoholic if MZ (48%) than if DZ (33%)

• Prescott et al. (2005) further supports the view that genetic factors have a major influence on alcoholism in both males and females

Weaknesses

• The study didn't actually measure "alcoholism", as temperance board registration only captured people arrested for public drunkenness, drink-driving, or alcohol-related crime

  • Therefore, many real alcoholics without a public incident would be missing from the data

• Zygosity (MZ vs DZ) was determined by self-report from the twins or a family informant, not DNA testing

  • Therefore classifications may have been inaccurate, which reduces the validity of the findings

• Biological explanations of addiction like this one can be misunderstood as deterministic

  • Inheriting certain genes doesn't make addiction inevitable, it increases vulnerability

    • The evidence shows both nature and nurture matter, so these explanations risk overstating how influential genes actually are