Anorexia nervosa: biological explanations

Genetics

Anorexia runs in families: family history of eating disorders is a risk factor (NHS). Twin studies compare MZ and DZ twins.

Holland et al. (1984) studied 34 twin pairs and one set of triplets (St George's and Maudsley Hospitals). Among the 30 female twin pairs, 9 of 16 MZ pairs (56.3%) and 1 of 14 DZ pairs (7.1%) were concordant for anorexia. The higher MZ concordance suggests a genetic contribution.

Serotonin

Serotonin is linked to mood, anxiety and appetite. Bailer and Kaye (2011) reviewed findings:

• When ill, people with anorexia have low levels of the serotonin metabolite 5-HIAA, which may partly reflect low dietary tryptophan (the building block of serotonin).
• After recovery, levels are raised, which suggests a possible underlying high serotonin activity linked to anxiety and perfectionism.
• Dieting lowers tryptophan; Kaye et al. (2003) found tryptophan depletion reduced anxiety in ill and recovered patients but not controls.

The proposed (speculative) model: restricting food lowers serotonin and eases anxiety, which may help explain why some people persist in starvation.

Evaluation

• Twin evidence supports genes, but samples are small and twins share environments.
• Concordance is not 100%, so environment matters (diathesis-stress).
• Cause or effect? Starvation itself changes brain chemistry, so serotonin differences when ill may be effects; studying recovered patients helps but cannot fully rule this out.
• Reductionist: ignores cultural pressure to be thin (Becker et al., 2002).
• Application: SSRIs (which act on serotonin) have not been shown to prevent relapse (Walsh et al., 2006), which weakens a simple serotonin account.

Worked example

Holland et al. found 9 of 16 MZ pairs and 1 of 14 DZ pairs concordant. Calculate the concordance rates and explain what they suggest.

1. MZ: 9 / 16 x 100 = 56.25% (about 56.3%).
2. DZ: 1 / 14 x 100 = 7.1%.
3. MZ twins share all their genes and DZ twins about half, so the much higher MZ rate suggests genes play a role.
4. But concordance is far below 100% for MZ twins, so environmental factors also matter.

Answer: 56.3% vs 7.1%: evidence of a genetic contribution, but not full genetic determination.

Check yourself

1. In Holland et al. (1984), 9 of 16 MZ twin pairs were concordant. What is the concordance rate? Give your answer to 1 decimal place.
   Show answer

   56.3 %. 9 / 16 x 100 = 56.25, which is 56.3% to 1 decimal place.

2. 1 of 14 DZ twin pairs were concordant. What is the concordance rate? Give your answer to 1 decimal place.
   Show answer

   7.1 %. 1 / 14 x 100 = 7.1%.

3. Why might restricting food reduce anxiety, according to the serotonin explanation?
   • Dieting raises dopamine
   • Dieting lowers tryptophan, reducing serotonin, which may ease anxiety
   • Restricting food increases cortisol
   • Hunger blocks NMDA receptors
   Show answer

   Dieting lowers tryptophan, reducing serotonin, which may ease anxiety. Tryptophan is needed to make serotonin.

4. Why do researchers study people who have recovered from anorexia?
   • Because ill patients cannot consent
   • Because recovered people have no genes
   • To see whether chemical differences remain once starvation is no longer affecting the brain
   • To measure their BMI
   Show answer

   To see whether chemical differences remain once starvation is no longer affecting the brain. Starvation changes brain chemistry, so recovered patients help separate cause from effect.

5. What did Bailer and Kaye report about 5-HIAA in recovered patients?
   • It was raised
   • It was low
   • It was absent
   • It was unchanged from when ill
   Show answer

   It was raised. Raised after recovery, low when ill.

6. Explain one weakness of the genetic explanation of anorexia nervosa.
   Show answer

   MZ concordance is not 100%, so genes cannot fully explain anorexia; environmental factors such as cultural pressure to be thin must also play a role.. Incomplete concordance points to environmental influence.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Psychology specification (issue January 2026)
• Pearson IAL Psychology component guide: WPS04 (Issue 2, November 2024)
• Holland, Hall, Murray, Russell and Crisp (1984), Anorexia nervosa: a study of 34 twin pairs and one set of triplets, British Journal of Psychiatry (abstract)
• Bailer and Kaye (2011), Serotonin: imaging findings in eating disorders, Current Topics in Behavioral Neurosciences
• NHS: Anorexia nervosa, overview
• Walsh et al. (2006), Fluoxetine after weight restoration in anorexia nervosa: a randomized controlled trial, JAMA 295

Last updated 8 November 2026. Spotted a mistake? Tell us.