Main symptoms Marked and persistent fear Anxiety response

• Main symptoms: – Marked and persistent fear – Anxiety response – Avoidance – Interferes with everyday activities – Duration 6 months or more • Affects 6 -11% of population (more F) • Lifetime problem in 10% of cases www. psychlotron. org. Phobia

• Biological – Genetics – Nervous system abnormalities • Psychological – Learning theory www. psychlotron. org. Explanations of Phobia

• Compare concordances of MZ & DZ twins Same genes Same environment MZ twins ü ü DZ twins û ü • Greater similarity of MZ twins is assumed to indicate a genetic influence www. psychlotron. org. Genetic Influences

MZ DZ Carey & Gottesman (1981) 13. 0 8. 0 Kendler et al (1992) 25. 9 11. 0 Kendler et al (2001) 15. 9 7. 7 www. psychlotron. org. Genetic Data (Simple Phobia)

www. psychlotron. org. Genetic Data (Simple Phobia) • MZ concordance usually significantly higher than DZ – Suggests genetic influence • MZ concordance always significantly lower than 100 – Indicates environmental influence is also in operation

• Commonly affected by several problems: – Small samples (NB: not always – check) – Unreliable zygosity tests (early studies only) – MZ twins tend to be treated more similarly than DZ, which may account for greater similarity – DZ twins can be M&F, which may be important with sex-differentiated disorders (like simple phobia) www. psychlotron. org. Twin Studies