Brief Note The Use of Discordant MZ Twins to Generate Hypotheses regarding Non-shared Environmental Influence on Anxiety in Middle Childhood Kathryn Asbury, Judy Dunn and Robert Plomin, King’s College Abstract Non-shared environmental influence (NSE) has been found to account for around 50 percent of anxiety variance, but specific NSE factors have not been identified. Discor- dant monozygotic (MZ) twins can be used to generate relevant hypotheses because MZ discordance can be caused by NSE, but not by genes or shared environment. Of 1590 MZ pairs teacher rated for anxiety at the age of seven, we selected the most discordant 19 pairs and conducted interviews with them and their mothers. In explain- ing discordant anxiety, the mothers emphasised negative school experiences, compari- sons within the twin relationship, and illness and accidents. During the maternal interviews neonatal life events, parent–child relationships and peer rejection also emerged as potential NSE sources. Keywords: monozygotic twin differences; anxiety; non-shared environment; qualitative interviews The monozygotic (MZ) twin differences method provides researchers with invaluable insight into the environment, because differences between MZ twins cannot be explained by genes or a shared environment. Such differences can only be explained by non-shared environmental (NSE) influence or measurement error, which is included within the NSE parameter in behavioural genetics analyses. For this reason, identifying within-pair differences in experience for behaviourally discordant MZ twins is a useful way of identifying environmental influences on behaviour. The MZ differences method has been used successfully in three previous quantitative analyses with large samples (Asbury, Dunn, Pike & Plomin, 2003; Caspi et al., 2004; Pike, Reiss, Hetherington & Plomin, 1996). All three articles found significant relationships between parent- reported parenting and parent-reported child behaviour. Non-shared environment is important to anxiety across the lifespan, showing greater influence than either shared environment or genes at all ages. In childhood and Correspondence should be addressed to Kathryn Asbury, Box P080, Social, Genetic, & Develop- mental Psychiatry Centre, Institute of Psychiatry, De Crespigny Park, London, SE5 8AF, UK. Email: spjgkaa@iop.kcl.ac.uk © Blackwell Publishing Ltd. 2006. Published by Blackwell Publishing, 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.