A 4-item suspiciousness scale was derived by factor analysis of the responses of 3,810 Australian twin pairs to the Eysenck Personality Questionnaire. Levels of suspiciousness were significantly higher in males than in females, and in divorced or separated v married individuals. Suspiciousness scores were negatively correlated with age and education, positively correlated with neuroticism and alcohol consumption (males only) and uncorrelated with extraversion. A biometrical genetic analysis was most consistent with a simple model in which genes and individual specific environment affected liability to suspiciousness, with a heritability of 41%. However, sex-specific genetic or shared environmental effects on the liability to suspiciousness could not be ruled out.