Background: The differential susceptibility hypothesis states that some genetic variants that confer risk in adverse environments are beneficial in normal or nurturing environments. The cholinergic system is promising as a source of susceptibility genes because of its involvement in learning and neural plasticity. The cholinergic receptor gene "CHRNA4" has been linked to characteristics related to the personality traits Neuroticism and Openness/Intellect. Methods: The effects of interaction between "CHRNA4" genotype and maltreatment status on child personality were examined in a well matched sample of 339 maltreated and 275 non-maltreated children (aged 8-13 years). Results: Variation in "CHRNA4" interacted with childhood maltreatment to predict personality in a manner indicating differential susceptibility. The interaction of "CHRNA4" and maltreatment status predicted Neuroticism and Openness/Intellect. Maltreated children with the rs1044396 T/T genotype scored highest on Neuroticism and showed no effect of genotype on Openness/Intellect. Non-maltreated children with this genotype scored lowest on Neuroticism and highest on Openness/Intellect. Conclusion: Variation in "CHRNA4" appears to contribute to personality by affecting degree of developmental sensitivity to both normal and adverse environments. (Contains 3 tables and 1 figure.)