An updated review of 34 human observational studies indicates that the length polymorphism of the serotonin transporter gene moderates the effect of environmental adversity in the development of depression. This finding depends on the use of contextual or objective methods to assess environmental adversity and is attenuated when self-report instruments are used. Inconsistent findings in male adolescents suggest a developmental stage and sex-specific protective mechanism. These systematic relationships between method and results should be followed up to specify causal mechanisms leading to depression.