Infants differ in HPA axis response to stress (i.e., increasers vs. decreasers).
COMTval158met polymorphism affects stress response, but no data exist on infants.
Infants were exposed twice to a double-exposure Still-Face procedure.
Met-carrier infants had higher negative emotionality to repeated stress exposure.
Among increasers, met-carriers had higher cortisol response than val-homozygotes.