ABSTRACTSkin conductance responses to a series of loud tones were measured bilaterally in 121 pairs of adult twins, 53 pairs reared together and 68 pairs reared apart. Subjects were given an absorbing task on which to focus their attention and instructed to ignore the meaningless tones. Rangecorrection eliminated hand and sex differences in SCR amplitude and also the correlation with age of this variable. For the combined group of 79 pairs of monozygotic (MZ) twins, the within‐pair correlations for Y‐intercept, the slope of the habituation curve, and the number of trials to habituation were near the limits of retest reliability for these variables. After range‐correction, the correlation for Y‐intercept for the 42 pairs of dizygotic (DZ) twins was about half the MZ value, suggesting that initial electrodermal reactivity is strongly genetic and can be interpreted according to a polygenic‐additive model. Biometric model testing indicated that stable individual differences in uncorrected SCR amplitude and in habituation slope are primarily determined by non‐additive genetic factors. About 40% of the total variance (and most of the stable variance) in number of trials to habituation is genetically determined. Co‐twins of MZ twins who were electrodermal nonresponders tended also to be hyporesponsive while co‐twins of DZ nonresponders tended to be