During the habituation phase, habituation was demonstrated by a decremental SCR with stimulus repetition from the first 2 presentations (mean, 0.38 microsiemens) to the last 2 presentations (mean, 0.23 microsiemens) (t342 = 15.48, P<.001). Fear conditioning was confirmed by significantly higher average response to the reinforced (CS+) than to the nonreinforced (CS−) stimuli (t342 = 16.66, P<.001). Supporting extinction, response discrimination between the reinforced (CS+) and nonreinforced (CS−) stimuli persisted (t342 =12.54, P<.001), but to a lesser extent compared with acquisition. The MZ twins did not differ from DZ twins in average response magnitudes for any of the experimental measures. In a separate sample, temporal stability in 28 subjects and internal consistencies in 223 subjects had been investigated.13 For the 5 measures of habituation, acquisition CS−, acquisition CS+, extinction CS−, and extinction CS+, the measured internal consistencies were 0.93, 0.91, 0.96, 0.90, and 0.93 and 1-month test-retest reliabilities were 0.72, 0.57, 0.85, 0.37, and 0.62, respectively.