For P3 amplitude, there was a significant main effect of externalizing (χ2 (1; N = 275) = 5.54, p = .019), wherein higher EXT was associated with significantly smaller P3 amplitude. The main effect of number of standards was significant, χ2 (3; N = 275) = 51.29, p < .0001. The greater the number of standards preceding the target, the larger the resulting P3 amplitude, with P3 increasing 1.5 μV as the number of preceding standards varied from 1 to 4 (the respective P3 amplitudes were 16.2, 16.9, 17.4, and 17.7 μV). A linear contrast of means was highly significant (p < .0001), whereas a quadratic contrast was not (p = .613). There was no interaction between EXT and number of standards. Higher EXT was associated with smaller P3 amplitude, and this relationship remained regardless of the number of standards preceding the target. Figure 1 illustrates this effect by comparing average ERPs for participants in the upper and lower deciles of the distribution of EXT factor scores, collapsing over “early” occurring targets (those preceded by 1 or 2 standards) and “late” targets (those preceded by 3 or 4 standards).
