Frontal P2 amplitude also reflected effects of gene– environment interactions. The effect of alcohol abuse, as measured with RAPI at age 18, was modified significantly by genetic influences on the P2 amplitude (β = −3.46, p < 0.05, SE = 1.44). At the same time, P2 amplitude correlated negatively with RAPI. Similar effects were found for variation in P2 frontal amplitude: the measure of high-density drinking MAX-D24 was significantly associated with variation in P2 amplitude and the association between alcohol use and variation in frontal P2 amplitude was modified significantly by genetic influences on frontal P2 amplitude (β = −0.39, p < 0.05, SE = 0.18).
