Factor interpretations were obtained by examining high and low factor loadings of behavioral measures on factors. Significant correlations among factors exists and is expected as no factor rotation procedure was applied, as factor rotation procedures are often employed to identify distinct factors. Results indicate that measures within test batteries load onto multiple factors, thereby supporting the hypothesis that each latent factor is associated with multiple behavioral measures and each behavioral phenotype measure is determined by variation along multiple trait dimensions (Supplementary information, Table S3). Factor 1 has high factor loadings for measures almost entirely from the Cocaine test battery and can be interpreted as injection stress-induced locomotor activity. Factor 2 represents morphine withdrawal measures such as jumps, defecation and urination. Factor 3 represents morphine activity/response. Factor 4 represents reactivity measures such as startle response and anxiety-like measures. Factor 5 predominantly consists of measures pertaining to locomotor activity in a novel environment. Factor 6 contains measures pertaining to conflict avoidance. Factor 7, like Factor 3, consists of measures with high factor loadings pertaining to morphine activity, but instead of response these
