Investigating client response to the active ingredients of motivational interviewing.

In spite of the generally good results produced by motivational interviewing (MI), the precise mechanisms of action associated with MI remain poorly understood and rarely conform to its theoretical model describing behavior change. This study, using data from protocol CTN-0013, “Motivational Enhancement Therapy to Improve Treatment Utilization and Outcome in Pregnant Substance Users,” investigated changes in motivational profiles of pregnant substance abusing women, half receiving MI and half receiving treatment as usual (TAU). Reported elsewhere, no main effect of treatment on later substance use was found between the MI and TAU conditions (Winhusen et al, 2008). One explanation for this finding is that the MI intervention failed to mobilize the intended change processes thought to produce positive outcome.

For this study, a total of 135 pregnant women were administered the 32-item stage of change tool, the URICA, and had complete data for the intake and end of treatment periods (MI: n=62; TAU: n=73). Hierarchical multiple regressions were done to determine if there were differential pre-post relationships between the four URICA scales by treatment group. The direction and magnitude of pre-post changes in precontemplation did not differ between TAU and MI (p < .09; $ combined = .48), and, controlling for intake precontemplation scores, the two groups did not differ in mean posttest precontemplation scores. In contrast, the slope for pre-post contemplation scores was significantly more positive for the TAU group ($ = .71) relative to the MI group, ($ = .49), p < .02, and MI participants reported, on average, lower contemplation scores at the end of treatment, p < .003. Finally, the slopes between pre-post scores for the action ($ = .49) and maintenance ($ = .63) scales did not differ between the TAU and MI clients. Unexpectedly, the TAU group reported significantly higher mean action and maintenance scores relative to the MI group at the end of treatment. Reasons for these differences are unclear but warrant future investigation. Findings offer partial support for the ability of MI to aid in the resolution of ambivalence, here characterized as contemplation.