Exp Brain Res (2011) 211:601–612 Prime Group had a signiﬁcant effect on ‘priming sentence error score’, we employed a 2 9 2 ANOVA with between- subjects factors Age Group and Prime Group. There was no signiﬁcant interaction between Age and Prime Group (F(1,86) = 0.25, P = 0.62) and no main effect of Age Group (F(1,86) = 0.42, P = 0.52) or Prime Group (F(1,86) = 0.47, P = 0.50). Hence, all groups exhibited equivalent per- formance on the Priming task. Imitation and effector priming tasks Error analysis Error rates (deﬁned as incorrect responses and reaction times less than 150 ms, and greater than 2,000 ms) were low in both the Imitation Task (7.9% of trials) and the Effector Priming Task (6.5% of trials). For the Imitation and Effector Priming tasks, errors on Incompatible trials are of theoretical relevance for the current study as they indicate a failure to inhibit responding on the basis of automatic imitation and effector priming, respectively. Therefore, errors on Incompatible trials were entered into a 2 9 2 ANOVA with factors of Age Group (Adult, Ado- lescence) and Prime Group (Pro-social, Non-social). For the Imitation Task, this ANOVA revealed a signiﬁcant main effect of Age Group (F(1,83) = 10.0, P = 0.002); however, the main effect of Prime Group and the interac- tion between Age and Prime Group were not signiﬁcant (both F(1,83) \ 1). The main effect of Age reﬂects an increased tendency to imitate in the Adolescent Group (Adolescent Group mean error rate 14.0% of Incompatible Trials, Adult error rate 7.3% of Incompatible trials) that was not affected by the Prime Group factor. The same ANOVA performed on the Effector Priming Task revealed a marginally signiﬁcant effect of Age Group F(1,83) = 3.9, P = 0.051) but the main effect of Prime Group and the Prime x Age Group interaction was not signiﬁcant (both F(1,83) \ 1). The main effect of Age reﬂects an increased tendency to respond on the basis of effector compatibility in the Adolescent Group (Adolescent Group mean error rate 9.8% of Incompatible Trials, Adult error rate 5.1% of 607 Incompatible trials) that was not affected by the Prime Group factor. RT analysis The magnitude of the Imitation Effect was calculated, for each participant, by subtracting RT on compatible trials from RT on incompatible Imitation task trials. Similarly, an index of Effector Priming Effect was calculated, for each participant, by subtracting RT in compatible trials from RT in incompatible trials from the Effector Priming Control Task. Table 1 shows the Imitation and Effector Priming Effects for each Prime Group in each Age Group. Differential effects of pro-social/non-social priming on imitation in adolescents compared to adults Paired samples t tests on compatible and incompatible RTs for Imitation trials demonstrated that each of the four groups exhibited a signiﬁcant Imitation Effect (Adolescent Non- social Group incompatible trials mean RT (SEM) = 547.87 (23.50), compatible trials mean RT (SEM) = 497.26 (18.05), t(16) = 4.22, P \ 0.001, 1-tailed; Adolescent Pro- social Group incomp mean RT (SEM) = 516.53 (21.49), comp mean RT (SEM) = 478.55 (17.01), t(16) = 4.12, P \ 0.001, 1-tailed; Adult Non-social Group incomp mean RT (SEM) = 508.89 (14.35), comp mean RT (SEM) = 470.91 (12.90), t(27) = 5.28, P \ 0.001, 1-tailed; Adult Pro-social Group incomp mean RT (SEM) = 573.29 (29.76), comp mean RT (SEM) = 502.18 (22.13), t(27) = 6.03, P \ 0.001, 1-tailed). That is, for each group, responses were faster when observing a lift of the same ﬁnger than when observing a lift of a different ﬁnger. To examine whether social priming differentially affects adolescents and adults, we conducted a 2 9 2 ANOVA with between-sub- jects factors Age Group (Adolescent, Adult) and Prime Group (Pro-social, Non-social). The ANOVA revealed a signiﬁcant interaction between Age and Prime Group (F(1,86) = 4.61, P = 0.02, 1-tailed). There was no main effect of Age Group (F(1,86) = 0.93, P = 0.34 or Prime Group (F(1,86) = 0.93, P = 0.34). Post hoc independent Table 1 Imitation and Effector Priming Effects for each Prime Group for each Age Group Mean Imitation Effect (SEM) Mean Effector Priming Effect (SEM) Adolescent Non-social 50.62 (11.99) 38.55 (8.02) Pro-social 37.98 (9.23) 35.23 (9.25) Adult Non-social 37.98 (7.19) 49.44 (8.28) The Imitation Effect is the RT difference between incompatible (e.g. Participant lifts index ﬁnger and watches a video of a middle ﬁnger lift) and compatible (e.g. Participant lifts index ﬁnger and watches an index ﬁnger lift) trials in the Imitation Task. The Effector Priming Effect is the RT difference between incompatible (e.g. Participant lifts index ﬁnger and observes a middle ﬁnger changing colour) and compatible (e.g. Participant lifts index ﬁnger and observes an index ﬁnger changing colour) trials in the Effector Priming Control task Pro-social 71.11 (11.79) 55.55 (14.84) 123