Exp Brain Res (2011) 211:601–612 (Shaw et al. 2008). Recent neuroimaging studies also suggest functional maturation of the social brain that con- tinues throughout adolescence. A number of studies have reported increased activation of the MPFC in adolescents relative to adults when representing the mental states (Blakemore et al. 2007; Wang et al. 2006) or the emotions (Burnett et al. 2009) of others. These studies suggest that parts of the social brain, which may be responsible for mediating imitation as a result of social priming, are still maturing throughout the adolescent years. Therefore, this experiment investigates development in adolescence of the modulation of automatic imitation by social attitudes. In the current experiment, we used a modiﬁed version of the Leighton paradigm. As in the study by Leighton et al., each participant completed two tasks: a social attitude priming task followed by an imitation task. In the priming task, participants formed sentences from sets of words that included either pro-social terms or non-social terms (two groups). In each trial of the imitation task, which was a modiﬁed version of the two alternative choice reaction time task used in Brass et al. (2000), participants were required to perform a lift action with either the index or middle ﬁnger as soon as they saw a cue appear on screen. Con- currently, participants observed a video of a ﬁnger lift. In 50% of trials, the observed effector was compatible with the effector that the participant was required to use (e.g. index ﬁnger response and observed index ﬁnger lift), and in 50% of trials, the observed effector was incompatible (e.g. index ﬁnger response and observed middle ﬁnger lift). We hypothesized that the effect of social priming on imitation would differ as a function of age group. In the original, naturalistic, experiments demonstrating a link between pro-social attitudes and imitation (Lakin and Chartrand 2003), it was unclear whether pro-social priming increased imitation speciﬁcally. Imitation was measured by scoring matching behaviour in naturalistic social interac- tions. In the majority of these studies, a participant inter- acted with a confederate who either shook their foot or rubbed their face with above average frequency. Judges measured the frequency of each action for each participant, yielding an ‘imitation score’. For example, imitation involved foot shaking in the presence of a foot shaking confederate but not in the presence of a face-rubbing confederate. However, this paradigm does not allow one to answer the question of whether social attitudes modulate imitation of another’s speciﬁc movements, or whether they just copy the degree to which the effector is moved. Observing a foot shaking model may result in increased performance of a variety of foot movement, which may indirectly increase the proportion of foot shaking. Non- speciﬁc effects of this kind are known in the motor control literature as ‘effector priming’ (Bach and Tipper 2007; Berger and Hadley 1975; Gillmeister et al. 2008). 603 In contrast, the effect reported by Leighton et al. was speciﬁc to the movement type required. Pro-socially primed adults were faster to conduct a hand-open response when observing a hand-open response rather than being faster to open their hand when observing any hand movement. Leighton et al. therefore argue that modulation of imitation by social attitudes does not occur indirectly via effector priming. To test this hypothesis, we added a control condition to our imitation task wherein no ﬁnger movement was present, instead a semi-transparent green mask was superimposed over either the compatible or incompatible ﬁnger. The green mask draws attention to the effector but does not afford imitation since no movement is present. It has previously been demonstrated that drawing attention to an effector, even if the effector is static, produces speeding of responses made with that effector. For example, Bach and Tipper (2007) showed that presenting participants with photographs of individ- uals with a coloured dot on either their hands or their feet speeded responses made by participants using the same effector. We hypothesized that adult participants primed with pro-social words would show a greater Imitation Effect than those primed with non-social words in the Imitation task but that performance on the Effector Priming Control task would be unaffected by social priming. Methods Participants The study included 90 participants divided into four groups: Non-social Adolescent Group (N = 17, mean age = 13.71, Standard Error (SEM) = 0.19); Pro-social Adolescent Group (N = 17, mean age = 13.82, SEM = 0.18); Non-social Adult Group (N = 28, mean age = 28.07, SEM = 2.58); Pro-social Adult Group (N = 28, mean = 25.79, SEM = 2.00). For each age group, the Non- and Pro-social groups were matched in terms of age (Adolescent Groups: t(32) = -0.46, P = 0.65 (2-tailed); Adult Groups: t(54) = 0.70, P = 0.49 (2-tailed)). Adults were recruited from the University College London Psy- chology volunteer database, and adolescents were recruited from two local schools. Within these age groups, the par- ticipants were randomly allocated to the Pro-social or Non- social Group. All participants were na¨ıve with respect to the purpose of the experiment. Informed consent was obtained from all adults and from the primary caregiver of each adolescent participant. The study was approved by the local ethics committee and performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki. 123