Page 91 - Like me, or else... - Michelle Achterberg

Heritability of aggression following social evaluation
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consisted of questions such as: ‘What is your favorite movie?’, ‘What is your favorite sport?’, and ‘What is your biggest wish?’. Children were informed that their profiles were reviewed by other, unfamiliar, children. During the SNAT the children were presented with pictures and feedback from same-aged peers in response to their personal profile. Every trial consisted of feedback from a new unfamiliar child. This feedback could either be positive (‘I like your profile’, visualized by a green thumb up); negative (‘I do not like your profile’, red thumb down) or neutral (‘I don’t know what to think of your profile’, grey circle). Following each peer feedback, the children were instructed to imagine that they could send a loud noise blast to this peer. We specifically instructed the children to imagine this to reduce deception, because it has been shown that imagined play also leads to aggression (Konijn et al., 2007). The longer they pressed the button the more intense the noise would be, which was visually represented by a volume bar (Figure 1). To keep task demands as similar as possible between the conditions, participants were instructed to always press the button, but they could choose whether they wanted a short noise at low intensity or a long noise at high intensity. Unbeknownst to the participants, others did not judge the profile, and the photos were created by morphing two children of an existing data base (matching the age range) into a new, non-existing child. Peer pictures were randomly coupled to feedback, ensuring equal gender proportions for each type of feedback.
Participants were familiarized with the MRI scanner during a practice session in a mock scanner. Then participants received instructions on how to perform the SNAT and the children were exposed to the noise blast twice during a practice session: once with stepwise build-up of intensity and once at maximum intensity. Participants did not hear the noise during the fMRI session, to prevent that they would punish themselves by pressing the button. To familiarize participants with the task, participants performed six practice trials. After the mock scanner session, one of the twins continued with the actual scan, while the other twin performed the WISC-III and other behavioral tasks. First-born and second-born children were randomly assigned to the scan session or behavioral tasks as their first task. When the first child completed the scan, he/she continued with the WISC-III and behavioral tasks while the other child participated in the scanning session.
The SNAT consisted of 60 trials, three blocks of 20 trials for each social feedback condition (positive, neutral, negative), that were presented semi- randomized to ensure that no condition was presented more than three times in a row. The optimal jitter timing and order of events were calculated with Optseq 2 (Dale, 1999). Each trial started with a fixation screen (500 ms), followed by social feedback (2500 ms). After another jittered fixation screen (3000-5000 ms), the noise screen with the volume bar appeared, which was presented for a total of 5000 ms. Children were instructed to deliver the noise blast by pressing one of the buttons on the button box attached to their legs, with their right index
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