This investigation was conducted according to the principles expressed in the Declaration of Helsinki. All experimental protocols are consistent with the Guide for the Experimentation with Humans and were approved by the Institutional Ethical Committee of Primate Research Institute, Kyoto University.

Participants

As participants, we recruited 75 typically developing 4-year-old boys from several kindergartens in Kyoto and Aichi prefectures, Japan. We obtained written informed consent from the parents of all participants involved in our study. They were randomly assigned to either of three groups, each of which was made up of 25 children, for subsequent experimentation. The experimental room was a sound-attenuated playroom (3.5 m X 5.5 m) familiar to all of the participants. It contained a one-way observation mirror and a low table on which the experimenter was able to display five toys. A ceiling speaker was installed in the ceiling of the room, just above the table. For the toys displayed, a total of 10 different miniature cartoon monster figures (known as “Pockemon” figures) were randomly chosen. They were randomly assigned to either of two sets, each of which comprised of five figures. All 10 figures were produced on the basis of images of monster characters that appeared in a TV cartoon, “Pocket Monster”. All of the toys were extremely popular with children in Japan, particularly with young boys and an opportunity to play with them was expected to be met with enthusiasm. Prior to the commencement of the experimental session, the experimenter spent several weeks at the kindergartens playing with the children, so that all of them could have known her well when the session started.

Procedure

In all, each participant was subjected to the experiments twice (the first and the second experiments) and to the control test once during the present study, i.e., each experiment was designated to evaluate the changes of the attractiveness of a toy to the participants when it was forbidden to play with it and the control test was designated to evaluate the change of the attractiveness of the toy to the participants when it was simply withdrawn. Both experiments were conducted for each child, with an interval of 14 to 15 weeks. The experiments were separated by the control test, which was undertaken after the first testing experiment with an interval of 7 to 8 weeks. One of the prepared two sets of toys was used for the first experiment and the subsequent control test whereas the other set was used for the subsequent second experiment.

In each of the first and the second experiments, the experimenter led each participant into the experimental room, closed the door and showed the participant the toys. She explained what each monster toy was and allowed the participant to play with it briefly before moving on to the next one. After the participant became familiar with all the toys, the experimenter suggested a “question game” following which the participant was provided with an opportunity to play with the toys. The experimenter placed all the toys on the floor and was seated on the opposite side of the low table from the participant. Having put two of the toys on the table (for example, Jorohda and Yonoire shown) she asked “Suppose you could play with either Jorohda [picking it up], or Yonoire [picking it up]. Which one would you rather play with?”

After the participant responded, the experimenter replaced the two toys on the floor, put two other monster toys on the table and continued until the participant made choices between all 10 possible pairs. By this procedure, a ranking was elicited, from the most preferred toy (rank 1) to the least preferred (rank 5) toy. As far as following this procedure, some inconsistencies in choices of the participant could have been expected (e.g., the participant preferred toy A over toy B, toy B over toy C, but also toy C over toy A). However, actually, these did not occur in the present experiment. After the participant ranked the toys, the experimenter picked up the second-ranked toy, placed it on the table in the center of the room, arranged the remaining toys on the floor and said: “I have to leave now for a few minutes to do an errand. But why don't you stay here and play with these toys while I am gone? I will be right back. You can play with this one [pointing], this one and this one. But I don't want you to play with [mentioning the name of the second-ranked toy].”

At this point, the experimental conditions were introduced. To each of the 25 children in one of the three groups that would experience a strongly worded suggestion, the experimenter continued: “I don't want you to play with [mentioning the name of the second-ranked toy]. If you played with it, I would be disappointed. I would have to take all of my toys and go home and never come back again. You can play with all the others while I am gone, but if you played with [mentioning the name of the second-ranked toy], I would think you were just a baby. I will be right back.”

To each of the children in the other two groups, who would experience a mildly worded suggestion, the experimenter instead continued: “I don't want you to play with [mentioning the name of the second-ranked toy]. If you played with it, I would be annoyed. But you can play with all the others while I am gone and I will be right back.”

The experimenter then left the room. As she was leaving the room, she switched on an audio player connected to a ceiling speaker in the room, if the child in the room was in one of the two groups of the participants who experienced a mildly worded suggestion, so that Mozart's sonata for two pianos in D major, K448 and Mozart's piano concerto No.23 in A major K488 was played, in the first experiment and in the second experiment, respectively (the sound pressure level: 65 dB in either experiment). The music continued to be played until the experimenter came back and switched off the player, whereas the children in the other two groups remained without such exposure to music during that period. To summarize, the overall design of the first experiment and that of the second experiment differed in only one respect: music was played in one of the three participant groups. In addition, the group of participants who had experienced a mild suggestion with music in the first experiment, subsequently experienced a strongly worded suggestion without music in the second experiment, while the group of participants who had experienced a mild suggestion without music in the first experiment, subsequently experienced a mild suggestion with music in the second experiment and the remaining group, consisting of those participants who had experienced a strongly worded suggestion without music in the first experiment, experienced a mild suggestion with music in the second experiment.