Funding for this research was provided by:
Japan Society for the Promotion of Science (17K13931)
Text and Data Mining valid from 2019-02-08
First Online: 8 February 2019
Compliance with ethical standards
: The authors declare that they have no conflict of interest.
: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study was approved by the Ethics Committee at the Department of Arts and Sciences, the University of Tokyo (Approval No. 513–2).
: Informed consent was obtained from all individual participants included in the study. Informed consent was also obtained from parents/guardians for high school or middle school participants.