International surveys estimate that it affects up to four out of 1000. There are no clear figures for Taiwan, but Chiu Yan-nan, a child psychologist at National Taiwan University, says that it seems to be becoming more and more prevalent.
根據國外統計，盛行率最高為千分之四，台灣沒有明確的數據，但「感覺上越來越多，」台大兒童心理衛生中心醫師丘彥南說。