The goal of this study is to synthesize quantitatively the results of studies of psychopathology among Chinese only children. Since 1979, China’s 1-child policy has generated large numbers of only children, especially in large urban centers, where the 1-child family has become a social norm. Motivated by concern for mental health, 22 studies, based on the SCL-90, have been published that compare the scores of only children to their peers with siblings. The raw effect sizes generated by each study underwent adjustments to enhance the reliability of the findings, including the identification and replacement of outliers, and weighting by inverse-sample size. In addition, analyses were conducted to evaluate the degree of publication bias exhibited by this collection of studies and the results from the SCL-90 studies were compared to studies using alternative measures of anxiety and depression. Overall, the synthesis found small, but significant advantages for only children compared to their peers with siblings, regardless of subscale. However, moderators of this only-child effect were also found: Only children as college students reported significantly fewer symptoms, regardless of subscale, while only children as military recruits reported more symptoms, although the findings about military recruits received less support from the analyses. Furthermore, the size of the only-child advantage was found to be greater for only children born after the policy. Conclusions based on this synthesis are limited by the fact that this body of studies is based on convenience samples of relatively successful youth.