In the last few decades, the spread of the issue of having an only child has indicated some changes in the childbirth of families as far as this phenomenon has been proposed as one of the effective factors of reduction of fertility rate. The psychological studies on having an only child include various topics and contradictory results. This research aimed to investigate the internal psychological studies on having an only child done during the last ten years (2005-2015) with a systematic review method so that the shortcomings of studies and research gaps were revealed. In order to find the research documents (research papers, academic dissertations, and presented papers in conferences), the databases of SID, Irandoc, and Civilica were searched by the keywords of "an only child", "a child with siblings", "having an only child", and "sister/brother". After three stages (identification, screening, and selection), with the help of two referees who examined the inclusive and exclusive criteria, lastly, 9 studies were selected for being reviewed. The findings showed that the children with sister/brother(s) are superior in terms of the theory of mind and some positive personality traits. According to the findings, the shortcomings of studies surveyed are as follows: hypothesis construction, sample heterogeneity, the reduction of sample size, and the inadequate control of nuisance variables. In addition, illogical hypothesis construction, the lack of native models, the lack of longitudinal studies, and the negligence of the scope of study are the research gaps in the internal studies.