Choice as the titular ruler
In 5 CE, Wang Mang, then already nearly unlimited in power as the imperial regent and fearful that the 13-year-old Emperor Ping, once grown, would retaliate against him for having slaughtered his uncles in 3 CE, murdered Emperor Ping by poison. Because the young emperor had not had any children by his wife Empress Wang (Wang Mang's daughter) or any of his concubines, there was no heir. Emperor Ping's grandfather, Emperor Yuan, had no surviving male issue—of his three sons, Emperor Cheng had no issue, and sons of the other two,
Liu Kang, Prince of Dingtao (劉康) and
Liu Xing, Prince of Zhongshan (劉興), had succeeded to the imperial throne (as Emperor Ai and Emperor Ping, respectively) and died without issue. The descendants of Emperor Ping's great-grandfather Emperor Xuan were therefore examined as possible successors.
There were 53 great-grandsons of Emperor Xuan then still living by this stage, but they were all adults, and Wang Mang disliked that fact—he wanted a child whom he could control. Therefore, he declared that it was inappropriate for members of the same generation to succeed each other (even though Emperor Ping had succeeded his cousin Emperor Ai several years earlier). He then examined the 23 great-great-grandsons of Emperor Xuan—all of whom were infants or toddlers.
While the examination process was proceeding, the mayor of South Chang'an submitted a rock with a mysterious red writing on it -- "Wang Mang, the Duke of Anhan, should be emperor." (During his regency, Wang, building a personality cult about himself, had made it an open secret that he encouraged the manufacturing of false prophecies that would call for him to have more and more power; this appears to be one of those instances.) Wang had his political allies force his aunt, Grand Empress Dowager Wang Zhengjun, to issue an edict granting him the title of "Acting Emperor" (假皇帝), with the commission to rule as emperor until a great-great-grandson of Emperor Xuan could be selected and raised.
In the spring of 6 CE, Acting Emperor Wang selected Ying—then just one year old—as the designated successor to Emperor Ping, claiming that soothsayers told him that Ying was the candidate most favored by the gods. He gave Ying the epithet Ruzi—the same epithet that King Cheng of Zhou had when he was in his minority and under the regency of Ji Dan, the Duke of Zhou—to claim that he was as faithful as the Duke of Zhou. However, Emperor Ruzi did not ascend the throne, but was given the title of crown prince. Empress Wang, still a young girl, was given the title empress dowager.