Most of the emperors of the Tang Dynasty lived relatively long, such as Tang Gaozong Li Yuan lived 69 years old, Tang Gaozong Li Zhi lived 56 years old, Tang Xuanzong Li Longji lived 78 years old, until the late Tang Dynasty, due to eunuch chaos, palace coups, etc., the emperor's life expectancy declined. The longer the lifespan, the better the physical fitness of the Tang Dynasty emperor, and he was naturally able to give birth to more sons. In contrast, the longevity of the Song emperors was slightly inferior.
The Tang Dynasty emperors had some Hu genes in their blood, which was verified by Mr. Chen Yinke, a historian, and most of the Tang Dynasty emperors liked sports and hunting, which could improve people's physical fitness. The emperor of the Song Dynasty emphasized literature over martial arts, and most of the time he stayed in the palace, living in the palace walls since he was a child, lacking exercise, and his physical fitness was poor. Moreover, the Tang Dynasty regarded fat as beauty, and men liked women with plump figures. It is said that Yang Guifei is relatively plump, and these women are fertile. In the Song Dynasty, foot binding had already emerged, and it may be that most of the emperors of the Song Dynasty liked delicate and weak women, who were good-looking, but their fertility was not strong, and even if they were born, they were weak and sickly.
Most of the descendants of Zhao Guangyi of Song Taizong are frail and sickly, with familial genetic diseases, Song Zhenzong only has one son alive, and Song Renzong's sons all died, and in the Southern Song Dynasty, Song Ningzong and Song Lizong's sons also died. Comparatively speaking, most of the emperors of the Tang Dynasty were in good health, especially the descendants of Wu Zetian and Li Zhi, due to Wu Zetian's long life and good physical fitness, which led to the later Tang Dynasty emperors' genes becoming better.
Of course, some people say that it is because the emperor of the Song Dynasty is inside the palace wall, and there are cinnabar and other things in the palace of the Song Dynasty, and long-term residence will lead to weak health or few children, and some people say that the emperor of the Song Dynasty likes to smell a kind of incense, especially the emperor of the Southern Song Dynasty, smelling a kind of incense leads to childlessness.