Boys’ breast development may be caused by physiological factors, medicinal factors, pathological factors and so on. 1. Physiological factors: most of the boys’ breast development is physiological factors, newborns are affected by placental estrogen, resulting in breast development, this situation usually does not last long, 2-3 weeks will disappear on their own, some of the boys are stimulated by growth hormone, adrenal hormones lead to mammary gland development, is generally a normal phenomenon. 2. Pharmacological factors: if the child takes hormone-containing drugs for a long period of time, it will lead to the rising of hormones in the body, which will lead to the development of mammary glands, and there may be disorders of the endocrine system and abnormalities in the metabolism of sex hormones. 3. Pathological factors: Boys’ mammary gland development may be caused by pathological factors, mainly due to endocrine disorders in the human body caused by the imbalance of the ratio of androgens and estrogens. If the symptoms continue to develop, parents are advised to take their children to regular hospitals for consultation, and under the diagnosis of doctors, adopt targeted treatment according to the condition of the children.