There were such a record in The Historic Records that the ancients didn＇t foster the infants who were born in May.Ancient and modern commentators of The Historic Records have been confused about or evaded the subject.After widespread investigation and careful analysis,we find that the emergence of this kind of custom has its complex factors,which include not only the reality environment reasons such as climate and insect epidemic diseases,but also the idea factor which were accumulated in the folk such as the infants born in May were disobedient and inauspicious.