Previous studies have shown the influence of social transformations such as urbanisation, education and income on the first birth interval. These researchers have argued that such social transformations provide more autonomy to young boys and girls in the late selection process. These changes could facilitate quicker intimacy among couples. Linking conjugal intimacy to coital frequency through romantic marriage, it has been argued that in the case of romantic marriage, (woman choosing her own spouse), the relationship between husband and wife will be very intimate, resulting in a higher level of coital frequency and thus greater risk of an early first birth. They termed it as the "quiet revolution" in Asia. Further evidence is seen of a decline in first birth interval among Tamangs residing in the high mountains in central Nepal which is attributed to marriage and cultural factors . Ethnic factors also play a role.
