MEAN FEMALE AGE AT FIRST MARRIAGE AND MEAN LENGTH OF FIRST BIRTH INTERVAL. CHINA. 1970-1985 24 First birth interval (right scale)20 Age at marriage (left scale) 18 19 Why the first birth intervalhas shortened 16 considerably since 1970? Late marriage caused by later longer and fewer policy More prevalent premarital sexual activity,I l970 1972 1974 l976 1978 1980 1982 1984MEAN FEMALE AGE AT FIRST MARRIAGE AND MEAN LENGTH OF FIRST BIRTH INTERVAL, CHINA, 1970-1985 Why the first birth interval has shortened considerably since 1970? ➢ Late marriage caused by later longer and fewer policy ➢ More prevalent premarital sexual activity