Suckling intensity directly correlates with the duration of the amenorrheal period following birth. Suckling intensity has several dynamic components: frequency of suckling, duration of the suckling bout, and duration of suckling in a 24 hour period. It is not clear which of these plays the most critical role in maintaining amenorrhea. Suckling intensity is highly variable across populations. Studies of U.S. and Scottish women show that at least six bouts per day and 60 minutes of suckling in a 24 hour period will typically sustain amenorrhea. Concurrent studies of !Kung women in Botswana and Gainj women in Papua New Guinea have shown that very frequent, very short suckling bouts of about 3 minutes, 40 to 50 times per day correlate with typical amenorrhea of up to two years postpartum.