PIP: Studies of the few remaining hunter-gatherer societies reveal that a birth spacing interval of up to 4 years can be realized without benefit of any modern contraceptives. The main explanation for this phenomenon is that breastfeeding is practiced in primitive societies for long durations and the infant suckles on demand (i.e., throughout the day and night) rather than on a schedule. Such a feeding pattern, coupled with an atmosphere that encourages breastfeeding, can postpone ovulation for 1-2 years, or possibly even longer. Suckling -- crucial to understanding how breastfeeding postpones ovluation -- induces the release of 2 hormones, namely, prolactin and oxytocin. Prolactin is involved in the production of milk and oxytocin works to eject the milk. 2 crucial factors involved in breastfeeding's contraceptive effect appear to be, in order of importance, the frequency of feeding episodes and the duration of its practice. More frequent suckling episodes produce more prolactin than do fewer episodes, and longer feedings produce more milk for future feedings. It is doubtful that prolactin directly inhibits ovulation; rather, its level may be an accurate index for some other factor that inhibits ovulation. Researchers do agree that the frequency and, to a lesser extent, the duration of suckling episodes, as well as the absence of supplemental feeding, are good predictors of how long ovulation will be postponed. If breastfeeding were totally instinctive to humans, perhaps its contraceptive effectiveness would only be dependent on physiological characteristics. Yet, even if a mother is willing to practice unrestricted breastfeeding, the physiological functions also are influenced by psychological factors. Breastfeeding is learned. Attitudes of hospital staff, relatives, friends, and the community towards breastfeeding can affect breastfeeding patterns and hence its contraceptive effectiveness. The "let-down reflex" is the Pavlovian response to suckling that sets milk production and ejection into motion. In some instances, the infant's cry for food can induce the reflex, pointing to the powerful interplay between emotions of the mother and her physical response. In developing countries, the main reason mothers give for ceasing to breastfeed is a lack of milk. The tragedy is that in most cases, the woman who believes that she has too little milk actually has too much. Environments that make breastfeeding appear backward, embarrassing, or inconvenient, however, can inhibit the let-down reflex and thus decrease the frequency and duration of suckling.