SummaryThe Gainj of highland Papua New Guinea do not use contraception but have a total fertility rate of only 4·3 live births per woman, one of the lowest ever recorded in a natural fertility setting. From an analysis of cross-sectional demographic and endocrinological data, the causes of low reproductive output have been identified in women of this population as: late menarche and marriage, a long interval between marriage and first birth, a high probability of widowhood at later reproductive ages, low (...) effective fecundability and prolonged lactational amenorrhoea. These are combined with near-universal marriage, a low prevalence of primary sterility and a pattern of onset of secondary sterility similar to that found in other populations. Of all the factors limiting fertility, by far the most important are those involved in birth spacing, especially lactational amenorrhoea. (shrink)