The relationship between mortality and fertility has been a matter of interest since the early days of population research. Concern with the nature and extent of the relationship has heightened since the formulation of demographic transition theory, which attributes central importance to the timing and interdependence of the secular declines in birth and death rates in Western demographic experience. Fertility can influence mortality in a variety of ways and in Chapter 4 the impact of the birth interval on infant mortality was examined. Most interest, however, has focused on the effect of mortality, and particularly infant and child mortality, on reproductive behavior. Recent conceptualization of this relationship has identified several different potential effects. These include an insurance (or hoarding) effect whereby couples have extra children in anticipation of child mortality, a replacement effect whereby couples have replacement births in response to their own actual (as opposed to anticipated) experience with child loss, and an involuntary physiological effect on the interval between births attributable to the interruption of breastfeeding and the consequent shortening of the postpartum non-susceptible period. At a different level, a societal effect is sometimes identified which operates indirectly (and presumably unconsciously) through social customs to adjust the community fertility level to the community mortality level.