Bongaarts’ aggregate model of the proximate determinants of fertility (cont.)
The Bongaarts model is a tool for examining the relative contribution of each of the four factors to the inhibition of fertility from its theoretical maximum. It is an aggregate fertility model which describes the relationship between fertility and the four principal proximate determinants: marriage; contraception; induced abortion; and postpartum infecundability (or postpartum insusceptibility). In other word, the model is not intended to estimate accurate total fertility rates.
The four proximate determinants are considered as inhibitors of fertility, because their action means that fertility is lower than its theoretical maximum. Fertility is lower than its maximum because of: delayed marriage and marital disruption; the use of contraception; induced abortion; and postpartum infecundability due to breastfeeding and postpartum abstinence. The following figure illustrates four levels of fertility and how they relate to the fertility-inhibiting effects of these four factors.
Natural fertility vs regulated fertility
In discussing proximate determinants, it may be useful to introduce the concept of natural and regulated fertility.
Natural fertility
Natural fertility has been defined by Henry (1961) as fertility that exists in the absence of any deliberate control. 5 That is fertility in the absence of any behaviour that is intentionally used to limit the number of births. Before the onset of fertility transition, populations are arguably at or near their natural level of fertility.6
The level of natural fertility may be well below the theoretical biological maximum because of factors which limit fertility due to postponement of marriage, prolonged breastfeeding and postpartum abstinence. The proximate determinants which affect the level of natural fertility are: marriage, fecundability, postpartum infecundability, spontaneous intrauterine mortality and permanent sterility. Among these, marriage (including being in sexual union) and postpartum infecundability are most important in explaining differences between populations in the level of natural fertility. The level of natural fertility varies between populations, but the age pattern of fertility is considered to be consistent.
Regulated fertility
While natural fertility refers to fertility when a couple does not stop having children after reaching certain number, regulated fertility is defined as fertility with deliberate parity-specific control. Fertility may be controlled prenatally by contraception, induced abortion or limiting the exposure to childbearing (e.g. abstinence or withdrawal). The age-specific pattern of fertility is found to vary greatly between populations with natural and regulated fertility since the use of deliberate measures to control number of births is not constant across the age range.
5 ’Natural’ may not be an ideal adjective (Bongaarts and Potter 1983; Henry 1961), but Henry selected the term natural fertility instead of biological or physiological factors, as there are many non-biological factors which influence natural fertility, such as marriage.
6 However, there is longstanding arguments over if couples deliberately controlled fertility before the onset of fertility transition or not. See more Carlsson (1966), Coale (1973), Coale and Watkins (1986), Mason (1997).