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TRANSCRIPT
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Microsimulation of Yoruba Fertility
M. G. SANTOW
National Blood Pressure Study, P.O. Box 691, Canberra City, A.C.T. 2601, Australia
Received I December 1977; revised 12 June I978
ABSTRACT
A microsimulation model is used to assess the extent to which the fertility of the
Yoruba of Western Nigeria may be affected by changes in the durations of lactation and
marital sexual abstinence. The simulations make no allowance for the compensatory use
of contraception. A series of preliminary simulations demonstrate the effect on fertility of
the length of the period of post partum non-susceptibility to conception, and an attempt is
made to duplicate the reported fertility of a large Ibadan survey (CAFNl). Input data are
then drawn from a number of recent Nigerian demographic surveys which enable the
separate simulation of the fertility of rural dwellers, poorer Ibadan women and richer
Ibadan women. The output indicates that, in the absence of contraception, urban fertility
is likely to exceed rural, and the fertility of richer urban women is likely to exceed that of
poorer urban women.
1. INTRODUCTION
The 1963 Nigerian census enumerated 11.3 million Yoruba, of whom
about 10 million inhabited the Western State (Lucas and Williams [13],
Orubuloye [IS]). About half now live in urban areas, with perhaps half of
these being concentrated in Lagos and Ibadan (Caldwell and Caldwell [6]),
although many urban residents live in traditional towns and are still
dependent on agriculture (Lucas and Williams [13]). In rural areas the crude
birth rate has been estimated at around 50 per 1000 and the crude death
rate at around 27 per 1000 (Lucas and Williams [13]).
The topic of this paper is the effect on Yoruba fertility of the partial
breakdown of the practice of marital sexual abstinence. Postpartum sexual
abstinence traditionally continued for at least as long as the mother
breastfed her child, and thus extended for two or three years. When
breastfeeding is prolonged for more than three or four months it is capable
of suppressing the return of ovulation and, therefore, the return of
menstruation. However, the average duration of amenorrhea is always
MATHEMATICAL BIOSCIENCES 42, 93-l 17 (1978) 93
BElsevier North-Holland, Inc., 1978
0025-5564/78/090087 + 25.SO2.25
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M G SANTOW
shorter than the average duration of lactation. The maximum contraceptive
effect of lactation is felt after about two years, beyond which point
prolonged lactation is no longer able to stave off the return of the menses
(see Buchanan [3]). Child spacing was therefore achieved through post-natal
sexual abstinence, as the first menstrual flow could be expected to occur
before sexual relations were resumed. Terminal abstinence was traditionally
adopted by married women in their forties, and affected fertility by truncat-
ing the reproductive span.
In some societies the optimistic belief is expressed that a lactating
woman cannot conceive (Santow [23]). In contrast, the Yoruba advocate
breastfeeding as a method not of preventing a new conception but of
ensuring the well-being of the child at the breast. Early weaning of a
Yoruba child will both leave him a prey to a variety of deficiency diseases
and synergistically lower his resistance to a host of other conditions. His
mother knows that such premature weaning may be induced by a new
conception too soon after his birth, while sexual abstinence prevents such a
conception and therefore protects her supply of milk. Thus Olusanya [17]
described the Yoruba belief that the milk of the lactating woman is made
harmful by intercourse, causing the child to fall ill and possibly die.
Moreover, a woman who became pregnant while she was still breastfeeding
was likely to be publicly criticized (Martin, Morley and Woodland [14]).
The short-term effect of abstinence is contraceptive in that it prevents a
conception. On the other hand, it should not be considered as contraceptive
in the long term, because its practice is unrelated to the number of children
already born to the family. Indeed, post partum abstinence should be
viewed as pro-natalist in intent, as it is the mothers method of maximizing
the number of her surviving children.
Caldwell and Caldwell [6] suggest that about two-thirds of abstinence
among Yoruba grandmothers is attributable to the strong belief that
grandmothers should not bear more children of their own. The demands of
such children for their mothers attention would jeopardize the strong link
between grandmother and grandchild. Such abstinence is plainly contracep-
tive in both intent and effect.
2. THE CASE FOR SIMULATION
The last Nigerian census was taken in 1963 and produced a sizeable
overcount (Lucas and Williams [ 131). More importantly, the establishment
Caldwell and Caldwell [6] write that the explanation for [the effect of a new
conception on the health of the unweaned child]
. .
s not always the correct one but the
incorrect explanation is often more vivid and probably more efficacious in enforcing
abstinence; amongst the Yoruba it is widely believed that the mans sperm actually enters
and poisons the milk which is being fed to the baby
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MICROSIMULATION OF YORUBA FERTILITY
95
of an effective system of vital registration in Nigeria is still in its infancy. AS
a result, fertility and mortality must be estimated from sample surveys,
which are subject to problems of omission and age misstatement. We shall
discuss these data defects in turn.
Respondents may fail to report a birth, particularly if, as is common, the
child is living with relatives rather than with his parents. Deaths, particu-
larly of very young children, may not be reported, and it may happen that
neither the birth nor the death of a child is reported if he died before he was
formally named. The failing memories of the more elderly (Brass [2]) may
be responsible for the omission of births and deaths by older women.
Deliberate omissions of births may occur in a situation similar to that
described by Lucas [l l] in which many Lagos respondents believed that
they would attract bad luck if they revealed the number of their children.
Soyinka [24] relates the response of an old woman, a character in a recent
African novel, to a question asking how many children she has:
Hush, we dont ask people how many children they have. It is not done. Children are not
goats or sheep or yams to be counted?
The second problem, that of age misstatement, may cause the misclassifi-
cation of reported vital events and therefore distort age-specific fertility and
mortality rates (see van de Walle [25]). All African demographic surveys
share the problem of trying to record the ages of people who do not know
their exact ages and are not fundamentally interested in knowing them
(van de Walle [26]). The most common source of error in all age reporting is
the overstatement of ages ending in certain preferred digits, with a corre-
sponding understatement of ages ending in other digits (Nagi, Stockwell
and Snavley [ 151). We shall return to this topic at a later stage of the paper.
Microsimulation provides a useful technique for measuring the effect on
fertility of rapid change amongst a group of intermediate variables, in this
case breastfeeding and post partum and terminal abstinence. For the sake
of clarity several areas of change were ignored in the simulations, namely,
infant and child mortality and contraceptive usage. Firstly, a recent
Nigerian study has demonstrated a very real differential between the infant
and child mortalities of two villages which differed principally in their
access to public health services (Orubuloye and Caldwell [19]). The im-
plementation of new health services is continuing and may be expected to
lead to an increase in the age-specific proportions of surviving children, as
the greatest effect of such services is the reduction of the exogenous
component of infant and child mortality. Secondly, although the level of
contraceptive use is low amongst the Yoruba of Ibadan, it has been
21n a Liberian study, Gay and Cole [8] observed that it is not proper to count aloud
even domestic animals lest some harm befall them.
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MICROSIMULATION OF YORUBA FERTILITY
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M. G. SANTOW
increasing over the last years (Caldwell and Caldwell [5]). The use of
microsimulation is indicated in this instance not only because of the
unreliability of the retrospective fertility data at our disposal, but as a
means by which to identify the effect on fertility of changes within one
group of linked intermediate variables in isolation from a number of others.
3. THE MODEL
The microsimulation model used in the following analysis is based on
those pioneered by Hyrenius and Adolfsson [lo] and Perrin and Sheps [20].
The time unit of the model is the so-called lunar month of 28 days, as this
is, on average, the longest period of time during which no more than one
conception can occur (Santow [23]). The reproductive span of each simu-
lated woman is defined by her ages at marriage and final sterility. Thus no
allowance is made for the possibility of pre-marital conception or marital
dissolution by divorce or the death of either spouse. As the Yoruba woman
generally remarries soon after her widowhood, the assumption that she
remains in some sort of sexual union for as long as she is fecund is not
unwarranted. The male partner does not appear explicitly in the model, but,
where appropriate, his contribution to fertility is represented by means of a
couple parameter. For example, the probability that a conception
terminates in a spontaneous abortion is a female parameter. On the other
hand, the probability that a conception occurred in the first place is a
couple parameter, because it is dependent on both of the marriage partners.
The first step in the simulation of each reproductive history is the
determination of the womans ages at marriage and final sterility. (Sterility
data are generally derived from the fertility non-experience of couples.)
The Monte Carlo technique is applied to the two relevant input distribu-
tions, and if the age at which sterility occurs does not exceed the age of
marriage, the woman exits from the simulation and the process is repeated
on the next woman.
Given that a positive number of reproductive cycles is initially allocated
to the woman under simulation, the Monte Carlo method is used to
determine whether she will conceive in the first cycle. If the first test is
unsuccessful it is repeated, and the number of such trials before a success
occurs gives the waiting time to conception in lunar months. The value of
the random number which identifies a conception also indicates whether the
conception is to terminate in a spontaneous abortion, a stillbirth or a live
birth. The lengths of the periods of gestation and of post partum
amenorrhea or sexual abstinence are determined from the input distribu-
tions specific to the pregnancy outcome.
The lunar-month counter is set to zero at marriage. After each Monte
Carlo test this counter is incremented either by one, in the case of an
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MICROSIMULATION OF YORUBA FERTILITY
99
unprotected cycle in which no conception occurs, or by the duration of
non-susceptibility after a conception. This latter term is calculated as the
sum of the durations of pregnancy and post partum amenorrhea or sexual
abstinence. After each such incrementation the time counter is compared
with the predetermined reproductive span, and as soon as the counter
exceeds this span the reproductive history is terminated. Its fertility data are
stored, and the next reproductive history is simulated. When the sample is
complete the final aggregates are made. All the simulations are of 1000
women.
4. INPUT DATA
The input data are of two types. The first comprises biological input not
drawn specifically from the Yoruba but compatible, as far as is known, with
their condition. The second consists of data drawn from a number of recent
Nigerian demographic surveys. The following examples demonstrate how
the input data are inserted into the model.
We wish to determine whether a non-pregnant, fecund woman of a
particular age will conceive. In this example she is 22 years old, and thus the
probability of her conceiving is 0.20. A random number is selected, and
according to whether it is less than or exceeds 0.20, a conception is said to
occur or not. In the former case the random number also determines
whether the conception will terminate in an abortion, a stillbirth or a live
birth. A second random number is selected and is compared with the
appropriate cumulative frequency distribution from Table 1 in order to
TABLE 1
Percentage Distributions of Duration of Gestation and Post Partum Amenorrheaa
Months
Abortion
Gestation
Stillbirth
Live birth
Post partum amenorrhea
Abortion
Stillbirth
1
2
3
4
5
6
I
8
9
10
11
12
Mean
40
21
19
8
3
2
1
2.2
60
4
30
49
10
31
12
3
1
15 1
19
4
30
44
36 48
3
9.9 10.5 1.5 2.6
*Source: Clinical studies surveyed in Santow [23].
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M. G. SANTOW
determine the length of the pregnancy. If this pregnancy is to end in an
abortion, a random number of 0.50 gives a duration of two months.
Similarly, a random number of 0.50 gives a stillbirth gestation duration of
10 lunar months, and a live-birth gestation of 11 months.
4.1. BIOLOGICAL DA TA
The fecundability input is a simple age-dependent function increasing
linearly from a value of 0.13 at age 15 to 0.20 at age 20, remaining constant
until age 25 and then declining linearly to a value of 0.03 at age 42, at which
value it remains constant (Santow [23]). Spontaneous abortions account for
15 per cent of all conceptions, and stillbirths for a further 2 per cent.
The sterility input is an exponential function derived from Pittenger [21]
with a radix of 5 per cent of women sterile at age 17.5 and a median of 41
years.3 Whereas fecundability is determined solely as a function of age and
is the same for all fecund women of the same age, the point at which
sterility overtakes each woman is determined by the Monte Carlo method.
4.2. SURVEY DATA
Table 1 contains the three input distributions of gestation and those of
post partum amenorrhea which follow an abortion or a stillbirth. The
duration of post partum amenorrhea following a live birth depends on the
duration of lactation and will be discussed in connection with the breast-
feeding data obtained from the survey material.
Table 2 presents the characteristics of a number of Nigerian surveys (see
Okediji, Caldwell, Caldwell and Ware [16]). Data from all the surveys
indicated that virtually all Yoruba women are married by the age of 30, and
the mean age at marriage ranged from 18 to 22 years.4 Only 3 per cent of
the women from the entire NF2 sample had been married more than once,
although only 48 per cent of the sample were married monogamously. Of
the CAFNI women, 6.6 per cent were no longer in contact with their
husbands either through widowhood, separation or divorce, and the per-
centage of monogamous marriages was the same as in the much smaller
NF2 sample.
Figure 2 presents the population pyramid of CAFNl respondents aged
between 15 and 50 years, and clearly illustrates the problem of age misstate-
ment discussed earlier. Nagi, Stockwell and Snavley [ 151 considered that, in
age estimates, the over-reported digits are those which are multiples of the
divisors of the base of the number system, and that the extent of the
heaping is related to the magnitude of these divisors. These predictions are
3The choice of this radix was justified by the discovery that 5 per cent of the CAFNl
women (see Table 2) in the 45-59 age group were nulliparous.
the context of this work marriage is taken to mean any form of permanent sexual
union.
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MICROSIMULATION OF YORUBA FERTILITY 101
nicely fulfilled by the reported age statistics of the CAFNl women. There is
massive heaping on ages terminating in a 5 or a 0 (particularly between the
highly fecund ages of 20 and 30) although more women reported that they
were 25 than reported that they were 20. Preference was then given rather to
even terminal digits than odd. More specifically, the digits 2 and 8 were
more popular than 4 and 6, since these latter digits are overshadowed by the
highly preferred 5. Similarly, the digits 3 and 7 were less unpopular than 1
and 9.
The eccentricities of this population pyramid cast doubi not only on
reported age-specific fertility data but also on the reported distributions of
age at marriage. After smoothing, the means of the distributions obtained
from the NF2-1, NF2-2 and NF2-3 surveys were respectively 22, 18 and 22
years, and their ranges were (15,26), (14,21) and (16,28). The mean age at
marriage of the CAFN 1 distribution was 21 years, and the range was
(17,26).
Figure 3 presents graphically the Nigerian Family Project distributions
of the length of lactation, the corresponding length of amenorrhea and the
length of post partum abstinence. The duration of amenorrhea was not
sought in any questionnaire but was estimated using data from studies
which link lactation and post partum amenorrhea.5 The graphs facilitate
such comparisons as the percentage of women in each sample who are no
longer effectively sterile on their babys first birthday. Through abstinence
this percentage increases from 7 (NF2-1) to 21 (NF2-2) to 74 (NF2-3), while
through breastfeeding, the percentage increases from 46 (NF2-1) to 56
(NF2-2) to 100 (NF2-3). The figure shows quite clearly that for the richer
women (NF2-3) it is nursing, rather than abstinence, which provides the
greatest contraceptive protection for the first six months after confinement.
In other words, although the erosion of traditional practices has affected the
lengths of time both for which women are willing to breastfeed and for
which they are willing to abstain, it is the custom of post partum abstinence
which has been affected, in this case, the more drastically.
The lowest strip on the graph compares three pairs of distributions which
were obtained by asking women both how long post-natal abstinence
should continue, and how long they themselves had abstained after their
last live birth. Two pairs are drawn from the Fertility and Family Limita-
tion Survey (FFL) and the third, represented by unjoined points, from the
large Ibadan survey (CAFNl). In both sets of Ibadan data the should
abstain distribution lies fairly consistently above the did abstain distrib-
ution. One explanation for this is that the respondent does not anticipate
the death of an unweaned baby when she answers the should abstain
e Santow [23] for an account of such studies.
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M G SANTOW
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FIG 2 Population pyramid for CAFNI women aged 15 to 50 years
question, but that such a death prematurely curtails the period of post
partum abstinence and thus influences her response to the did abstain
question. With an annual infant mortality rate in Ibadan as high as 100 per
1000 live births (Santow [23]), it is likely that sufficiently many respondents
had recently experienced a child death for the did abstain distribution to
be depressed below the should abstain. [This does not explain why the
opposite trend is apparent in the Ekiti rural data (FFL-l).]
One feature common to all four sets of distributions is the irregularities
that occur at six-monthly intervals. For example, 32 per cent of NF2-2
women reported that they had stopped breastfeeding by the seventeenth
month post partum, but as many as 56 per cent reported that they had done
so by the eighteenth. Similarly, only 40 per cent had resumed sexual
relations by the twenty-second month, but 68 per cent had done so by their
childs second birthday. It seems likely that this heaping is not completely
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MICROSIMULATION OF YORUBA FERTILITY
.
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M. G. SANTOW
analogous with that observed in the reported age statistics, as the durations
of abstinence and nursing are often linked directly to the childs age. For
instance, a woman may resume sexual relations during a particular festival
at harvest time, recalling that she gave birth during this festival two years
before.6 The data presented in Figure 3 provide input distributions of post
partum amenorrhea and sexual abstinence specific to each survey, using a
time unit of half a year.
In the CAFNl sample, 55 per cent of women aged 40-44 years had
terminated all sexual relations, while 69 per cent had done so at ages 45-49,
and 83 per cent at ages 5&54. A further survey (NF3) was devoted
exclusively to female terminal abstinence and provided clear differentials in
the ages at which rural, poorer and richer Ibadan women became terminally
abstinent. By the age of 44 years the number of terminally abstinent women
comprised 53 per cent of the rural women (NF3-1 and NF3-2), 30 per cent
of the poorer Ibadan sample (NF3-3) and 30 per cent of the richer Ibadan
women (NF3-4). The average ages of commencement of terminal absti-
nence varied only from 36.4 years @F3-3) to 40.7 years (NF3-l), but the
age distributions of entrance into the abstinent state showed considerable
variation.
The NF3-1 and NF3-2 data were aggregated to provide an age distribu-
tion of entry into the terminally abstinent state compatible with NF2-1
data. Similarly, the NF3-3 terminal abstinence data were applied to the
NF2-2 simulation, and the NF3-4 data to NF2-3. Single-year distributions
were constructed, and as the NF3 sample included no women older than 44
years, the proportions of terminally abstinent women at higher ages were
obtained by linear extrapolation, with all women terminally abstinent by
the age of 50.
5. FERTILITY AND THE PERIOD OF POST PARTUM NON-SUS-
CEPTIBILITY TO CONCEPTION
A series of simulations were performed to test the effect on fertility of
increasing the duration of post partum non-susceptibility to conception
from two lunar months to three years. A constant marriage age of 17 years
was employed, and the remaining input data were used as described in the
previous section.
Table 3 presents the means, and Fig. 4 the distributions, of live births
corresponding to seven distinct periods of post partum nonsusceptibility.
contrast, the preliminary analysis by R. Lesthaeghe and H. Page of the Lagos
Parity Study data on breastfeeding, post partum amenorrhea and abstinence detected
considerable heaping in the retrospectively obtained distributions, but not in the prospec-
tive distributions. This suggests that in Lagos (which is a modem city by comparison with
Ibadan) such heaping is due more to classical misstatement than to a true link between the
childs age and the durations of breastfeeding and abstinence.
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MICROSIMULATION OF YORUBA FERTILITY
107
TABLE 3
Average Parity by Duration of Post Partum Non-susceptibility to Conception
Non-susceptibility
2
5 9
13
18 25 35
(lunar months)
Average parity
12.94 11.52 9.92
8.66
7.55 6.56 5.32
Width of 95
confidence
intervala
0.54
0.47 0.42 0.38
0.31 0.27 0.23
aThe width of the 95 confidence interval is obtained by dividing twice the sample
standard deviation by the square root of the sample size, and multiplying the result by
1.96.
FIG 4 Distributions of live births by fixed duration of abstinence in lunar months.
Progressively greater increments in the duration of non-susceptibility need
to be selected in order to demonstrate a fertility differential of at least one
live birth.
A simple formula demonstrates the approximate relationship between
fertility and the length of the period of post partum non-susceptibility. If
we set
a as the sum of
the waiting time to conception and
the gestation period,
k as the length of non-susceptibility,
R as the reproductive span, and
F as the average fertility,
This trivial example ignores such factors as fetal loss.
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M G SANTOW
A further set of simulations tested the effect on fertility of the use of a
distribution of non-susceptibility rather than a constant duration. Neither
average fertility, nor the width of the corresponding 95 per cent confidence
interval, was affected by the change in the form of the non-susceptibility
input. As the duration of non-susceptibility increases it tends to dominate
the other factors which contribute to the determination of ultimate fertility.
This occurs because the proportion of the time expended in producing a live
birth which is contributed by the non-susceptible period is, in terms of the
analytic model,
&-1-s.
(2)
Consequently it is this factor, rather than the use of a constant duration of
post partum non-susceptibility, which is responsible for the shrinking of the
confidence intervals.
6.
PRELIMINARY TESTING-CAFN 1 DATA
The next task assigned to the model was to attempt to simulate the
fertility of the CAFNl sample. This survey sought no information on
breastfeeding, but we have seen that the proportion of women still breast-
feeding exceeds the proportion still abstaining only during the first six
months post partum of the richer Ibadan women (NF2-3). Caldwell and
Caldwell [6] noted that
in rural areas, the period of abstinence is shorter than the period of lactation even now in
only about one case in twenty where the child survives, and further investigation usually
reveals that such atypical behaviour is exhibited only by persons who have broken
substantially with the traditional culture.
Consequently, in all but the most exceptional cases it is abstinence, rather
than post partum amenorrhea, which provides the greatest post partum
protection against conception, as the duration of lactation exceeds that of
post partum amenorrhea.
Two simulations were performed which incorporated the CAFNl did
abstain and should abstain distributions, and the CAFNl estimated
distribution of female age at marriage. The 95 per cent confidence intervals
around the average completed fertilities of these runs were, respectively,
(5.14,5.42) and (5.25,5.54), and their means were 5.28 and 5.40 live births.
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109
An attempt was then made to estimate completed fertility directly from
the 400-odd CAFNl respondents in the 45-59 age group who stated that
they had never used any contraception.* This condition removed nearly 10
per cent of the women in this age group, but was deemed necessary because
the only artificial constraints which the simulation imposed on fertility
were the age at marriage and the length of post-natal abstinence. Despite
this adjustment, the fertility estimate is still a crude one because it embodies
the assumptions that the respondents have completed their childbearing,
that they have reported their reproductive histories with complete accuracy,
and also that the fertility of these fifteen birth cohorts remained constant.
The fertility estimated in this way was 5.15 live births. Slightly over half
of these women were in the 50-59 age group, and their average fertility was
5.39 live births. This differential may be caused by the falsity of the
assumption that women in the 45549 age group have completed their
childbearing, or by a decline in cohort fertility.
It is encouraging to find such agreement between the simulated and
reported average fertilities. However, one is confronted with an obstacle
when one tries to make sense of the recorded age-specific fertility data,
because of the unreliability of the age data on which they are based (see
Fig. 2). As an alternative approach, therefore, the distribution of live births
was estimated from the testimony of the CAFNl women in the 4549 age
group.
This distribution is compared with the two simulated distributions in Fig.
5. The modal frequency of the reported distribution is lower than those of
the simulated distributions, and the range is slightly greater. Moreover, the
reported distribution is characterized by jagged irregularities caused by the
tendency to report an even number of children. Except for the percentage
of women who reported nine births, the percentage of women who reported
an odd number of births is less than the percentages who reported either
one more or one less than this number.
A similar preference for even parities was demonstrated by the responses
in another survey to a question on desired family size (Santow [23]). It
appears that the Yoruba find it easier to conceptualize a family of an even
number of children than an odd number, and that older women, at least, do
not perceive a question on achieved parity as being obviously less theoreti-
cal than a question on desired parity. A simple analytic treatment demon-
strates the effect of this even preference on the reported completed fertilities
*Contraception here includes the use of charms or medicines sold by a native doctor,
rhythm, withdrawal, condoms, jellies, creams, sQline pessaries, douching, diaphragm,
foam, internal ring, orals, I.U.D., the sterilization of either partner and abortion. See
Caldwell and Caldwell [5].
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M. G. SANTOW
FIG. 5.
Distribution
of live births of real and simulated populations with long periods
of post partum sexual abstinence.
of fertile women. If we set
i as the number of births where i = 1,2,..,n for n=2k+ 1,
4
as the number of women with i births,
n, as the number of women with i births who report i 1 births,
n,- as the number of women with i births who report i - 1 births, then
the average fertility is
We assume that ni+ = nip =0 when i is even, and that the extent of
individual misreporting does not exceed one birth. Then the reported mean
fertility, FR,
can be expressed as
4)
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MICROSIMULATION OF YORUBA FERTILITY
Consequently, FR F when
111
(5)
Thus, if the total number of odd-parity women who report one extra birth
equals the number who report one fewer, that is, if the direction of the
distortion is random, then the average reported fertility will still represent
the average achieved fertility.
A stronger condition for equality is that
nZ+
I = nzi+3
for i=O, l,..,k- 1.
(6)
7. BREASTFEEDING, ABSTINENCE AND FERTILITY-NF2
DATA
The sizes of the NF2 sub-samples preclude the direct estimation even of
average completed fertility. On the other hand, the distributions of post
partum non-susceptibility to conception are no more irregular than that
derived from the CAFNl data. Indeed, this latter distribution is similar to
the abstinence distribution obtained from the poorer urban sample (NF2-2).
TABLE 4
Age group
Simulated Age-Specific Fertility per loo0 Women*
NF2-1
NF2-2 NF2-3
Lactation Abstinence Lactation Abstinence Lactation Abstinence
10-14
lSl9
20-24
25-29
3 34
35-39
4549
5 54
18 20
455
376
912 814
247 271
1369 1010
1990 1521
1610 1566
1862 1258 1902 1421 2515 2319
1598 1105
1618
1293
2143 1952
1122
810
1145
897
1433 1394
458 362
426
394
527
474
82 65
81 67 105
97
4
3
6 7
7
6
Total
6950
4989 8098
6434 8587
8079
Mean post partum
14.6
29.9 14.0
23.6 6.7
8.4
infecundable period
Mean marriage
age
22
22 18
18 22
22
Modalnumberof 7 6 9 7 10 8
live births
Using NF2 data on distributions of lactation and abstinence.
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M G SANTOW
Table 4 presents the age-specific fertilities of the six simulations of the
NF2 sub-samples which incorporate independently the distributions of
lactation and abstinence as depicted in Fig. 3. A number of inferences may
be drawn from the table. The mean lengths of lactation amenorrhea are
almost the same for the rural and poorer groups (NF2-1 and NF2-2
respectively), but the women of the latter group marry about four years
earlier than those of the former. This earlier marriage age adds, on average,
over one live birth per woman to the completed fertility of the first group.
Similarly, the rural and richer women (NF2-1 and NF2-3) marry, on
average, at the same age, but the richer women are amenorrheic for less
than half the period which is usual amongst the rural women. This has an
even greater effect on fertility than the first comparison, as the richer
women each produce over one and a half more live births, on average, than
the rural women.
When one examines the simulations which incorporate post partum
sexual abstinence, one finds that the marriage differential between the rural
and poorer samples, combined with the shorter period of abstinence of the
latter, produces an extra one and a half live births. Moreover, the richer
women each produce about three mole babies than the rural ones because
they abstain for a much shorter period. In comparative terms, this fertility
increase is 62 per cent of the fertility of the rural women.
Figure 3 reveals that, amongst the NF2-1 and NF2-2 women, the
importance of the duration of breastfeeding is everywhere negated by the
duration of sexual abstinence. However, in the NF2-3 sample it is the
duration of nursing which is the dominant factor for the first few months
post partum. Having demonstrated the individual effects of the distributions
of lactation and sexual abstinence on the fertility of the NF2 sub-samples,
we next aggregate these distributions to produce just one distribution of
post partum non-susceptibility for each sub-sample. For the NF2-1 and
NF2-2 samples, therefore, this new distribution is identical with the old
abstinence distribution, but a truly new distribution was constructed for the
NF2-3 sample from Fig. 3 by taking successively the minimum percentage
of the distributions of abstinence and lactation amenorrhea.
Figure 6 presents the live birth distributions of these three new NF2
simulations. There is a very clear shift from the steepest distribution of the
NF2-1 sample to the flatter one of NF2-3, and a corresponding increase in
the range of possible completed family sizes.
Table 5 compares the age-specific fertilities of the runs shown in Fig. 6
with those of a set of new simulations which incorporate terminal absti-
nence. As the input data provide for only 1 per cent of women to be
terminally abstinent by the age of 34, one looks for the first effects of such
abstinence in the fertility of the 35-39-year-olds. Indeed, there is a drop of
about 10 per cent in each of the three runs in this age group, but in the
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MICROSIMULATION OF YORUBA FERTILITY
113
FIG. 6. Distribution of live births of the three Yoruba simulations.
40-44 age group the declines are respectively 47 per cent, 30 per cent and 32
per cent.
The variations in the fertility decreases exemplify once again the dif-
ferential erosion of a traditional practice, for although the fertility of the
urban women has declined, the extent of the decline is not as great as that
of the rural women. The mean completed fertility of each run is signifi-
cantly lower at the 0.05 level than that of its parent run which does not
TABLE 5
Simulated Age-Specific Fertility of NF2 Sub-samples Showing the Effect of Terminal Abstinence
Original runs
NF3 Terminal abstinence
Age group
NF2-1 NF2-2
NF2-3 NF2-1 NF2-2
NF2-3
10-14 0 20 0
0 17 0
15-19 376 814 251
369 798 228
2 24 1010 1521 1496
973 1486 1456
25-29 1258 1421 2188
1259 1438 2186
30-34 1105 1293 1923
1139 1241 1875
35-39 810 897 1274
738 798 1146
362 394 520 192 274 352
4549 65 67 83
25 16 39
5G54 3 7 7
0 0 0
Total 4989 6434 7742
4695 6068 7282
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M. G. SANTOW
allow for terminal abstinence, with the falls in mean fertility increasing from
about 0.3 live births for the rural women to 0.4 for the poorer Ibadan
women and to 0.5 for the richer Ibadan women.
8. CONCLUSIONS
The demonstration of the impact on fertility of the length of the period
of post partum non-susceptibility to conception, and the comparison of
simulated and reported CAFNI fertility data, encouraged an attempt to
simulate the fertility of the three groups of Yoruba women, namely, rural,
poorer ibadan and richer Ibadan. The preliminary simulations were refined
by the inclusion of patterns of terminal abstinence appropriate to each
sub-sample.
We may regard the three final simulations as static representations of the
fertility experience of a population at different stages in a process of
increasing Westernization accompanied by a weakening of the force of
traditional practices. Viewed in this way, the simulations possess dramatic
implications: the rural fertility of 4.7 live births increases to the poorer
urban fertility of 6.1 iive births, to the richer urban fertility of 7.3 live births.
In centers other than Ibadan one would expect these figures to be slightly
different because of different patterns of marriage, lactation and sexual
abstinence, but the overall conclusion would not change. As women move
from the villages and towns into the city, they can be expected to produce
nearly one and a half more babies than their sisters who stayed at home. As
the financial situation of these urban women improves they can be expected
to produce, on average, slightly more than one additional child, and this
fertility increase would be even greater but for the fact that these women
marry about four years later than their poorer urban counterparts.
In her study of a central Javanese village Hull [9] found a positive
correlation between the average numbers of ever born and surviving
children, and income. Women in the 25-44 age group were differentiated by
two patterns of abstinence according to income, with lower-income women
abstaining for about five months longer than upper-income women. There
was no difference in abstinence levels by income for women in the 15-24
age group, although the periods of stated abstinence were about six months
shorter, at ten months, than those reported by the lower income women in
the 25-34 age group. However, in contrast with this trend, Hull [9] noted
that a later age at marriage of women with higher levels of schooling was
seen to affect cumulative fertility averages of current 2624 year olds.
That urbanization and economic development lead to a reassessment of
personal goals (or even to the first such assessment) and hence to a
gFor example, Adegbola, Page and Lesthaeghe (1) discovered much shorter periods of
breastfeeding and post partum abstinence in Lagos than Ibadan.
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MICROSIMULATION OF YORUBA FERTILITY
115
reduction in fertility, is a theme which permeates much demographic
work.O If one manages to free ones mind from this preconception, one
need not be puzzled by the existence of a positive relation between eco-
nomic class and urbanization on the one hand and fertility on the other. In
the Nigerian case, increasing urbanization and economic development are
facets of a general process of change which is also affecting the adherence
to customs whose effect is the spacing of births and whose intent is the
maximizing of surviving fertility. One is led to the conclusion that the gap
left by the breakdown of traditional methods of spacing births is ready to
be filled by the efficient use of contraception. In the words of Dow [7],
as such [African pronatal spacing] intentions have not changed greatly over time, con-
traception would represent merely a substitution of means in the pursuit of relatively
constant ends or values.
One might predict that the use of contraception for spacing purposes might
lead to its use for limiting purposes.
An examination of studies on contemporary Nigeria recalls us from such
realms of speculation. The shortening of the abstinence period by means of
family planning was deemed a good thing only by 31 per cent of rural
women (NF2-1) and 16 per cent of poorer urban women (NF2-2), but by 80
per cent of richer urban women (NF2-3). Wares [27] analysis of Ibadan
survey data found that only 16 per cent of all Yoruba women personally
wish to have four or fewer children. Moreover,
universal acceptance of the four-child family would not drastically reduce population
growth rates, but it would represent a significant step towards the modernization of family
size ideals.
On the positive side, however, the proportion of women who have never
used family planning fell from 91 per cent of the rural women and 94 per
cent of the poorer urban women, to 30 per cent of the richer urban women.
While only 16 per cent of CAFN 1 women have ever used modem con-
traception, Caldwell and Caldwell [5] report not only a low dropout rate
amongst users, but also that the level of contraceptive usage has doubled
every four years during the previous twenty. The simulations indicate the
magnitude of the gap left by the breakdown of marital sexual abstinence
which needs to be filled by the use of modem contraceptives if Yoruba
fertility is not to undergo a dramatic increase.
I am grateful to Professor J. C. Caldwell of the Demography Department,
Australi an National University, and to the late Professor F . 0. Okedgi of the
OSee.Robinson [22] and Caldwell [4].
Lucas and Ukaegbu [12] cite a comparable figure of 3 per cent of the rural Ngwa
Ibo.
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M. G. SANTOW
Soci ology Department , Uni versig of I badan, for permi ssi on t o use data from
t he N i geri an surveys of t he Changi ng A fri can Famil y project . I am grateful t o
t he ormer for permi ssi on t o use data rom t he Ni geri an Fami ly Proj ect surveys,
and to M r. I . 0. O rubuloy e, al so of t hi s department , f or permi ssion t o quote
data from his Fertility and Fami Limitation surveys.
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