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Biological and behavioral determinants of fertility in Tierra del Fuego.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 127:105–113 (2005)
Biological and Behavioral Determinants of Fertility in
Tierra del Fuego
J. Pascual,* C.E. Garcı́a-Moro, and M. Hernández
Unitat d’Antropologia, Facultat de Biologia, Universitat de Barcelona, 08028 Barcelona, Spain
KEY WORDS
Chile
human fertility; path analysis; proximate determinants; biodemography;
ABSTRACT
The reproductive history of 182 women in
postreproductive life or near menopause from the Chilean
part of Tierra del Fuego was traced back by means of
familial interviews. These postmenopausal women represent the population since almost the beginning of the
settlement, and their reproductive years were spent on
the island. Path analysis was applied to analyze fertility
determinants of these women and to propose a complex
model of interconnections among factors. The reproductive history of these women is characterized by a long
fertile span, a short childbearing period, and low fertility.
Age at menarche is relatively late, and the age of the
women at first birth is mainly determined by their late age
at marriage. The use of contraception is related to both
spacing and stopping behaviors. The late age of women at
marriage, the rhythm of conception, and practices of contraception are proposed as the main determinants of fertility in Tierra del Fuego. Am J Phys Anthropol 127:
105–113, 2005. © 2004 Wiley-Liss, Inc.
An approximation to the analysis of fertility determinants in a human population is a multivariate
study of the connections among several events of the
reproductive histories of women. Correlation coefficients are often used to estimate the connection between several factors, even though multivariate
techniques such as hazards analysis are in use. Most
studies of fertility in human populations choose a
few factors estimating the correlation matrix; after
that, a consistent explanation of results is written by
the researchers. These results can be analyzed in an
independent way, but they do not provide an overall
view of the reproductive model of the population.
One of the goals of this article is to design a complex
model of interconnections that incorporate various
factors considered in the reproductive history of
women surveyed in Tierra del Fuego. This model
should explain as much as possible the interconnections among factors, using the method of path analysis.
Geneticists and breeders have long known and
have been using the method of path coefficients developed in 1920 –1921 by the pioneer population geneticist Sewall Wright. In last decades, the path
analysis method has been used in population genetic
studies and other anthropological sciences, especially in the estimation of both heritability and family transmission of characters (Salces et al., 2001).
Moreover, the method of path analysis is an interesting procedure to build and evaluate models about
connections among factors. Nevertheless, the use of
path analysis in fertility studies of human populations is rare (Crognier, 1996; Westoff, 1990; Loebner
and Driver, 1973) and, in general, in biodemography
studies (Shen and Williamson, 1999; Gober, 1997;
Clay and Vander Haar, 1993). Recently, Madrigal et
al. (2003) concluded that heritability of fertility is
moderated, and they used path analysis to show
that the anthropometric biological characteristics of
mothers explain small variability in a population’s
fertility. Factors that better explain differences in
the fertility of women in Tierra del Fuego should be
found among other biological and sociocultural characteristics of the population.
This biodemographic study of the population of
the Chilean part of Tierra del Fuego attempts to
establish a fertility model. A statistical method that
allows us to integrate an important number of variables and to define both a consistent and a structured model was used.
©
2004 WILEY-LISS, INC.
POPULATION AND METHODS
The Chilean province of Tierra del Fuego is situated at the southern tip of the South American continent (52.5–55° latitude), which conditions its environmental and climatic characteristics. The Chilean
territory of Isla Grande de Tierra del Fuego was
*Correspondence to: Joel Pascual, Unitat d’Antropologia, Facultat
de Biologia, Universitat de Barcelona, Avda. Diagonal 645, 08028
Barcelona, Spain. E-mail: joelpascual@ub.edu
Received 6 July 2003; accepted 12 February 2004.
DOI 10.1002/ajpa.20065
Published online 31 August 2004 in Wiley InterScience (www.
interscience.wiley.com).
106
J. PASCUAL ET AL.
inhabited by the aboriginal Selk’nam populations
(Garcı́a-Moro et al., 1997a) until the end of the 19th
century, when a quick reduction in these populations and their near-extinction happened with the
arrival of European expeditions. The colonizing population arrived, at the end of the 19th century, from
other Chilean regions (particularly Chiloé) and diverse European countries, especially Britain and
Croatia, but also Germany, Spain, and Italy. The
majority of inhabitants live in Porvenir, the administrative capital of the province (2002 census: 6,904
inhabitants). From the beginning of the settlement,
this community has been a semirural developing
population, where ranching activity was predominant (Pascual et al., 2000).
A survey was carried out in the austral spring of
2000 in Porvenir. The reproductive history of women
was traced back by means of familial interviews,
including personal information, data on stable
unions, and biological characteristics of the women
and information on their children. Women were selected randomly, and all neighborhoods and social
classes of Porvenir are represented. The women surveyed are predominantly of Chilean origin, mainly
from Tierra del Fuego, other locations of the Magellanic Region, and the Chiloé Islands. They show a
semirural economy (most are from families dedicated to ranching activity), and they have a general
low level of education (68% of the women studied
less than 7 years of school, and only the 6.6% of the
women had university studies). Interviewed women
met two previous conditions: 1) to be more than 45
years old at time of survey (they were born between
1912–1956, so the population since almost the beginning of the settlement is represented), and 2) to
be born in Porvenir or to have lived there since
childhood or youth, so that their reproductive period
was spent in Tierra del Fuego. Local authorities
gave support to the researchers to obtain a census of
women who fit the study criteria.
The possibility to subdivide women into several
subgroups to analyze temporal trends of factors or
changes in patterns exists, but the reduction of sample size increases the heterogeneity of variables;
assuming this, path diagrams would be less consistent and, therefore, moot structures. Thus, we decided to keep together the whole group of surveyed
women.
The method of path coefficients (Li, 1975) is a form
of structured linear regression analysis with respect
to standardized variables in a closed system. To
apply this method, the first step is to create a list of
biological, demographic, and sociocultural factors
that will be considered in the analysis. In this case,
we made a special effort to choose the most important and direct factors a priori, and this selection
was based on the knowledge of the population and
factors suggested by the literature. After that, a
structure concerning the interconnections among
variables is assumed. To do this, factors were classified according to the time sequence in reproductive
life of a woman, and finally, we decided on paths and
directions of the connections. Low multicollinearity
among variables is indispensable for including several factors in the same path. When the path diagram is done using correlations among factors, the
mathematical analysis quantifies with different coefficients (path coefficients) the importance of every
connection established in the model. To work with
correlations assumes that variables included in diagrams are normally distributed; however, most factors related to fertility are non-normal variables. In
this case, the strength of results will depend on the
factors’ involvement in women’s fertility.
The path coefficient is defined as the ratio of the
total variability that can be explained by only one
factor when all the rest of the variables are unchanged (Li, 1975). In other words, a path coefficient
represents the net influence of a variable on another
one. The residual (r) includes heterogeneity not explained by factors (not confounded with the correlation coefficient).
The squares of the path coefficients measure the
degree of determination by each factor (direct determination), but an indirect determination of each
factor can also be quantified by means of the other
variables considered (combined determination).
Then the direct determination quantifies the influence of variable A on variable B across the considered path. At the same time, A can influence B
across other different paths, and this is quantified
by means of the combined determination. The unexplained variability (1 ⫺ total determination) can be
estimated when the total determination (defined as
the sum of both direct and combined determinations) is known.
The correlation between two variables is the sum
of all connecting paths between them. Then, a correlation can be decomposed into various components
(path coefficients), each corresponding to a path. The
path coefficient coincides with correlation coefficient
only when you consider one component.
In summary, path analysis enables us to 1) hypothesize a complex causal structure, 2) draw a visual display of the model, 3) calculate the strengths
of the various connections, and 4) insert their coefficients into the causal diagram. Moreover, the
method provides a general procedure to explore the
indirect effects of a determining variable on a dependent variable in a multivariate path model, which
seems more informative than partial correlation
analysis (Loebner and Driver, 1973).
On the other hand, whenever we assume a structure about the interconnections among a number of
variables, it is unavoidable to involve some degree of
subjective judgment: two investigators, given the
same data on the same variables, may very well
come up with two different understandings of the
variables and propose two different path diagrams
for analysis. Hence, the most important disadvantage of path analysis is the dependence on an a
priori causal scheme (Loebner and Driver, 1973).
107
FERTILITY IN TIERRA DEL FUEGO
TABLE 1. Abbreviations of variables in text
Variable
Abbreviation
Variable
Abbreviation
Menarche
Age at marriage
Mean birth interval
Age of mother at first birth
Age of mother at last birth
Childbearing period
M
AAM
MBI
AMFB
AMLB
CP
Years using contraceptives
Years of fertile marriage
Pregnancies
Abortions
Number of live births
CONTRAC
YFM
P
ABOR
NLB
However, path analysis requires that results must
be consistent throughout the structure and compatible with the observed data on all variables involved
in the structure (Li, 1975).
Twelve variables of the reproductive life of women
were initially considered in the analysis, and abbreviations used in text are expanded in Table 1.
Among the diversity of schemes which explain interrelationships among variables, two path diagrams
were selected, considering the proposed connections
in the literature and the considerable percentages of
variability that they explained. It is possible that
some connections among variables suggested in the
literature are not expressed in these particular path
diagrams. This is because they are less quantitatively important than others in Tierra del Fuego,
and the absence of arrows ensures no connection in
the diagram. These unconsidered connections are
included in indirect paths and residual components.
We used SPSS 10.0 and Mathematica 4.0 to estimate statistical and algebraic values.
The age at menarche and age at menopause were
estimated by a retrospective method. The limitations of this method were described elsewhere (Madrigal, 1991). However, several studies confirmed
that women remembered their menarcheal age with
a minor error (Bergsten-Brucefors, 1976; Tryggvadóttir et al., 1994). Therefore, a 0.5-year midpoint
correction factor was added to the reported age,
when estimating both mean age at menarche and
menopause.
RESULTS
Main reproductive characteristics
The main events that determine the reproductive
life of postmenopausal women in Tierra del Fuego
are shown in Table 2. The age at menarche and age
at menopause were recently analyzed in Pascual et
al. (2003). Their main conclusion is that women from
Tierra del Fuego show a long fertile span, even
though the mean age at menarche is relatively late.
Moreover, 32.9% of women lost their fecundity due
to pathological or voluntary surgery, and these
women cannot be included in the sample of the fertile span. The natural fertile span, estimated as the
difference between age at menopause and age at
menarche, is 34.12 years; nevertheless, the childbearing period is more informative than the fertile
span because this is the real period dedicated to
reproduction. The childbearing period, estimated as
the difference between the age of women at first and
TABLE 2. Main reproductive events in reproductive histories of
women from Tierra del Fuego1
Variables
Mean
SD
N
Women’s age
Age at menarche
Age at marriage
Age at first birth
Age at last birth
Age at menopause
Fertile span
Childbearing period
Birth interval
Pregnancies
Live births
62.06
13.85
23.03
23.66
32.11
48.38
34.12
8.89
3.69
3.77
3.31
11.35
1.59
5.22
5.39
6.29
4.85
4.86
5.53
2.75
2.15
1.91
168
162
159
156
155
99
73
147
142
168
168
1
All measures are estimated in years.
last birth, is 8.89 years. Thus a long theoretical
fertile span does not entail high fertility, because the
practical reproductive span is short.
In a multifactorial view, the number of live births
in a woman’s reproductive life is determined by a
few proximate variables (Bongaarts and Potter,
1983). Late age at marriage of women reduces by 10
years the theoretical period to bear children. The
age of the mother at first birth is much related to age
at marriage. Both variables are very close, because
intercourse before marriage is frequent in Tierra del
Fuego, and in many cases marriage takes place after
a period of cohabitation. On the other hand, the
mean length of birth intervals is relatively long and
the age of the mother at last birth is too early for a
natural fertility population (Bongaarts and Potter,
1983). The extensive use of contraception and the
importance of definitive sterilization are suggested
by these results.
Fertility is relatively low in the Chilean part of
Tierra del Fuego. Postreproductive women experience an average of 3– 4 pregnancies, and they have
an average of three live births. The total fertility
rate (number of live births per woman) is 3.3 (including 7% of childless women), and the total marital fertility rate (number of live births per married
woman) is 6.4 births per woman. In summary, this
low fertility could be the consequence of a late age at
marriage, a short childbearing period, and the use of
contraceptives.
Table 3 shows the correlation matrix between
variables considered in the model. Path coefficients
were estimated from these values. Suggested connections among factors and their associated path
coefficients are represented in Figures 1 and 2 and
Tables 4 and 5. Models 1 and 2 are similar, and they
only differ in the final section of the diagrams.
108
J. PASCUAL ET AL.
TABLE 3. Intercorrelations among variables assumed to determine number of offspring
M
M
AAM
MBI
AMFB
AMLB
CP
CONTRAC
YFM
P
ABOR
NLB
AAM
MBI
AMFB
AMLB
CP
CONTRAC
YFM
P
ABOR
NLB
⫺0.109
⫺0.084
⫺0.034
0.131
0.766
0.028
⫺0.038
0.452
0.499
0.528
⫺0.155
⫺0.214
0.516
⫺0.348
0.628
0.048
⫺0.255
0.043
⫺0.314
⫺0.277
0.018
0.219
0.250
0.162
0.210
0.038
0.255
0.343
0.014
⫺0.251
⫺0.237
⫺0.310
0.214
⫺0.471
⫺0.085
0.150
⫺0.061
⫺0.035
0.043
0.030
⫺0.024
⫺0.080
⫺0.113
0.079
0.383
⫺0.006
⫺0.245
⫺0.289
⫺0.347
0.243
0.544
⫺0.062
0.164
0.923
0.045
TABLE 4. Model 1: degrees of determination by variables of
reproductive history of women
Determination
AMFB
Age at menarche
Age at marriage
AMLB
AMFB
MBI
CP
Contraceptives
AMLB
Pregnancies
Age at marriage
CP
Fig. 1. Model 1. Path diagram shows suggested interconnections and path coefficients among factors.
Determination
Path models of fertility
The first event of the childbearing period of a
woman (i.e., age of mother at first birth; AMFB) is
almost completely determined by two parameters of
a different nature: age at menarche (a biological
trait), and age at marriage (a sociocultural trait).
Nevertheless, age at marriage is clearly the most
explanatory factor, because it explains 76.6% of the
variability, while age at menarche explains only
5.1% of the variability. This may be related to the
late mean age at marriage of these women, which
moves the biological event that marks the beginning
Combined
Total
Unexplained
0.051
0.766
⫺0.022
0.795
0.205
0.264
0.235
0.014
0.513
0.487
0.043
0.471
⫺0.079
0.435
0.565
0.138
0.331
⫺0.092
0.377
0.623
TABLE 5. Model 2: degrees of determination by variables of
reproductive history of women
AMFB
Age at menarche
Age at marriage
AMLB
AMFB
MBI
CP
Contraceptives
AMLB
Pregnancies
Age at marriage
YFM
CP
Fig. 2. Model 2. Path diagram shows suggested interconnections and path coefficients among factors.
Direct
Direct
Combined
Total
Unexplained
0.051
0.766
⫺0.043
0.774
0.226
0.264
0.235
0.014
0.513
0.487
0.043
0.471
⫺0.079
0.435
0.565
0.107
0.042
0.352
⫺0.111
0.389
0.611
of fertile life away from the next biological event of
the reproductive history of a woman (age at first
birth, that marks the beginning of childbearing period). Only 20.5% of the variability of AMFB is unexplained by these two factors (see Tables 4 and 5).
Two cuasi biological variables, maternal age at
first birth and mean birth interval, explain together
more than 50% of the variability of the age of a
mother at last birth (both menarcheal and nuptial
ages explain little additional variability). The beginning of the childbearing period of a woman and the
rhythm of conception of the offspring (mainly determined by breastfeeding period, the use of contraceptives, and either temporal or definitive spousal separation) determine maternal age at last birth. Path
coefficients are very similar to correlation coefficients, which show that these two factors act in an
independent path. In a general way, a woman would
109
FERTILITY IN TIERRA DEL FUEGO
have last birth at a late age if she had her first birth
at a late age, and her children are separated by long
birth intervals. In the same way, a woman would
finish her childbearing period at early age if she had
her first birth at an early age and her mean birth
interval was short. The residual value indicates the
importance of unconsidered determinants in the
variability of maternal age at last birth.
The length of the childbearing period is mainly
determined by the age of the mother at last birth
and the use of contraceptives, in such a way that the
higher the age of a mother or the use of contraceptives, the longer the childbearing period. The length
of childbearing period depends more on the endpoint
of the interval (age of mother at last birth explains
47.1% of the variability) than the beginning of the
span (age of mother at first birth could explain directly only 12.11%). A path diagram that includes
simultaneously both a direct effect of AMFB and a
direct effect of AMLB would be inconsistent due to a
low multicollinearity among variables; this is because the length of the childbearing period is not a
natural event of women, but is a variable defined as
the difference between AMLB and AMFB. Contraception is an important determinant of the length of
the childbearing period. Thus its use could be not
only a stopping strategy, but also a spacing control
action.
Determinants of the number of pregnancies are
different in both models presented here. The age of
marriage and the length of the childbearing period
explain 37.7% of the variability of the number of
pregnancies in model 1. The negative sign of path
coefficients indicates an opposite connection between variables. In general, women who married at
late ages and who enjoyed a long childbearing period
would have a number of live births lower than those
women who married early and who had a short
childbearing period. In this sense, a long separation
between first and last birth does not provide a
greater number of children. In other words, the beginning of married life, the rhythm of conception,
and maternal age at last birth are the main determinants of the number of pregnancies of Tierra del
Fuego’s women.
The age of marriage, the fertile years during marriage, and the length of the childbearing period are
the main determinants of fertility in model 2. The
three variables explain 38.9% of the variability of
the number of pregnancies. Differences between
models are low. Model 1 considers the event that
defines the beginning of a stable connection (age at
marriage), whereas model 2 adds the duration of
marriage (considering definitive separation of
spouses, divorce, widowhood, repeated marriages, or
definitive loss of fecundity by surgery).
Moreover, the sterile age for women (age when
women experience a natural or an artificial menopause) was added as the most explanatory factor of
the variability of fertile years during marriage. In
this sense, the endpoint (and not the beginning of
TABLE 6. Analysis of variance showing influence of age at
marriage in number of live births1
Age at marriage
⬍20 years
20–24 years
25–29 years
ⱖ30 years
Mean of NLB
SD
N
3.89
3.48
3.37
2.38
1.96
1.89
1.46
1.0
56
52
35
16
Difference between means is significant (P ⬍ 0.05) between
women married before 25 years old and women married after 30
years old.
1
the marriage) explains the variability of the fertile
years during the marriage. Differences in both age of
marriage and birth interval are the most explanatory factors of the sterile age for women who experience an artificial menopause or sterilization (explaining 73.4% of variability), but these factors only
explain roughly 24% of the sterile age of women with
natural menopause.
Finally, the number of pregnancies of a woman is
obviously the main determinant of her number of
live births, because it explains 85.2% of the variability. Abortions and stillbirths explain the residue.
Additional results
Several additional analyses were included to complete the general view of the influence of some factors in the fertility of the population. First, an analysis of variance that shows the influence of age at
marriage in the number of live births is given in
Table 6 (F ⫽ 3.145; df, 3,155; P ⫽ 0.027). Women
with late age at marriage have fewer live births than
women who get married early.
On the other hand, women who live with less than
10 years of fertile marriage (3.28 live births, SD
1.20, n ⫽ 32), women who live from 10 –19 years
(3.43 live births, SD 1.57, n ⫽ 30), and women who
live with more than 19 years of fertile marriage (3.82
live births, SD 2.14, n ⫽ 44) do not show statistical
differences in their fertility (F ⫽ 0.970; df, 2,103;
P ⫽ 0.383). Finally, Table 7 shows no significant
differences (F ⫽ 0.754; df, 2,116; P ⫽ 0.473) in age of
the mother at last birth according to definitive loss
of fecundity of women. There are no significant differences (F ⫽ 0.700; df, 2,125; P ⫽ 0.498) in mean
number of live births by reason for becoming infertile.
Finally, the effect of contraception in fertility of
women from Tierra del Fuego is pointed out with
two simple comparisons. First, birth intervals where
women use contraception show a mean length of
4.068 years (SD 3.439, n ⫽ 94), and birth intervals
where women do not use contraception show a mean
length of 2.099 years (SD 1.416, n ⫽ 191). The differences are statistically significant (t ⫽ 5.335; df,
283; P ⫽ 0.000). On the other hand, the fertility of
women who affirm the use of contraceptives (X ⫽
3.33, var ⫽ 1.85, n ⫽ 114) is not significantly different (t ⫽ 0.614; df, 154; P ⫽ 0.540) from the fertility
110
J. PASCUAL ET AL.
TABLE 7. Mean age of mother at last birth and mean number of live births according to reason for definitive end of fecundity
Age at last birth
SD
N
NLB
SD
N
32.55
32.31
30.90
6.84
6.45
4.44
66
22
31
3.27
3.09
3.65
2.11
1.31
1.38
74
23
31
Natural menopause
Menopause due to surgery
Sterilization
of women who do not use them (X ⫽ 3.60, var ⫽ 7.42,
n ⫽ 42).
DISCUSSION
The recent origin of the colonizing population of
the Chilean part of Tierra del Fuego, together with
the socioeconomic characteristics of this austral population (semirural economy, and general (female)
low level of education), lead us to think that the
studied population keeps similar reproductive patterns from other colonizing populations or developing societies (such as early age at marriage, a long
childbearing period, or high fertility) (Bongaarts
and Potter, 1983; Cleland et al., 1984; Wood, 1994;
Castro Martı́n and Juarez, 1995). Instead of this
preliminary assumption, results confirm that the
population of Tierra del Fuego has experienced a
swift modernization of these reproductive patterns
throughout the 20th century, which place the population in an advanced phase of the fertility transition (Bongaarts and Potter, 1983).
The age at menarche in human populations is
described as between 11–15 years, with maximum
values of 17 years in poor nutritional populations
(Livi-Bacci, 1993; Thomas et al., 2001). Early ages at
menarche are more common in developed populations, where both nutritional and sanitary conditions are better. The age at menarche of Tierra del
Fuego women is one of the latest ages described in
South America. South American populations of European origin show earlier ages at menarche than
native populations, due to the genetic pool or better
socioeconomic development (Pascual et al., 2003;
Garcı́a-Moro et al., 2000; Greksa, 1990; DankerHopfe, 1986). In contrast, human populations show
mean ages at natural menopause between 42–54
years (Leridon and Menken, 1979; Leidy, 2001).
Mean age at natural menopause of women from
Tierra del Fuego is a central value inside the range.
The average length of fertile span was described in
human populations as between 26.8 years in Agta
women from the Philippines (Goodman et al., 1985)
and 35.9 years in a sample of Minnesota women
(Treloar, 1974). Women with natural menopause
from Tierra del Fuego show a long fertile span (34.12
years). Nevertheless, an important proportion of
women (32.97%) reduced their fertile span by
36.08% due to surgery, with an expected strong consequence in their final fertility. However, this repercussion is not observed in the population, as we
show later.
Age at marriage is considered one of the main
determinants of fertility, especially in natural fertility populations (van de Walle, 1978; Varea, 1993).
Age at marriage is a significant determinant of the
duration of marriage, and a close connection between duration of marriage and fertility is frequently described in human populations (Busfield,
1972; Bumpass and Mburugu, 1977; Bongaarts and
Potter, 1983; Aryal, 1991). This connection is not
significant in Tierra del Fuego. When this connection exists, it becomes an indirect sign of nonexistent or infrequent contraceptive practices, in such a
way that women have children during all their reproductive years (Loebner and Driver, 1973). In fact,
duration of marriage can be considered an indirect
measure of the exposure to risk of pregnancy, in
populations where illegitimate children are scarce
(Josephson, 2002). On the other hand, maternal age
at marriage is also important in these populations,
because a woman’s reproductive capacity is greatest
in her late teens and early twenties. Age at marriage
is an important determinant of fertility if it is not
late and the time lag between wedding and cohabitation is short (Loebner and Driver, 1973). The age
at marriage of women from Tierra del Fuego is related to their final number of live births (Table 5),
but this connection is not seen across the duration of
marriage. The comparison of total fertility rate (3.3)
and total marital fertility rate (6.4) shows that the
specific features of marriage are one of the main
determinants of fertility in Tierra del Fuego. The
late mean age at marriage and the frequent breakup
of marriage reduce the potential fertility.
Age of the mother at first birth is mainly determined by sociocultural regulations in the majority of
developed societies and other rural populations like
Aitourir, Morocco (Salvat and Crognier, 1993), or
the Mapuche population of Rı́o Grande, Argentine
(Crognier et al., 1996). Menarcheal age or high levels of fecundability were related to the beginning of
the childbearing period as well (Komlos, 1989; Udry
and Cliquet, 1982). The age at marriage is the main
factor which determines the maternal age at first
birth in women from Tierra del Fuego.
On the other hand, most of the childbearing periods in human populations have been described as
between the 5.2 years of Goessel Mennonites (USA)
(Stevenson et al., 1989) and the 16.64 years of the
Amizmiz population (Marrakech, Morocco) (Varea,
1990). Among South American populations, lengths
of childbearing period of 13 years, 13.1 years, and
14.95 years were described, respectively, in the
Shipibo Peruvian Amazon (Hern, 1994), the Aymara
peasant society of the Bolivian Altiplano (Crognier
et al., 2002), and Mapuche women of the province of
Rı́o Negro (Argentine) (Crognier et al., 1996).
Women from Tierra del Fuego show a relatively
FERTILITY IN TIERRA DEL FUEGO
short childbearing period if the length of the interval
is compared with the long fertile span. In fact,
women from Tierra del Fuego only use 26.05% of
their fertile span, to reach 3.3 live births per woman.
Available data in other human populations show
that women from Maragateria (Spanish rural region; Bernis, 1990) use 30% to bear 4.4 live births,
and women from Amizmiz (Morocco; Varea, 1993)
use 59% to have 8 live births. The mean length of the
childbearing period is closely related to both long
birth intervals and low fertility.
The relatively widespread use of contraceptives in
the Chilean population of Tierra del Fuego was unknown before this study. Contraception is widely
extended in developed populations (Frejka and Ross,
2001), but this is not the case in developing societies,
as studied elsewhere (Fort, 1989; Parker and Ross,
1992). Their use was described both as a stopping
behavior (to limit the size of progeny) and as a spacing behavior (to separate births).
The study of the age of the mother at last birth
provides information about the stopping behavior of
the population. The mean age at last birth is remarkably invariant in natural fertility populations.
With few exceptions, the means fall in the 39 – 41year range (Bongaarts and Potter, 1983; Desjardins
et al., 1994). These results suggest that the timing of
the onset of natural sterility varies little among
populations, except in cases where sexual infectious
diseases are widespread. Then, factors such as taboos against intercourse at older ages (Leidy, 1993),
disease-induced sterility (Pennington and Harpending, 1991; Jenkins, 1993), or the use of contraception
and sterilizations (Leidy, 1999; Barroso et al., 2003)
can vary the values in human populations. In Tierra
del Fuego, women who were sterilized showed an
earlier age at last birth than natural menopausal
women, but the differences were not statistically
significant (Table 6). Nevertheless, the early mean
age of a mother at last birth (even in natural menopausal women) suggests that the use of contraception in Tierra del Fuego limits the number of live
births. On the other hand, with regard to spacing
behavior, the length of birth intervals is longer
(nearly 2 years) for women who use contraceptives
than for women who do not use them. The results
also suggest the use of contraception to space births.
The fertility of women who affirm the use of contraceptives is not significantly different from the
fertility of women who do not use them. Nevertheless, contraception is infrequent in women with a
high number of live births, and women who have
used contraceptives in their reproductive lives
mainly reach a number of three or four live births.
This progeny size is also the most represented in
sterilized women. At the same time, natural menopausal women do not have a greater number of live
births than sterilized women. These results are indirect evidence of the use of contraceptives and of
sterilization to regulate women’s fertility to reach
their desired family size. In conclusion, contracep-
111
tion has become an important determinant of fertility in the population, especially influencing both the
rhythm and the endpoint of the childbearing period.
In conclusion, the fertility model of Tierra del
Fuego moves away from described models of traditional natural fertility populations (such as the
Amish or Mennonites; Felt et al., 1990; Hewner,
1998; Greksa, 2002) or European historical populations (Henry, 1961; Knodel and Wilson, 1981). In
that sense, comparisons with other colonizing populations (Campbell and Wood, 1988) show that rural
Canadians during the early eighteenth century
(Charbonneau et al., 1987), the Hutterites during
the late nineteenth and early twentieth centuries
(Eaton and Mayer, 1953), the Mormons during their
phase of westward expansion in the nineteenth century (Mineau et al., 1979), and the colonizing population of California (Garcı́a-Moro et al., 1997b) share
the main fertility determinants described for those
natural fertility populations. The age at marriage
(the beginning of the marital life) and marriage duration are the main determinants of fertility in this
kind of population. The dissolution of a marriage
before the end of the reproductive period and the
absence of contraception are also considered important determinants of fertility.
On the other hand, among the closest geographical populations, the reproductive patterns of the rural Mapuche population of Rı́o Negro (Patagonian
Argentine) offer the only fertility model known
(Crognier et al., 1996). The use of contraception is
widespread in the population, but is mainly limited
to reproductive years after couples reach the desired
number of children. Furthermore, there is no regulation of fertility access, because marriage is not
universal in the population. Finally, path analysis
was applied in a few studies of human fertility.
Woman’s fertility is largely a direct function of both
age at marriage and duration of marriage in populations from Central India (Loebner and Driver,
1973); in contrast, the birth interval and the span of
reproductive life (defined as the interval between
age at marriage and age at last birth) are the main
determinants of fertility in an Amizmiz Berber population (Crognier, 1996).
Women from the Chilean part of Tierra del Fuego
show a different pattern from natural fertility populations. The duration of marriage was considered
by means of fertile years of marriage, and this factor
only explains 4.2% of the variability in the number
of pregnancies. This result suggests an extensive
and effective use of contraceptives that reduces the
importance of duration of marriage. On the other
hand, the beginning of the marital union is a significant determinant of fertility in many of the cited
populations (including Tierra del Fuego), because
marriage is almost universal and fertility is mostly
limited to these unions. An example of the nonexistence of this connection is the Mapuche population of
the Patagonian Argentine. Finally, Tierra del Fuego
and the Amizmiz populations share some reproduc-
112
J. PASCUAL ET AL.
tive characteristics. Fertility analyses of both populations reveal the importance of the beginning of the
conjugal union, the rhythm of conception, and the
end of the childbearing period.
CONCLUSIONS
In summary, the method of path analysis applied
to analyze the fertility determinants shows a fertility pattern defined by the importance of biological
and sociocultural factors. A late age of women at
marriage, the rhythm of conception, and practices of
contraception limiting and spacing births are the
main determinants of fertility in the Chilean part of
Tierra del Fuego. Our next efforts will study in
depth 1) some aspects of the reproductive histories
of women from Tierra del Fuego, and 2) the quantitative importance of fertility determinants in the
population.
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