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Who keeps children alive?
A review of the effects of kin on child mortality
REBECCA SEAR1
London School of Economics
RUTH MACE
University College London
Running headline: who keeps children alive?
Word count (text): 9,025
1 Corresponding author
Department of Social Policy, London School of Economics
Houghton St, London WC2A 2AE, UK
Tel: + 44 20 7955 7348 Fax: +44 20 7955 7415 Email: r.sear@lse.ac.uk
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Abstract 1
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Children pose a problem. The extended period of childhood dependency and short
inter-birth intervals mean that human mothers have to care for several dependent children
simultaneously. It has long been argued that this is too much of an energetic burden for
mothers to manage alone, and that they must enlist help from other relatives to share the
burden of raising children. Which kin help is the subject of much debate. Here, we review the
evidence for whether the presence of kin affects child survival rates, in order to infer whether
mothers do receive help in raising offspring and who provides this help. These 43 studies
come from a variety of (mostly) natural fertility populations, both historical and
contemporary, across a wide geographical range. We find that in almost all studies, at least
one relative (apart from the mother) does improve the survival rates of children, but that
relatives differ in whether they are consistently beneficial to children or not. Maternal
grandmothers tend to improve child survival rates, as do potential sibling helpers at the nest
(though the latter observation is based on rather few studies). Fathers and paternal
grandmothers, however, are sometimes beneficial to children and sometimes not, suggesting
that help from paternal kin is facultative. Overall, this review suggests that while help from
kin may be a universal feature of human childrearing, who helps is dependent on ecological
conditions.
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Introduction 22
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Human life history suggests that human females do not raise their children without
help. The human interbirth interval of about 3 years in natural fertility populations, is out of
line with that of other great apes of similar body size. The orang-utan, for example, has an
interbirth interval of about 8 years, and the chimpanzee 4-5 years (see Galdikas & Wood,
1990 for a review; Gibson & Mace, 2005). Ape mothers rear their offspring without help. If
human females are capable of such rapid reproduction (albeit possibly with a higher rate of
infant mortality too), most anthropologists agree that this is due to the support they receive
from other family members. The ‘traditional view’ has been that this help comes from the
father – hence the human pair-bond is based on mutual interdependence of husband and wife
to raise their children (e.g. Lovejoy, 1981). In hunter-gatherer societies, the division of labour
is nearly always such that men bring back meat to the band, whereas women gather.
However, the importance of the male contribution to the subsistence of the women and
children has been questioned (Hawkes, 1990). The observation that the number of calories
brought back from gathered foods often exceeds that from hunting, combined with the fact
that meat is often shared widely throughout the band rather than strictly within the nuclear
family (Hawkes et al., 2001; Kaplan & Hill, 1985), has lead to the suggestion that women are
not as dependent on men to raise their family as once thought (Hawkes et al., 1997).
Alternatives to paternal investment are provided by characteristics of human life
history. Unusually, human females spend a relatively high proportion of their lives in a non-
reproductive state. Both pre- and post-reproductive individuals may be available to help
mothers in raising offspring, as they can do so at relatively little cost to their own
reproduction. Grandmothers, in particular, are often proposed as an alternative to male care.
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If grandmothers are helping to support their daughter’s children, then two unusual features of
human female life history – menopause and high birthrates - can potentially be explained in
one go. Both may arise because menopause is an adaptation to enable grandmaternal support,
which in turn enables a high human birth rate (Hawkes et al., 1998). Mothers may also use
the labour of their older children, particularly daughters, to spread the costs of raising
offspring. The extended juvenile period of human young is another unusual characteristic of
our species. Helping behaviour has not been proposed as an explanation for the evolution of
childhood in humans (though it has been argued that the use of children's labour allows
parents to underwrite the costs of high human fertility rates: Kramer, 2005; Lee & Kramer,
2002). Instead, explanations for an extended childhood tend to centre on the benefits of a
lengthy period of growth that are reaped during adulthood, though whether a long childhood
is beneficial primarily in terms of increased adult production (Kaplan et al., 2000; Kaplan et
al., 2003) or reproduction (Bogin, 1997; Charnov, 1993; Gurven & Walker, 2006; Hawkes et
al., 1998) is debated. But whether or not helping behaviour was a significant factor in the
evolution of childhood, this long juvenile period may provide a pool of potential ‘helpers at
the nest’.
Who supports the family in hunter-gatherer societies?
How might empirical studies help us to distinguish between the two views of the
human family: that the pairbond with the father is key, or that other kin, especially
grandmothers, are more important as allocarers? Empirical studies on hunter-gatherer
communities are data-limited, due to both the very small number of such societies that
survive, and the very small number of individuals living in something approaching a hunter-
gatherer lifestyle within those societies. This may have contributed to the fact that a
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consensus view on the relative importance of fathers as compared to grandmothers has not
emerged.
The main line of evidence in this debate came from nutritional studies. Hawkes et al
(1997) point out that in the Hadza of Tanzania, children with older female relatives in their
band are better nourished, and their data suggest that the hunting season is not actually a
particularly good time of year for children. Studies on foraging strategies in the Ache of
Paraguay and in the Hadza highlight the fact that total calories and energy return rates from
gathering often equal or even exceed that from hunting (Blurton Jones et al., 2000; Hill et al.,
1987; Marlowe, 2003). Isotope studies on pre-historical Californians suggest that male and
female diets were so different that they appeared to be almost on different trophic levels
(Walker & Deniro, 1986); the males appeared to have been living almost entirely off marine
resources whereas the females must have been eating food almost exclusively terrestrial in
origin. This suggests that food sharing between the sexes was minimal. But Hill, Kaplan and
others (see e.g. Gurven & Hill, 1997; Gurven & Kaplan, 2006; Hill, 1993; Kaplan &
Lancaster, 2003) have argued that the nature of the food brought back by males is superior
and very important, leading them to conclude that the contribution of males to family
nutrition is very significant. As an extreme example, Arctic hunters like the Inuit are almost
entirely dependent on hunted food brought in by men. In the coldest areas, babies and young
children could barely survive outside for much of the year, and thus females are almost by
definition dependent on their spouse for almost everything. And Marlow (2003) shows that
male provisioning occurs at very important times in the Hadza, such as when a woman’s
foraging is handicapped because she recently gave birth.
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These findings suggest that the ecology of the system influences the relative
importance of fathers, grandmothers, and potentially other kin such as siblings or older
offspring, in the rearing of human children. This should come as no surprise to evolutionary
ecologists. The variability in hunter-gatherer ecology further highlights the fact that data from
just one type of population cannot answer the question of whether humans are co-operative
breeders. We will argue here that it is not necessary or sufficient to restrict our studies to
extant hunting and gathering communities, none of which are necessarily cases of special
importance in human history. Furthermore, very few hunter-gatherer studies can generate
large enough sample sizes to estimate important determinants of rare events like mortality, or
low variance measures like fertility. There are a small number of natural fertility and natural
mortality populations for which large sets of demographic data are available, some of which
are historical populations. These are now being analyzed to enhance our understanding of
which kin have an influence on the fitness of their descendants. Most of these populations are
of farmers, but farmers with high workloads, high disease burdens and high reproductive
rates. Whilst most of these populations are/were growing rather than stable, the same can be
said of contemporary hunter-gatherers populations too. We need to use as much data as is
available to us to untangle the full story of the evolutionary ecology of human family life.
Kin effects on child mortality in a range of natural fertility/natural
mortality populations
There are many studies on the contributions of various relatives to childcare, nutrition
and other aspects of development (Hewlett et al., 2000; Hurtado & Hill, 1992; Ivey, 2000)
that contribute greatly to our understanding of social networks and child-rearing, but it is not
always easy to determine from these studies the extent to which such help enhances the
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fitness of the beneficiary. In this review we shall concentrate solely on studies that have
examined the effects of kin on one specific component of fitness: child mortality. For women,
at least, child survival may be the most important determinant of reproductive success
(Jones, 2005; Strassmann & Gillespie, 2002), since women (compared to men) have
relatively low variance in fertility. Improving the survival chances of a woman’s children
may be the most important thing relatives can do to increase her reproductive success.
This review includes 43 populations in which the impact of at least one category of
kin on child mortality has been investigated. Most populations had little or no access to
modern medical care, including contraception. A few studies do include data from
populations which are moving through the demographic transition, so cannot strictly be
described as natural fertility, natural mortality populations, but are nevertheless from
societies in which child mortality is sufficiently high to demonstrate variation according to
the presence or absence of kin. Such studies correlating the presence (often approximated by
the survival status) of relatives with the survival of children do, of course, need to be
interpreted with caution. Correlational studies are helpful, but suffer from the usual problem
of attributing causation. Given that kin can share not only genes but frequently much of the
same environment, there is a high possibility that confounding variables, not included in the
analysis, are of great significance. Appropriate statistical analysis, such as event history
analysis, needs to be employed to minimise the chance that confounding factors will obscure
genuine kin effects or result in false positives (Allison, 1984; Singer & Willett, 2003). As not
all studies which have investigated this topic have used such adequately controlled statistical
analysis, we have divided the sample into two groups. The statistically valid sample (n=29)
includes only those studies in which properly controlled statistical analysis was used. The
supplementary studies (n=14) present data on the impact of relatives but either do not attempt
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statistical analysis to demonstrate associations, or have not adequately controlled for possible
confounding variables (i.e. only univariate analysis was used).
We have presented the data in three sets of tables. Tables 1a and 1b give details of the
effect of the presence of the mother on child survival (Table 1a shows the statistically valid
sample, Table 1b supplementary data). Tables 2a and 2b demonstrate the effects of other kin
on child survival (Table 2a the statistically valid sample, Table 2b supplementary data). In
these tables, ‘+’ indicates that the presence of a particular relative improves child survival, ‘–‘
that the relative lowers survival and ‘none’ the relative has no effect. Brackets indicate that
the relationship was of borderline significance (0.05>p>0.1), or only applied to certain
children. Blank cells indicate that category of relative was not included in the study. Table 3
provides a numerical summary of the previous 4 tables, and shows the number of studies
which have found positive, negative or no effects of each relative on child survival.
Who keeps children alive?
It comes as no surprise that in all populations in which the association between
mother’s death and child death has been investigated, the death of the mother is clearly
associated with higher child mortality (Tables 1a and 1b). That this effect exists is expected.
What we wanted to determine from this analysis was firstly, how long this association lasted
(i.e. is it seen throughout the whole period of childhood, or do mothers only matter to young
children?), and secondly, can even young children survive the loss of their mothers? If this
association is confined to young children, and if young children are able to survive the loss of
their mother, this would indicate that other relatives are stepping in to help children out, if
their mothers die.
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Tables 1a and 1b indicate that the mother effect is strongly dependent on the age of
the child. The consequences of losing a mother in very early life are catastrophic, as
evidenced by the tiny proportion of children who survive if their mothers die giving birth to
them: only 1.6% of Swedish children survived such a maternal death in the 19th century, and
5% of Bangladeshi children in the late 1960s (although by the 1980s, 26% of children
survived maternal deaths in the same Bangladeshi population). But a child’s survival chances
appear to improve rapidly with age. Much higher proportions of children manage to survive
the death of their mothers if it occurs during their first year of life in some populations: 35%
in 19th century Caribbean and 40% in 1920s US (though these studies only investigated
survival to age 1 yr); 50% in Burkina Faso, 40% in historical Sweden and 48% in historical
Germany (all looked at survival of the child to at least age 6 years). Studies which have
statistically investigated the timing of the mother effect confirm that the effect of mother’s
death on child survival weakens or even disappears entirely after children are weaned.
Almost half of the studies which have tested whether the mother effect varies with age of
child found that mothers only appear to be important for the first two years of a child’s life or
less. Four of the remaining six find that the magnitude of the effect declines substantially
with age, although the loss of the mother does still result in higher mortality after the age of
two years.
Clearly, two year old children are not self-sufficient, so the good survival prospects of
children who lose their mothers in later childhood must be due to other individuals taking
over childcare and provisioning. Tables 2a and 2b suggest who those individuals might be. In
all but two of the studies which have examined the impact of the extended family on child
survival, at least one relative (apart from the mother) has a significant impact on child
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survival. The remaining two cases only investigated mortality up to age two years (when the
mother is of paramount importance), and only investigated the impact of one relative: the
paternal grandmother. This widespread importance of kin apart from the mother supports the
hypothesis that women are cooperative breeders, sharing child-rearing with other family
members. But which relatives help is less consistent that the fact of help itself.
How much do fathers matter?
Every study that has compared the effects of the loss of mother and father on child
survival has found that the loss of the father has substantially less impact than the mother’s
death on the survival prospects of the child. Indeed, Tables 2a and 2b demonstrate that fathers
frequently make no difference to child survival. Table 3 shows that in 46% of the populations
studied using appropriate statistical techniques, there is no association between the death of
the father and the death of the child (if supplementary studies are included this proportion
rises to 70%). And in at least one case where an association was found, the relationship was
more likely to have been caused by mortality crises that killed family members
simultaneously (such as infectious disease) rather than any causal effect of the loss of the
father: Beekink et al (2002) found that child mortality was only increased for one month after
the death of the father (whereas the effect of the mother’s death lasted considerably longer).
We interpret this as an indication that paternal investment in children is facultative,
and dependent on ecological conditions. Even where fathers are important for child survival,
it is not clear that the benefits they bring to children are those of provisioning and economic
support. Hurtado and Hill (1992) compared the effects of fathers on the child survival in two
South America hunter-gatherer groups. The loss of the father had a significant impact on
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Ache children, where marriages are unstable, meat widely shared among the group and
fathers little involved in childcare, but no effect on Hiwi children, who are raised in nuclear
families, with considerable input from the father in terms of both provisioning with meat and
direct childcare. The importance of Ache fathers may instead lie in protecting their children
from other males, rather than direct provisioning (infanticide of orphans was not uncommon
in this group).
Indirect evidence that the importance of fathers lies at least partly in protecting
children from other males comes from studies of the impact of the mother’s divorce and
remarriage. Divorce and remarriage have been shown to increase a child’s risk of dying
(Alam et al., 2001; Bhuiya & Chowdhury, 1997; Sear et al., 2002). It is often not clear how
much of this is due to father absence, to step-father presence or to mother absence (divorcing
women may be unwilling or unable to take children with them), or indeed to the stress and
violence of the divorce itself. But step-children have been found to be at greater risk of
homicide than children living with natural parents (Daly & Wilson, 1988), and have higher
stress levels than children living with both biological parents (Flinn & England, 1995).
Fathers may also provide more diffuse benefits. Illegitimate children in historical
Europe had consistently lower survival chances than legitimate children (see van Poppel,
2000 for a review). Though some of these analyses may be confounded by poverty or
demographic factors such as maternal age and birth order (illegitimate children are more
likely to be first born children of younger women, both risk factors for child mortality:
Hobcraft et al., 1985), higher mortality has been observed among illegitimate children even
controlling for these variables (Brändström, 1996). Lack of male support is also known to be
a correlate of infanticide in societies beyond historical Europe (Daly & Wilson, 1984). Some
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kind of socially-sanctioned male support seems therefore to be important for child survival,
even if the data presented in Tables 2a and 2b suggests that in many cases fathers are not
actively providing their children with useful benefits within socially recognised unions. But if
fathers are not always beneficial to children within marriage, then this does beg the question
of why children without male support should suffer higher mortality rates. The answer may
lie in female reproductive strategies: the benefits of male support may be reaped by women
rather their children. If marriage is beneficial to women and if women’s marriage chances are
harmed by the presence of existing children, then single mothers may be less inclined to keep
children alive than married mothers (Voland, 1988). Interestingly, there is some suggestion
that kin support (from maternal grandparents) can alleviate the detrimental effects of
illegitimacy, indicating that there are interactions between the presence of a father and
extended kin (Blaikie, 1998).
This then leads on to the question of why marriage is beneficial to women. Again, the
‘traditional’ view has been that women and men form long-term monandrous associations
because both sexes benefit from the increased survival of children who are provisioned by
both mothers and fathers. Men monopolise their wives’ reproductive behaviour because they
do not want to invest heavily in provisioning children who are not genetically related to them.
But if men in many societies do not provision their children, then this explanation is
inadequate. Instead, Hawkes, Blurton Jones and colleagues (Blurton Jones et al., 2000;
Hawkes, 2004; Hawkes et al., 2001) have argued that mate guarding may be a more
convincing rationale for human pair-bonds and marriage: male competition for paternity is
sufficient to explain male desire to monopolise women, even in the absence of paternal
investment.
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That paternal investment is not universal in our species makes adaptive sense given
that child mortality is probably not the most important determinant of male reproductive
success. Under some circumstances at least, men are likely to achieve significantly greater
gains in fitness by directing their efforts towards gaining additional mates rather than
investing in existing children. In polygynous societies, men have the option of spending their
resources on attracting additional wives. This could account for some of the variation: for
example, the absence of a father effect in polygynous Gambians or Kipsigis (Borgerhoff
Mulder, 2005; Sear et al., 2002), but a significant positive effect of fathers in monogamous,
historical Quebec (Beise, 2005). Even if successfully polygynous men were inclined to
provide for children, they would find it rather difficult to provision all of their offspring; men
with multiple wives can father considerable numbers of children (the most reproductively
successful man in our Gambian population had 36 children). Polygyny tends to be associated,
by necessity, with productive enough subsistence systems that women can raise children
without help from their husbands (Low, 2005). This provides a testable hypothesis for
identifying under which circumstances men help to raise children: resource-poor
environments are likely to be those in which men help.
These studies suggest that the importance of fathers in provisioning their young
children has previously been overestimated, but it does not mean that men are unimportant in
raising children. Firstly, these analyses may well underestimate the importance of fathers.
Many of the studies focus on the mortality of young children. The mortality risks of young
children are likely to be highly dependent on the quality of care received (including
lactation). Fathers can take no part in lactation, and in most populations take relatively little
part in direct childcare (though there are exceptions: Hewlett, 1992), so may have little
opportunity to affect the survival chances of young children, with the exception of protecting
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them from other males. Fathers may play more important roles in the lives of older children,
teaching them subsistence skills and perhaps enhancing their marriage and fertility chances.
There is some evidence that women in traditional societies who lack fathers have later first
births than those with fathers, suggesting fathers may be instrumental in arranging marriages
for women (Allal et al., 2004; Waynforth, 2002). And Marlowe (2001) has found that male
contribution to diet is positively correlated with female reproductive success in a cross-
cultural study of hunter-gatherers, although male contribution was not associated with chid
survival. A full investigation of how much fathers (and other kin) matter requires analysing
the effects of fathers on all components of reproductive success.
Secondly, the lack of a father effect may be because what fathers do for children can
be more easily substituted than the services mothers provide to children. The care given to
young children by reproductive aged women is usually directed exclusively towards the
women’s own children (i.e. lactation). There are rare cases of a lactating woman adopting and
feeding an infant after the mother’s death, but lactation is energetically costly and also
inhibits conception, so that reproductive aged women can usually gain more from investing in
their own offspring than looking after less closely related children. In contrast, the productive
work or protection that men provide for children can more easily be directed towards children
other than their own. Though evidence does suggest that men are disinclined to invest in the
progeny of other men (hence the role fathers play in some societies as protectors against other
men), there are strategies that can be used to spread the ‘fathering’ role amongst other men.
Hrdy (2000), in a review of the ethnographic literature on mating behaviour, suggests that
women are more polyandrous than has been traditionally supposed. This polyandry functions
in part to improve child survival by confusing or spreading paternity in order to protect
children from potentially infanticidal males and/or encourage several males to invest in
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mothers and children. For example, in some South American hunter-gatherer communities,
paternity is considered to be ‘partible’, i.e. any man who has sex with the mother around the
time of conception and pregnancy is regarded as a father of the child. In both the Ache and
among Bari hunter-gatherers of Venezuela, children with multiple fathers do better than those
with only one (Beckerman et al., 2002; Hill & Hurtado, 1996) – though Ache children with
many fathers did less well than those with one primary and one secondary father. An
alternative strategy for spreading the fathering role may be patriliny, where patrilineally
related men and their wives may live and work in close proximity. In such societies,
patrilineally related males may take over the father’s responsibilities if a child’s father dies,
especially where the levirate is practiced (women marrying their husband’s brother after
widowhood). In the Gambia, patrilines live in extended family compounds, and the levirate is
common (around 40% of widows married their dead husbands’ brothers). Children may
therefore suffer little after the death of their fathers, as any services provided by the father
can be taken over by other related males in the compound.
“A home without a grandmother is like a house without a roof”
If fathers are not universally beneficial to children, then does this review provide any
evidence that grandmothers are more helpful to mothers (confirming the belief of the Serer of
Senegal that grandmothers are of vital importance to family life: Aubel et al., 2004)? In
contrast to the variability shown by fathers, maternal grandmothers appear to be more
consistent helpers for women raising children. In eight of ten cases (ten of twelve including
supplementary data), maternal grandmothers improved child survival. The one study which
found a detrimental effect of maternal grandmothers was of a resource-stressed, matrilineal
population where maternal kin may have been in competition for the main resource of land
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(maternal aunts also increased the risk of child death: Sear, 2003). Furthermore, this dataset
did not include grandmaternal effects for children whose mothers had died, and in such cases
anecdotal evidence suggested maternal grandmothers play a crucial role. On the other hand,
paternal grandmothers are similar to fathers in that they show rather more variation in their
effects on child survival. While just over half of the studies in Tables 2a and 2b demonstrate a
positive effect of paternal grandmothers on child survival, five suggest no effect and two
indicate that paternal grandmothers are detrimental to the child’s prospects of surviving. This
greater variation may be in part explained by the greater age of paternal than maternal
grandmothers, due to females reproducing earlier than males (though maternal age, and
sometimes age of grandparents, is controlled for in those studies in Table 2a). Or it may
reflect their lower level of genetic relatedness to their patrilineal descendants (due to
paternity uncertainty). Separating out the effects of maternal and paternal relatives on female
fitness is clearly important, as maternal and paternal kin may therefore differ in both their
ability and inclination to invest in children. This may explain why two of the three studies
which have not separated out the effects of maternal from paternal grandmothers have found
no effect.
A closer inspection of the timing of these grandmaternal effects suggests evidence
that maternal and paternal relatives have different roles to play in the lives of mothers and
children. In some populations, maternal grandmothers appear to have the strongest effect
around the age of weaning (Beise, 2002; Beise, 2005; Sear et al., 2002). Weaning is a
dangerous time for children. It increases their exposure to pathogens in food, and is often
associated with the arrival of a younger sibling, when mothers divert their attention away
from weaned children and to their new babies. Maternal grandmothers may be stepping in to
protect children from the dangers associated with this stage of childhood (see Sear et al.,
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2002; Thompson & Rahman, 1967 for an example of how maternal grandmothers might do
this in the Gambia). Paternal grandmothers, in contrast, often appear to have the strongest
effect (whether beneficial or detrimental) during the first month of a child’s life (Beise, 2002;
Beise, 2005). Mortality in this period is less dependent on exogenous causes (such as quality
of care received) and more dependent on endogamous causes (such as low birthweight:
Mosley & Chen, 1984). Birthweight is correlated with the condition of the mother during
pregnancy (Andersson & Bergstrom, 1997; Kirchengast & Hartmann, 1998). Paternal
grandmothers may therefore affect child mortality by affecting the condition of the mother
during pregnancy. This effect may be beneficial (perhaps by helping out with domestic or
productive tasks) or detrimental (stress and harassment may lead to worse maternal condition
and higher neonatal mortality rates). Beise (2005) illustrates the difference between the roles
of the maternal and paternal grandmothers by distinguishing between the ‘helping’ and the
‘helpful’ grandmother; the former – maternal – helps out with direct childcare, the latter –
paternal – affects the condition of the mother by helpful (or occasionally harmful) behaviour
during pregnancy.
Most of the studies in this paper have only used correlational evidence to infer helping
behaviour from kin, but Gibson and Mace (2005) also collected time budget data to establish
what relatives were actually doing for one another. This analysis provides further support for
the suggestion that maternal and paternal relatives perform different functions in women’s
lives. Maternal grandmothers were found to help women out with heavy domestic tasks, thus
freeing mothers for childcare. Paternal grandmothers, on the other hand, were more likely to
help women with agricultural work, an activity from which they may gain a direct benefit
(i.e. a share in the harvest).
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Grandfathers are much less important to children. Even where associations are found
they tend to be of borderline statistical significance (in four of the six cases where paternal
grandfathers have an impact on child survival the effect is borderline or applies only to
female children).
Data on the effects of related reproductive-aged adults on child survival (apart from
parents, such as aunts and uncles) is relatively scarce. The little evidence available suggests
the effects of such relatives are very mixed. The children of Kipsigis agropastorialists in
Kenya appear to do better if they have either paternal or maternal uncles (Borgerhoff Mulder,
2005). Chewa children in Malawi have higher survival in the presence of paternal aunts, but
lower survival if maternal aunts or maternal uncles are around (Sear, 2003). Venetian
children apparently neither gain nor suffer from aunts or uncles (but neither maternal nor
paternal, nor aunts and uncles were distinguished: Derosas, 2002). In historical China, the
presence of reproductive aged females (usually paternal aunts) increased mortality for
motherless children (Campbell & Lee, 2002). 19th century Mormon children may have
benefited from maternal uncles and either kind of aunt (Heath, 2003). Reproductive-aged
adults may be in a position to help one another with childcare, domestic tasks or productive
activities, but also may either be too concerned with the well-being of their own small
children, or actively in competition with each other for resources to be consistently
beneficial. In a study of childcare arrangements in Efe hunter-gatherers, Ivey (2000) found
that children were frequently looked after by individuals other than their mothers but these
allocarers were rarely other women who had nursing infants of their own. Data from
historical studies do however suggest that one category of reproductive-aged women may be
beneficial for child survival: step-mothers. Despite numerous folk tales warning of the
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dangers of the wicked stepmother, both Andersson et al (1996) and Campbell and Lee (2002)
found that children with stepmothers had similar risks of dying to those children who still had
their own mothers (though such analyses need to be interpreted with care, as children with
stepmothers will be older and have experienced the death of their mothers further in the past
than most motherless children). If this is not a statistical artifact, such philanthropic
behaviour on the part of step-mothers may be a form of mating effort, as has been suggested
for step-parental behaviour in non-human animals (Rohwer et al., 1999).
Helpers at the nest
Rather few studies have investigated the effect of potential sibling ‘helpers at the nest’
on child survival, despite the widespread observation that the labour of older children is used
by parents both for domestic work (including childcare) and productive activities (Borgerhoff
Mulder & Milton, 1985; Cain, 1977; Kramer, 2002; Kramer, 2005; Weisner & Gallimore,
1977). The effects of older siblings, however, are complicated by competitive relationships.
Several studies have found that older siblings increase, rather than decrease, the risk of death
for children (e.g. Das Gupta, 1987; Muhuri & Preston, 1991). These effects are usually
interpreted as parental manipulation of the size and sex composition of their families for
optimal allocation of limited family resources. Here, we only present studies which have
investigated the effect of older siblings who are potential helpers, rather than competitors, by
restricting the analysis to those children several years older than the focal child (at least 3 yrs
older, and often more, depending on the study). Only six studies have analysed helping at the
nest, but five of these studies find potential helpers have a positive effect on child survival
(and the sixth study only investigated the effects of adult siblings, who may have been
occupied with children of their own). In some cases this effect is specific to older sisters,
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suggesting the domestic responsibilities of juvenile girls (including childcare) are important,
but in other cases the sex of helpers does not matter, suggesting all activities contributed by
pre-reproductives are beneficial.
Confounding effects
Some of the studies in the sample found that kin effects are not straightforward. In a
few populations, the effect of a particular category of kin was only seen for children of one
sex. Mothers themselves are known to invest differentially in children according to sex and
birth order. Other kin may mirror the investment decisions of mothers, by investing in
similarly favoured children. The reproductive interests of kin are not necessarily identical to
those of the mother, however. Sorenson Jamison et al (2002) highlight the possibility that
paternal grandmothers in Japan are influenced by concerns of lineage, which means that
certain children (such as later born boys who may be unwelcome competitors for favoured
male heirs) are particularly disadvantaged, whereas other grandchildren may be supported.
Such sex-specific and birth order biases, which are found in a number of wealth-inheriting
societies, would confound attempts to label individual kin relationships as always positive or
negative for child survival. Such grandmothers would, nonetheless, be attempting to promote
their lineage, albeit at the expense of certain unfortunate grandchildren.
Availability of resources also seems to alter kin effects. Both Borgerhoff Mulder
(2005) and Leonetti and Nath (2004) found interactions between kin effects and wealth. In
the Kipsigis, patrilineal kin appeared to be most important for rich children, though the effect
was seen in all families (Borgerhoff Mulder, 2005). But maternal kin only appeared to be
important in poor families. In India, husbands were more likely to help women out in poorer
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households (Leonetti & Nath, 2004). In the latter study, the condition of the mother also
mattered. There was a tendency for men to be more helpful to women with fewer resources
(both economic and physiological: shorter women were more likely to be helped by
husbands). There were also interactions between help given by husbands and grandmothers
(more help from grandmothers correlated with less help from husbands). These complications
to the story of kin help suggest that help from any category of kin may be facultative to some
extent, depending on resources, the mother’s ability to rear children, the presence of other
kin, etc.
A final word about confounding effects. A common criticism of studies which find a
correlation between the survival of a particular relative and child survival is that these effects
might simply be due to shared genes or environment, i.e. certain children come from
‘healthy’ families where both they and their relatives have good survival prospects, and
others come from ‘unhealthy’ families where their own survival chances are low, as is the
probability that their relatives have survived long enough to help care for them. While such
explanations cannot be ruled out in all cases, the results presented in Tables 1 and 2 suggest
that shared genes or environment is unlikely to be the full explanation in all cases. For
example, if such confounding effects were important we would expect to see positive
relationships between children and all categories of grandparent. Instead we see considerable
variation between relatives and between populations in which kin are important for child
survival. The effects of kin are also often dependent on the age of the child. Again, if shared
genes or environment were responsible for these results then the survival of kin should be
correlated with child survival throughout the child’s life. Thirdly, several studies have
controlled for shared environment between relatives by including statistical controls for
economic factors (e.g. Borgerhoff Mulder, 2005; Gibson & Mace, 2005; Leonetti et al.,
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2005). Significant kin effects are still seen even using such controls. Finally, the authors of
these studies are frequently aware of this potential confound and have often used additional
analysis or ethnographic evidence to interpret the results of their correlational analysis, to
provide assurances that these results are unlikely to be entirely due to shared genes or
environment.
Discussion
Evolution and the human family
What does this review tell us about the evolution of the human family? Clearly, there
is a problem using data on current populations to infer anything about evolutionary history.
Certainly the study of a single society tells us little about evolution of a particular trait. In the
Gambia, we found positive effects of maternal grandmothers and no effect of fathers on child
survival, but this does not constitute strong evidence in favour of the importance of older
women and the unimportance of men in the human family. These results could have arisen
due to some peculiarities of Gambian ecology. Cross-cultural analysis is essential to
determine which traits are common across societies and which vary according to
environmental conditions (see e.g. Walker et al., 2006 for an example on growth). This
review offers hints about which features of the human family may have been common
throughout our evolutionary history, and which are adaptations to local environments. We
conclude from this review that kin support in rearing offspring does appear to be a human
universal. Support from maternal kin (especially grandmothers) appears to be more reliable
than that from paternal kin, which is rather variable. Support from fathers also appears to be
facultative, and dependent on ecological conditions.
22
But does even this cross-cultural review tell us anything about the evolution of the
human family? This review covers a variety of human cultures, but examining the impact of
relatives on child mortality is a data intensive exercise. This means that the dataset has
relatively few hunter-gatherers, and is biased towards those who made at least some of their
living from farming. Is it possible that throughout most of our history we have lived in
relatively stable (perhaps nuclear) families where fathers assume more importance in
provisioning children, or even where mothers were better able to provision their children
alone? The variation we see among extant populations may be, at least in part, a response to a
shift in subsistence and demographic patterns to a set of conditions which make helping by
extended kin more favourable. For example, if agricultural populations have higher fertility
and lower adult mortality than hunter-gatherers, this might make kin (such as grandmothers
and older children) both available and necessary as helpers. Draper and Harpending (1987)
have suggested that paternal involvement and sibling care may differ systematically between
foraging and farming communities, with father involvement much more common among
foragers and sibling care more frequent among farmers (see also Hewlett, 1991). Kaplan and
Lancaster (2003) have also argued that shifts in subsistence strategy during human history
have been accompanied by shifts in optimal family structure. In particular, they assert that the
move from foraging to horticulture and agriculture was accompanied by a significant
reduction in the importance of male provisioning to children.
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If there are such systematic differences in the family structures of farmers and
foragers, then our sample may well overestimate or underestimate the importance of certain
relatives. However, it seems unlikely to us that one particular family structure has been of
paramount importance throughout human history. Existing hunter-gatherer populations are
hardly uniform in either their subsistence strategies or demographic patterns. Hunter-gatherer
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populations have, after all, been used to illustrate both the importance of fathers (Ache), and
the importance of grandmothers (Hadza). This particular debate might reflect differences
between Old World and New World foragers, since foragers in the Old World tend to rely
relatively more on gathering and have lower male contributions to the diet than New World
foragers (Marlowe, 2005). There are also problems in using extant hunter-gatherer
populations as models for past hunter-gatherers as many of the remaining hunter-gatherers
occupy marginal environments unsuitable for farming activities (though this view has
recently been questioned: Marlowe, 2005). This variability shown by hunter-gatherer
populations is unlikely to have been of recent origin, given that recent estimates suggest
hominins have had a wide geographical distribution (i.e. outside of Africa) for nearly 2
million years (Dennell & Roebroeks, 2005). If early hominids had a wide geographic
distribution then they probably occupied a variety of different environments, with associated
plasticity in behavioural characteristics.
It seems more parsimonious to us to assume that human family systems have always
been somewhat flexible and responsive to ecological conditions, as are those of many other
primates. After all, as Hrdy (2005) points out, relying exclusively on a single category of kin
(such as fathers) seems a rather risky strategy, given the improbability that one particular
relative will survive and be able to help throughout a woman’s reproductive career.
Evolution of human life history
We introduced this paper by describing the unusual features of human female life
history – late puberty, short birth spacing and menopause. Does this review tell us anything
important about the evolution of human female life history characteristics? We’ve found
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unmistakable support for the hypothesis that women receive help from kin in raising children
in extant populations, but can we infer from this that characteristics of human life history can
be explained by the cooperative nature of human reproduction? Again, it is difficult to draw
conclusions about the evolution of a particular trait by examining existing populations. For
example, grandmothers (of one kind or another) do appear to be universally beneficial across
societies in improving the fitness of their relatives. This provides some support for the
grandmother hypothesis for menopause, but we still cannot be entirely certain that
menopause evolved because of its fitness benefits. It may be that grandmothers invest in their
grandchildren because they are unable to continue having children of their own, and investing
in grandchildren is better than investing in nothing at all. Rather than relying solely on
statistical investigations of patterns of behaviour in modern populations, mathematical
modelling may be necessary to get at the evolution of particular traits, by quantitatively
testing whether a particular trait is likely to have evolved given a set of parameters.
Most attempts to build quantitative models in which women can compensate for lost
fertility in later life through enhancing the fitness of children and grandchildren have failed to
find fitness benefits sufficiently large to favour menopause at 50 (Grainger & Beise, 2004;
Hill & Hurtado, 1991; Rogers, 1993). Shanley and Kirkwood (2001) argue that menopause at
a slightly older age could be favoured if a range of selective forces are combined, including
an increase in maternal mortality with age, as well as grandmaternal effects both on
grandchild survival and on their daughters’ fertility (and these latter effects need to be large).
When parameterising this model with data from the Gambia (Shanley et al in prep), we find
that maternal and grandmaternal effects on child survival are particularly important, and
parental contributions to daughters’ fertility are less important. But again, realistic parameter
values suggest a late age menopause is adaptive, which implies that some important effect
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may still be missing from the model. However, what this approach highlights is an important
point about the relative contributions to fitness of grandmothers and mothers. The loss of the
mother clearly has a larger impact on a child’s survival chances than the loss of its
grandmother, especially in early years; but because adult mortality is so low in humans, the
chance of a child losing its mother in early childhood is actually very low. The loss of a
grandmother may have a smaller effect on child survival, but the chances of losing a
grandmother in early childhood are quite high. Therefore the impact of grandmaternal deaths
on population levels of infant mortality is greater than that of mothers, and thus the relative
fitness benefits of grandmaternal care are more significant than those of maternal care –
grandmothers give children that competitive edge.
That these quantitative analyses suggest marginal, if any, benefits of menopause at 50,
has contributed to a belief that grandparental and parental care are a significant selective
force on human longevity, but not necessarily on the timing of menopause (Hawkes et al.,
1998). Recent work has focussed on modelling the mortality schedules and aging patterns of
our species, rather than a specific component of human life history such as menopause. These
models have suggested that many of the peculiarities of human life history, including a long
juvenile period, long lifespan and postreproductive life, may hinge on intergenerational
transfers in general (not specifically those from grandmothers, but including all transfers
from older to younger individuals: Kaplan & Robson, 2002; Lee, 2003). The mathematical
framework needed to address these problems continues to develop. Such models would also
benefit from more information on the parameters needed to inform these models: effect sizes
for kin help across a number of different populations would illustrate the relative importance
of mothers, fathers and grandmothers. Whether elaborations of these models using realistic
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human parameters can explain menopause, as well as other human life history characteristics,
better than existing models awaits further analysis.
Next steps
This review has found some commonalities but also substantial variation between
populations in which kin help women raise children. Several studies have also found
variation within populations on who helps and when, depending on resource availability,
availability of alternative helpers and the condition of the mothers themselves. The next step
is to explain this variation within an evolutionary ecological framework. This could involve a
meta-analysis of those studies which have investigated this issue, testing hypotheses about
the circumstances under which particular kin help, preferably using appropriately
phylogenetically controlled methods (Mace & Pagel, 1994: not all studies in this sample can
be considered independent data points, since several come from similar populations). We
suggest the following list of potential factors may affect the level of help offered by particular
relatives. (1) Subsistence strategy, which may affect: the degree to which certain kin may
help (e.g. children may be economically productive in some agricultural societies, but less so
hunter-gatherer communities); and the division of labour between sexes, which affects what
kind of help kin can provide and the extent to which help is necessary. (2) Demography: the
probability of having a particular relative around to help depends on a number of
demographic factors such as adult sex-specific mortality rates, age-specific fertility rates and
age difference between spouses. (3) Marriage systems: polygynous men are likely to find it
difficult to invest in children from several mothers, and will also have alternative mating
opportunities which make mating effort more productive than parental effort. (4) Residence
patterns: which will affect which kin are most available for help. (5) Inheritance patterns:
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which may result in selective helping of certain children. (6) Condition of the both the mother
and potential helpers: the former will affect the payoffs to relatives of investing in children,
the latter their ability and inclination to invest.
This study has only examined statistical correlations between the survival of kin and
survival of children. While we have attempted to separate out studies which are likely to have
demonstrated genuine correlations from those which have not adequately controlled for
potentially confounding factors, even those studies which have used appropriate statistical
analysis have not demonstrated a causal relationship between the presence of kin and the
survival of children. A better understanding of the pathways by which kin help would
improve our understanding of why these associations are found (and provide reassurance
these effects are not merely statistical artifacts). The studies that are presented here suggest
that the pathways through which kin influence reproductive success may well differ between
relatives. Men and women appear to help in different ways, because of sexual division of
labour within societies (e.g. help in direct childcare is much more likely to come from female
kin than male kin). There also appear to be differences in the kinds of help offered by
maternal and paternal kin in their helping behaviour (and not only in the frequency with
which they offer help: Beise, 2005; Gibson & Mace, 2005). Pathways may also be more
variable for fathers than for other kin. Fathers can potentially provide a variety of services to
children including provisioning with food, providing protection from other males, childcare,
and other social benefits. Female kin tend to confine their roles to lifting energetic burdens
from women by helping out with childcare, domestic and subsistence activities. This review
has also highlighted that not all kin are beneficial. Suggestions for the detrimental effects of
relatives on child survival have included competition for resources (Campbell & Lee, 1996)
and conflicting interests between women and their husband’s kin (Beise, 2002; Voland &
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Beise, 2005). These results suggest that any models which attempt to investigate the
evolution of certain life history traits need to take into account differences between maternal
and paternal kin, as well as potential conflicts between relatives.
Relevance to current family policy debates
Finally, we conclude with a brief discussion of the relevance of such evolutionary
analysis to family policy. There is a tendency for policymakers in Western countries to
believe that the nuclear family (often the male breadwinner) model is most beneficial for
individuals, children and society, and that the decline in marriage and increase in divorce and
single motherhood in recent years marks an unprecedented decline in the family in human
history. Policy theorists have claimed that all welfare states were initially predicated on the
male breadwinner family, and most still subscribe to some degree to the male breadwinner
model (Lewis, 1992; Sommestad, 1997). There is an enormous literature arguing that father
absence has detrimental consequences for children (see Sigle-Rushton & McLanahan, 2004
for a review), reinforcing the view that marriage is good for children, divorce is bad, and that
children should grow up in a home with both biological parents. But this review shows that
the human family is clearly a diverse entity, and that the nuclear family system may not be
the normative solution to the problem of raising children in all circumstances (though it may
be in others). What is clear is that the male breadwinner model is a rather unusual family
system, and there is little evidence that it has been common throughout our evolutionary
history. The two features which make the male breadwinner model particularly unusual are
that women are expected to raise children alone, and are not expected to contribute any
productive labour. These studies and others demonstrate that mothers do not raise their
children alone in many societies, but receive substantial help from others, and that it is not at
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all unusual for children to receive care from other kin and group members. It is also
extremely unusual for women to take no part in productive activities. Hewlett, in a table titled
‘the myth of the male breadwinner’, tabulates the contribution of women to the family diet
from 90 societies worldwide and observes that in half the societies the breadwinner role was
shared roughly equally between men and women, and that the number of societies in which
men were the main breadwinners was equalled by the number of societies in which females
contributed the majority of the family diet (Hewlett, 2000). Not dissimilar results are seen if
only hunter-gatherers are considered (Hewlett, 1991; Marlowe, 2005).
What fathers do for children is also not entirely clearcut. While they are undoubtedly
extremely important in many societies, the idea that humans are a species with obligate and
substantial paternal investment is simply not supported by this review. This review highlights
that there is some variation within, as well as between, societies in kin (including paternal)
investment, according to level of available resources and other factors. These observations of
considerable variation in optimal family structure suggest it might be useful for policymakers
to take a slightly less rigid approach when considering what is the best environment to raise a
child.
This does raise the question of exactly how such evolutionary analyses can be used to
inform family policy, if at all. For example, knowledge that the best kind of family to raise a
child can take several forms may not be necessarily useful to policymakers trying to
formulate policies at a national level. A recent attempt to use evolutionary psychology to
inform family policy appeared to conclude that evolutionary approaches are useful because
they allow us to understand better the preferences of individuals, so that social policy can be
directed towards fulfilling these preferences (Browne, 2002). However, an evolutionary
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perspective also tells us that the preferences of individuals may be well in conflict: the
preferences of men may not coincide with the preferences of women; the preferences of
children may not coincide with those of parents; and the preferences of the family may very
well be in conflict with those of institutions such as employers, government, etc.
Evolutionary analyses can be used to gain a better understanding of human behaviour, but
cannot be used to provide easy policy solutions.
Conclusion
We have presented evidence that human children benefit from an extended family and
that kin support can enhance female reproductive success. There are several studies on
focussing on components of reproductive success that further support this view, but we
narrowed our discussion here to those that could identify a kin effect on child survival, an
unambiguous determinant of reproductive success, so that we could unpick differing
influences within the family. This unpicking reveals that different kin help in different ways.
Fathers’ contributions to child survival are variable, possibly being more important in
monogamous societies. Maternal grandmothers and other matrilineal kin tend to improve
child survival. Elder sibling ‘helpers-at-the-nest’ also seem consistently beneficial. Paternal
grandmothers and other patrilineal kin sometimes improve child survival, but are not
universally beneficial and may even be harmful under certain circumstances. These different
strategies could emerge as a response to different relatedness to the mother. Overall, this
review suggests we may not be obligate cooperative breeders, but the most convincing
explanation for a range of theoretical and empirical analyses of our life history suggest that
we are evolved to raise children as an extended family enterprise.
31
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46
Table 1a: Studies of the effect of mother’s presence on child survival
Population Authors Effect
of
mothers
Age of
children
studied
Timing of
mother
effect
%age surviving
mother’s death Notes
Nepal (Sarlahi)
1994-97 (Katz et al., 2003) + 0-24
weeks 0-24 weeks Effect size increased with age of infant. Maternal deaths1
only
Caribbean (St Barthélemy)
1878-1976 (Brittain, 1992) + 0-1 yr Not tested 35% %age survival to 1 yr after mother’s death in first year
Gambia (4 villages)
1950-74 (Sear et al., 2000; Sear et
al., 2002) +
0-5 yrs < 2 yrs
only Nutritional status also lower without mothers
Kenya (Kipsigis)
1945-90 (Borgerhoff Mulder,
2005) + 0-5 yrs Not tested
Burkina Faso (Nouna)
1992-99 (Becher et al., 2004) + 0-5 yrs 0-5 yrs 50% %age survival in follow-up period (0-5 yrs) after mother’s
death in first year. Effect weakens with age
Sub-Saharan Africa2
1980s-2000 (Zaba et al., 2005) + 0-5 yrs <2 yrs only Effect limited to first yr after mother’s death. Relationship
holds for HIV –ve children
Canada (Quebec)
1680-1750 (Beise, 2005) + 0-5 yrs 0-5 yrs Mother effect weakens with child’s age
Poland (Bejsce)
1737-1968 (Tymicki, 2006) + 0-5 yrs 0-5 yrs
Guinea-Bissau
1990-98 (Masmas et al., 2004) + 0-8 yrs <2 yrs only Low HIV prevalence, so effect not due to mother-to-child-
transmission of HIV
Paraguay (Ache)
1890-1971 (Hill & Hurtado, 1996) + 0-9 yrs 0-9 yrs Weak evidence (p=0.09) that mother effect declines with
child’s age
Netherlands (Woerden)
1850-1930 (Beekink et al., 1999;
Beekink et al., 2002) + 0-12 yrs <6 mths /
0-12 yrs 1999 paper suggests effect only seen <6 mths; 2002 paper
effect seen up to age 12, though weakens with child’s age
Italy (Tuscany)
1819-59 (Breschi & Manfredini,
2002) + 0-12 yrs Not tested
Canada (Quebec)
1625-1759 (Pavard et al., 2005) + 0-15 yrs 0-15 yrs Mother effect weakens with age of child. Neonates
excluded. Effect stronger on girls after age 3 yrs
Sweden (Sundsvall)
1800-1895 (Andersson et al., 1996) + 0-15 yrs <1 yr only 40% %age survival to 15 yrs after mother’s death in first year
Japan (Central)
1671-1871 (Sorenson Jamison et al.,
2002) + 1-16 yrs Not tested Effect stronger for boys (but seen in all children)
China (NE)
18th & 19th C (Campbell & Lee, 1996;
Campbell & Lee, 2002) + ~1-15 yrs Strongest
~6-10 yrs Timing of effect only tested for boys
1 Definition of maternal death may differ between studies but broadly refers to death due to childbirth
2 Pooled data from 3 cohort studies in Tanzania, Malawi and Uganda
Table 1b: Supplementary data on the relationship between survival of mother and child survival (but not fully controlled statistically)
Population Authors Effect of
mothers
Age of
children
studied
Timing of
effect %age surviving
mother’s death Notes
US (New York State)
1936-38 (Yerushalmy et al.,
1940) + 0-1 mth Maternal deaths only considered
Bangladesh (Matlab)
1967-70 (Chen et al., 1974) + 0-1 yr 5% %age survival to 1 year after maternal death
Bangladesh (Matlab)
1976-85 (Koenig et al.,
1988) + 0-1 yr 25.9% %age survival to 1 year after maternal death. Deaths
among older siblings <3 yrs not affected by maternal
death
US (8 cities)
1920s (Woodbury, 1926) + 0-1 yr 40% %age survival to 1 yr after mother’s death in first month
Tanzania (Hadza)
1985-1990 (Blurton Jones et
al., 1996) + 0-5 yrs
Uganda (Rakai)
1994-2000 (Bishai et al., 2003) + 0-6 yrs Effect holds for HIV –ve children
Bangladesh (Matlab)
1983-85 (Over et al., 1992) + 0-9 yrs Effect substantially stronger for girls
Spain (Aranjuez)
1870-1950
(Reher &
González-
Quiñones, 2003)
+ 0-9 yrs <2 yrs only Effect strongest for boys in neonatal period; girls at older
ages. Effect increases over calendar time. Nutritional
status also lower without mothers
Italy (Venice)
1850-69 (Derosas, 2002) + 0-10 yrs
Germany (Ostfriesland)
1668-1879 (Voland, 1988) + 0-15 yrs 48.5% %age survival to 15 yrs after loss of mother in first year
Sweden (7 parishes)
19th C (Högberg &
Broström, 1985) + 0-15 yrs <5 yrs only 1.6%, 3%, 13% %age survival to age 5 if lost mother at birth, during first
year and between 1-5 yrs respectively
UK (Cambridgeshire)
1770-1861 (Ragsdale, 2004) + 0-15 yrs Loss of mother within 2 yrs of birth of child
47
48
Table 2a: Studies of the effects of fathers, grandparents and older siblings on child survival
Population Authors Age of
child
Effect of
fathers Effect of
maternal
gms
Effect of
paternal
gms
Effect of
maternal
gfs
Effect of
paternal
gfs
Effect
of older
sibs
Other effects and notes
Gambia (4 villages)
1950-74 (Sear et al., 2000;
Sear et al., 2002) 0-5 yrs none + none none none + Elder sisters only increase survival (not
brothers), and only at 24-59 mths; divorce -
Canada (Quebec)
1680-1750 (Beise, 2005) 0-5 yrs + + + + (+) + Fathers improve survival 1-23 mths; pgms
in first month; mgms 12-35 mths; mgfs 36-
59 mths; pgfs 36-59 mths but only for girls
Malawi (Chewa)
1997 (Sear, 2003) 0-5 yrs none - + none none + Elder sisters only increase survival; mat
aunts - ; mat uncles - (borderline); pat aunts
+ (borderline); divorce -
Kenya (Kipsigis)
1945-90 (Borgerhoff
Mulder, 2005) 0-5 yrs none (+) + (+) + Mgm and mgf effects only in poor families;
pat kin (except pgm) have stronger effects
in rich families; mat and pat uncles +
Poland (Bejsce)
1737-1968 (Tymicki, 2006) 0-5 yrs + + + + + All grandparental effects seen only in first
year; father effect seen at all ages
Japan (Central)
1671-1871 (Sorenson
Jamison et al.,
2002)
1-16
yrs none (+) (-) none (-) Mgm effect borderline; pgm effect only
seen for boys; pgfs only for girls
Ethiopia (Oromo)
1993-2003 (Gibson & Mace,
2005) 0-5 yrs (+) (+) none none Mgms have borderline effect; pgm effect
only seen for girls;
Germany (Krummhörn)
1720-1874
(Beise, 2002;
Voland & Beise,
2002)
0-5 yrs + - none none Pgm effect seen in first month; mgm effect
esp pronounced 6-12 mths
Italy (Venice)
1850-69 (Derosas, 2002) 0-10
yrs none (+) none (-) Pgm effect only seen in orphaned children;
pgf effect only <1yr; both effects
borderline; no effect aunts/uncles; reported
that loss of father has no effect (no statistics
presented)
India (Khasi)
1980-2000 (Leonetti & Nath,
2004; Leonetti et
al., 2005)
0-10
yrs none + Mgm effect seen in first yr only
Sweden (Sundsvall)
1800-95 (Andersson et al.,
1996) 0-15
yrs none Stepmother +
Italy (Tuscany)
1819-59 (Breschi &
Manfredini, 2002) 0-12
yrs none Death of father increased risk of emigration
Japan (NE)
1716-1870 (Tsuya & Kurosu,
2002) 2-14
yrs +
Netherlands (Woerden)
1850-1930 (Beekink et al.,
1999; Beekink et
al., 2002)
0-12
yrs (+) Fathers only had effect within 1 mth of
their deaths
India (Bengali)
1980-2000 (Leonetti et al.,
2005) 0-10
yrs + Pgm effect only seen in children 1-9 yrs
India (Uttar Pradesh)
1992-3 (Griffiths et al.,
2001) 0-2 yrs + Pgm effect only in first mth
India (Tamil Nadu)
1992-3 (Griffiths et al.,
2001) 0-2 yrs none
India (Maharashtra)
1992-3 (Griffiths et al.,
2001) 0-2 yrs none
Bolivia (Aymara)
1960s-90s (Crognier et al.,
2002) 0-15
yrs + Elder brothers and sisters improve survival
Morocco (Berber)
1930-80 (Crognier et al.,
2001) 0-15
yrs + Elder brothers and sisters improve survival
Finland (5 communities)
18th &19th C (Lahdenpera et
al., 2004) 0-15
yrs + Pat and mat gms not distinguished; effect
only seen 2-15 yrs, and only for gms <60
yrs old
Paraguay (Ache)
1890-1971 (Hill & Hurtado,
1996) 0-9 yrs + none none none Mat and pat grandparents not distinguished;
elder sibs only include adult sibs
China (NE)
1774-1873 (Campbell & Lee,
1996; Campbell
& Lee, 2002)
~2-15
yrs (+) none - Father effect only in girls; pat and mat
grandparents not distinguished; presence of
‘adult women’ increases mortality for boys
if no mother or stepmother present.
Stepmother +
49
Table 2b: Supplementary evidence for the effects of fathers, grandparents and older siblings on child survival (not statistically controlled for
confounding factors)
Population Authors Age of
children
Effect of
fathers Effect of
maternal
gms
Effect of
paternal
gms
Effect of
maternal
gfs
Effect of
paternal
gfs
Effect of
older
siblings
Other effects and notes
UK (Cambridgeshire)
1770-1861 (Ragsdale, 2004) 0-15 yrs none + none none none
Utah (Mormons)
19th century (Heath, 2003) 0-1 yr + none none (+) Pgf effect borderline; mat aunts,
mat uncles and pat aunts +
Tanzania (Hadza)
(Blurton Jones et
al., 2000) 0-5 yrs none Father absence tested (including
death and desertion)
Venezuela (Hiwi)
~1980s (Hurtado & Hill,
1992) 0-5 yrs none Father absence tested (including
death and divorce)
Uganda (Rakai)
1994-2000 (Bishai et al., 2003) 0-6 yrs none
Bangladesh (Matlab)
1983-85 (Over et al., 1992) 0-9 yrs none
Spain (Aranjuez)
1870-1950
(Reher &
González-
Quiñones, 2003)
0-9 yrs none Fathers improve nutritional
status
Italy (Venice)
1850-69 (Derosas, 2002) 0-10 yrs none
50
Table 3: summary of kin effects on child survival (figures in brackets represent percentages)
Statistically valid Supplementary Total
Number
of studies
+ve
effect -ve
effect No
effect Number
of studies +ve
effect -ve
effect No
effect Number
of studies +ve
effect -ve
effect No
effect
Mothers 16 16
(100) 0
0
11
11
(100) 0
0
27 27
(100) 0
0
Fathers 13 7
(54) 0
6
(46) 7 0
0
7
(100) 20 6
(30) 0
14
(70)
Maternal gms 10 8
(80) 1
(10) 1
(10) 2 2
(100) 0 0 12 10
(84) 1
(8) 1
(8)
Paternal gms 13 8
(62) 2
(15) 3
(23) 2 0 0 2
(100) 15 8
(54) 2
(13) 5
(33)
Non-specific gms 3 1
(33.3) 0 2
(66.7) 0 0 0 0 3 1
(33.3) 0 2
(66.7)
Maternal gfs 9 3
(33) 0 6
(67) 2 0 0 2
(100) 11 3
(27) 0 8
(73)
Paternal gfs 9 3
(33) 2
(22) 4
(45) 2 1
(50) 0 1
(50) 11 4
(36) 2
(18) 5
(46)
Non-specific gfs 2 0 1
(50) 1
(50) 0 0 0 0 2 0 1
(50) 1
(50)
Older sibs 6 5
(83) 0
1
(17) 0 0 0 0 6 5
(83) 0 1
(17)
51
