Grandparental investment: Past,
present, and future
David A. Coall
School of Psychiatry and Clinical Neurosciences, University of Western
Australia, Fremantle, Western Australia 6160, Australia
david.coall@uwa.edu.au
http://www.uwa.edu.au/people/david .coall
Ralph Hertwig
Department of Psychology, University of Basel, 4055 Basel, Switzerland
ralph.hertwig@unibas.ch
http://www.psycho.unibas.ch/hertwig
Abstract: What motivates grandparents to their altruism? We review answers from evolutionary theory, sociology, and economics.
Sometimes in direct conflict with each other, these accounts of grandparental investment exist side-by-side, with little or no
theoretical integration. They all account for some of the data, and none account for all of it. We call for a more comprehensive
theoretical framework of grandparental investment that addresses its proximate and ultimate causes, and its variability due to
lineage, values, norms, institutions (e.g., inheritance laws), and social welfare regimes. This framework needs to take into account
that the demographic shift to low fecundity and mortality in economically developed countries has profoundly altered basic
parameters of grandparental investment. We then turn to the possible impact of grandparental acts of altruism, and examine
whether benefits of grandparental care in industrialized societies may manifest in terms of less tangible dimensions, such as the
grandchildren’s cognitive and verbal ability, mental health, and well-being. Although grandparents in industrialized societies
continue to invest substantial amounts of time and money in their grandchildren, we find a paucity of studies investigating the
influence that this investment has on grandchildren in low-risk family contexts. Under circumstances of duress – for example,
teenage pregnancy or maternal depression – there is converging evidence that grandparents can provide support that helps to
safeguard their children and grandchildren against adverse risks. We conclude by discussing the role that grandparents could play
in what has been referred to as Europe’s demographic suicide.
Keywords: child development; demographic transition; empathy; grandmother hypothesis; grandparental investment; grandparental
solicitude; intergenerational transfers; kin altruism; maternal depression; reciprocal altruism
1. Introduction
What is it about grandparents that is so lovely? I’d like to say that
grandparents are God’s gifts to children. And if they can but see,
hear and feel what these people have to give, they can mature at a
fast rate.
— Bill Cosby
You have to do your own growing no matter how tall your grandfather
was.
— Irish proverb
The best babysitters, of course, are the baby’s grandparents. You feel
completely comfortable entrusting your baby to them for long
periods, which is why most grandparents flee to Florida.
— Dave Barry (Babies and Other Hazards of Sex, 1984, p. 54)
From birth to adulthood children are gas-guzzlers. Across
three traditional South American mixed-economy hunter–
gatherer and horticulturist societies, Kaplan (1994) found
that a child from birth to age 18 years consumes between
10 and 13 million more calories than the child itself
produces. Who foots the bill? In cooperative breeding
species such as humans, where child-rearing is not the
exclusive domain of the parents, alloparents – that is,
helpers other than the biological parents (Hrdy 1999;
2009) – can step up. According to the cooperative breed-
ing hypothesis, ancestral mothers evolved in groups
“where a broad range of individuals – not just the
genetic father – assist the genetic mother in protecting,
carrying, or provisioning offspring, thereby permitting
DAVID COALL is a Research Assistant Professor in the
Community, Culture, and Mental Health Unit, School
of Psychiatry and Clinical Neurosciences at the Univer-
sity of Western Australia. Still early in his research
career, he has authored 11 publications across the
fields of human biology, psychology, and pediatrics.
He is especially interested in applying evolutionary
theory to understand variation in human health and
disease with a particular focus on intergenerational
effects and the childhood environment.
R
ALPH HERTWIG, Professor of Cognitive and Decision
Sciences at the University of Basel, Switzerland, is the
author of more than 75 publications in the areas of be-
havioral decision research and the methodology of
social sciences. He received the German Society for
Psychology’s Young Scientist Award in 1996 and
Early Career Award in 2006.
BEHAVIORAL AND BRAIN SCIENCES (2010) 33,1–59
doi:10.1017/S0140525X09991105
# Cambridge University Press 2010 0140-525X/10 $40.00 1
her to produce and rear costlier, slower-maturing offspring
than otherwise would survive” (Hrdy 2005a, p. 69).
Indeed, Kaplan observed that a child’s “excessive” con-
sumption is offset by adults who, between 20 and 40
years of age, produce an average surplus of 2,000 to
4,000 calories a day, which is generally maintained until
65 years of age (see also Kaplan 1997; Kaplan et al. 2000).
Who are these seeming altruists? Although it is still being
debated whether the key sources of this and other help are
fathers, grandmothers, grandfathers, siblings, or children
(e.g., Hawkes et al. 1989; Hill 1993; Hrdy 1999; Ivey
2000; Kramer 2005a), both formal and informal models
point to the evolutionary significance of intergenerational
transfers (e.g., Bogin 1997; Hamilton 1966; Lee 2003a;
2008). Specifically, because individuals can increase their
fitness by reproducing themselves or by helping kin who
share common genes by descent (Hamilton 1964), kin
selection theory (Maynard Smith 1964) predicts that it is
most likely that these helpful individuals in the mother’s
social group will be close kin, such as children, siblings,
uncles, aunts, and, crucially, grandparents. Indeed,
although humans are similar to other cooperative breeding
animals in many respects, it is the presence of knowledge-
able, postreproductive helpers that sets them apart (Hrdy
2005b). During the prolonged postreproductive period,
grandparents may have the opportunity and the motive
to boost their own fitness by investing resources in their
children’s and grandchildren’s survival and reproduction.
The opportunity for grandparents to care for their kin
has never been as great as it is today. With increasing
human life expectancy in industrialized societies – for
example, in the United States, women’s life expectancy
in the 20th century has increased from 50.7 (born 1900)
to 79.7 (born 2000) years (see Arias 2006) – today’s grand-
parents’ lives and those of their grandchildren overlap
markedly, thus providing them with greater opportunity
than ever before to play a significant role (Bengtson
2001). Empirical evidenc e supports the considerable role
that grandparents play in taking care of their grandchil-
dren. In Switzerland, for instance, it is estimated that
unpaid grandparental childcare amoun ts to at least two
billion Swiss francs per year (Bauer & Strub 2002). In
the United States, 28% of employed women rely on their
parents or in-laws to provide childcare for their young chil-
dren (Guzman 1999). Across ten European countries, 58%
of grandmothers and 49% of grandfathers provided some
care for their grandchild during a 12-month period
(Hank & Buber 2009; similar percentages are found in
the United States: Guzman 2004). A sample of Germans
aged 55 to 69 years was observed to spend, on average,
12.8 hours each month supervising their grandchildren
(Kohli et al. 2000).
Despite grandparents’ historically unparalleled oppor-
tunity to care for their grandchildren, the impact of their
investments may never have been smaller than it is
today, at least when measured against classic components
of fitness, such as number of offspring and child mortality.
Specifically, with fertility rates below replacement and a
delayed age at first childbirth in most industrialized
societies, the probability of becoming a grandparent is
falling. Moreover, low childhood mortality rates mean
that the high-risk times of infancy – conditions under
which grandparental investment may have evolved –
have largely been removed.
In light of these changes in grandparenthood in indus-
trialized societies, we ask: Does the help that grandparents
provide, which may have benefited grandchildren in
traditional and historical populations, still yield benefits
for grandchildren in industrialized societies? Our theoreti -
cal and empirical review in this article is structured as
follows. Section 2 defines grandparental investment,
spells out its underlying evolutionary rationale, and sum-
marizes evidence from natural fertility societies. Section
3 turns to recent extensions of research on grandparental
investment in terms of the notion of grandparental solici-
tude. Specific ally, we review several likely determinants of
grandparental investment across human populations. In
Section 4 we describe economic and sociological accounts
of grandparental investment. Sometimes in direct conflict
with each other, the economic, evolutionary, and socio-
logical accounts explain some of the empirical data, but
none explain all. Section 5 examines a range of recent
demographic changes that render the environment in
which contemporary grandparents find themselves
unique in human history, and that have profoundly
altered the basic parameters of grandpar ental investment.
In Sections 6 and 7, we review evidence addressing the
issue of whether grandparental care, which benefited
grandchildren in traditional and historica l populations,
still yields benefits for grandchildren in industrialized
societies. Finally, in Section 8, we discuss public health
and policy implications of grandparental investment.
2. Grandparental investment: Definition,
evolution, and evidence
2.1. What is grandparental investment?
Resources invested in one’s offspring are referred to as par-
ental investment (see Geary 2000). Parental investment
was originally defined as “any investment by the parent in
an individual offspring that increases the offspring’s
chance of surviving (and hence reproductive success) at
the cost of the parent’s ability to invest in other offspring”
(Trivers 1972, p. 139). Trivers’ key point was that the cur-
rency of parental investment is its cost to the parent’s
ability to invest in other offspring. Therefore, the definition
of parental investment was highly restrictive. However, a
broader definition has evolved that incorpo rates:
any characteristics or actions of parents that increase the
fitness of their offspring at a cost to any component of the
parent’s fitness ..., including any costs of parental care to
the parent’s subsequent mating success ... survival ... fecund-
ity ... growth .... In addition, it is logical to include costs to
fitness of other offspring or any ... other relatives. (Clutton-
Brock 1991, p. 9)
We interpret parental investment and, by extension,
grandparental investment using this broader frame (see
also Dawkins 2006; Ge ary 2000; Hertwig et al. 2002).
Proxy outcome measures of parental investment and – to
the extent that grandparents influence these outcomes –
grandparental investment that have been used include
child mortality, birth weight, breast-feeding, infanticide,
homicide, abuse, attachment/bonding between parents
and infants, inheritance, interbirth interval, and edu-
cational investment (Hagen et al. 2006). The pathway
through which grandparental investment affects a child’s
development can be direct (e.g ., direct support, advice,
Coall & Hertwig: Grandparental inve stment
2
BEHAVIORAL AND BRAIN SCIENCES (2010) 33:1
or as a role model) and indirect via a third party (most
often the grandchildren’s parents).
2.2. Why grandparental investment?
Cooperative breeding species such as humans can increase
their inclusive fitness directly by reproducing themselves
or indirectly by helping their relatives, who share a pro-
portion of their genotype by descent, to transmit their
genes into future generations (Hamilton 1964). Grandpar-
ents can boost their inclusiv e fitness by devoting resources
to their kin, helping to ensure the conception, birth, survi-
val, growth, development, and reproduction of their rela-
tives. The opportunity to increase indirect fitness is
afforded by the long human lifespan in general, and in par-
ticular by the extensive postreproductive lifespan of
human females that is unique among cooperative breeding
animals (Hrd y 2005b). Although various forms of repro-
ductive termination occur across a range of species
(Cohen 2004; Packer et al. 1998 ; Paul 2005), humans are
the only primate species in which reproductive termin-
ation is a distinct and universal trait (Caro et al. 1995;
Pavelka & Fedigan 1991). The survival of an organism
beyond reproduction represents a biological anomaly:
“[T]here should be little or no postreproductive period
in the normal lifecycle of any species” (Williams 1957,
p. 407). Attempting to explain this anomaly, Williams
hypothesized that it would be adaptive to cease reproduc-
tion before death if the benefits to the woman’s survival –
that is, removal of the risks associated with dying during
childbirth, and the increased parenting effort that she
could devote to her living children and grandchildren –
outweighed the benefits to reproductive fitness brought
to the woman by further reproduction.
Couched in terms of “Hamilton’s Rule” (Hamilton
1964), postreproductive helping behavior will evolve
when an individual can help a related individual, who
has a probability r (i.e., coefficient of relatedness) of
sharing this same gene by common descent, to survive
and reproduce. This holds as long as the benefit (B, i.e.,
benefit to the relative receiving help), multiplied by the
probability of sharing that gene (r), outweighs the cost
(C) to the helper, in terms of his own survival and
forgone reproductive opportunities. To wit, postreproduc-
tive helping behavior should evolve when rB . C.Itis
likely that this condition held for human ancestors
because (a) grandparents are closely related to their grand-
children (grandparents, on average, share 25% of their
genes with each of their grandchildren, over and above
the large proportion of genes that all humans share in
any case; Dawkins 2006), (b) helping comes with a rela-
tively low opportunity cost, in terms of the grandparent’s
own reproductive success (especially in the case of postre-
productive grandparents), and (c) there is a potentially
large benefit to grandchildren in terms of survival (and
subsequent reproduction) during the high-risk times of
infancy and childhood.
1
Instigated by Williams’ (1957) suggestion of the poten-
tial adaptive value of a pos treproductive period, two
related lines of reasoning have emerged, both emphasizing
the vast investments required by the long childhood devel-
opment period: the good-mother hypothesis and the
grandmother hypothesis (reviews by Jamison et al. 2002;
Peccei 2001a; Voland et al. 2005).
2
The good-mother
hypothesis holds that burdened with the long dependence
of human children, a postreproductive period shields the
mother from survival risks (i.e., pregnancy and giving
birth) and the child from health risks (e.g., Down syn-
drome), and enables the mother to free up the resources
to keep her ch ildren, particularly the youngest and most
helpless, alive. This investment contributes to the
mother’s direct reproductive success by ensuring that her
existing children survive. Thus, one would expect to see
the presence of a mother and her increased longevity
improving the survival chances and health state of her
children, and we do.
3
The grandmother hypothesis suggests that a pos trepro-
ductive grandmother can foster her inclusive fitness by
supporting her reproductive daughter and her grandchil-
dren, thus increasing her daughter’s fertility and improv-
ing her grandchildren’s chances of surviving (e.g., Hrdy
1999). Moving the provisioning of children from exclu-
sively the mother (i.e., breast-feeding) to allomothers, as
the grandmother hypothesis proposes, has immediate
reproductive consequences that could translate into
inclusive fitness advantages for the allomothers (grand-
mothers): For example, ceasing to breast-feed term inates
the mother’s lactational amenorrhea (Ellison 1990), thus
potentially redu cing the interval to the daughter’s next
pregnancy. Comparative data confirm that humans wean
their children at a younger age and have shorter inter-
birth intervals than do orangutans, gorillas, and chimpan-
zees (Hawkes et al. 1998; Kramer 2005a; Lancaster et al.
2000). The notion of the helpful grandmother, however,
should not be conceptualized as selection to blindly maxi-
mize the number of offspring, because any increase in
number of descendents is often accompanied by a
decrease in investment per offspring and potentially survi-
val (e.g., Coall et al. 2009; Gibson & Mace 2005; Walker
et al. 2008). Thus, we would expect to see the presence
of a grandmother and her increased longevity improving
the fertility of her daughter, but not at the expense of
the survival and health of her grandchildren.
Although the good-mother hypothesis and the grand-
mother hypothesis, which are not mutually exclusive,
predict a selective advantage associated with a postrepro-
ductive lifespan, attempts at modeling this advantage in
contemporary, largely natural fertility, human populations
have failed to support these hypotheses (e.g., Hill &
Hurtado 1991; 1996; Rogers 1993; see also Austad 1994).
The benefits of early reproductive cessation in women –
diverting resources from reproduction to maternal care
and avoiding the costs of continuing to reproduce – do
not appear to outweigh the benefits of having additional
children. However, consideration of additional costs,
such as the decline in fertility with older age, increased
perinatal mortality risk associated with giving birth at an
older age, and increased competition with increased
family size, as well as the variety of additional benefits –
earlier reproduction in daughters, improved growth and
survival of grandchildren – suggests that the benefits of
switching from reproduction to parenting could indeed
outweigh the costs (Hrdy 2005a; Mace 2000; Turke
1997). Moreover, a recent analysis of data from two
Gambian villages (1950–1975) showed that menopause
may have a fitness advantage when the combined influence
of both the incre ased maternal mortality associated with
giving birth at an older age and the increased childhood
Coall & Hertwig: Grandparental investment
BEHAVIORAL AND BRAIN SCIENCES (2010) 33:1 3
survival associated with having a maternal grandmother
present are taken into account (Shanley et al. 2007; see
also Sear et al. 2000; Shanley & Kirkwood 2001). Thus,
the joint pos itive influence of a “good mother” and a
“helpful grandmother” on survival during childhood may
have created a selective advantage for postreproductive
survival in women.
2.3. Why postreproductive lifespan may not be adaptive
In contrast to the aforementioned views, there are evol-
utionary theories proposing that the early cessation of
reproduction, which implies the availability of postrepro-
ductive grandmothers, is an exaptation. It is not or was
not originally adaptive (for reviews, see Peccei 2001a;
Voland et al. 2005). On this view, menopause occurs
because a woman has lived lon g enough to deplete the
supply of primary oocytes with which she was born
(Ellison 2001; but see Johnson et al. 2004; Skaznik-
Wikiel et al. 2007). Several theories of senescence
propose that small fitness benefits early in an individual’s
life will be selected for even if they exact a cost to fitness
later in life. The reduced strength of natural selection at
older ages permits these negative effects to remain unchal-
lenged (Kirkwood 1977; Medawar 1952; Williams 1957).
Therefore, rather than being advantageous itself, meno-
pause may be a by-product of the adaptive benefit of
producing, protecting, and storing a complete stock of
primary oocytes at birth (Pavelka & Fedigan 1991). Con-
sistent with this interpretation is the similarity of human
and chimpanzee ovarian follicle depletion rates (Jones
et al. 2007; for gorillas see Atsalis & M argulis 2008),
which suggests that menopause may be a phylogenetic
legacy and not unique to humans.
Even if this is the case, and menopause is a non-adaptive
epiphenomenon that occurs because of humans’ longevity,
its presence creates the stage for helpful grandmothers.
Bogin and Smith (1996) proposed a biocultural model in
which menopause is non-adaptive but helpful grand-
mothers still play a central role. Once women started
living past their reproductive ceiling, the only way that
they could improve their inclusive fitness was by caring
for their children and grandchildren. Whether human long-
evity and specifically the postreproductive longevity associ-
ated with menopause is adaptive or an epiphenomenon,
grandparents are still in the right place, at the right time.
2.4. Does grandparental investment enhance fitness?
Evidence from natural fertility societies
The influence of helpful grandm others has been most
intensively studied in historical and contemporary
natural fertility societies with little or no access to
modern medical care, including contraception. In these
societies, grandparental investment matters for mothers’
reproductive success. Sear and Mace’s (2008) review of
45 studies investigating the presence of kin (father, grand-
parents, older siblings) supports the beneficial influence of
postreproductive relatives: The presence of a maternal or
paternal grandmother was associated with an increase in
her grandchildren’s probability of surviving in 69% (9 of
13 studies) and in 53% (9 of 17 studies) of cases, respect-
ively. For illustra tion, in their time series analysis of the
Oromo agro-pastoralists of southern Ethiopia, Gibson
and Mace (2005) found that the presence of a hard-
working maternal grandmother, who helped the mother
with heavy domestic tasks (e.g., grinding maize), increased
the probability of a grandchild surviving to three years of
age by 25%. This effect is comparable to that ach ieved
by installing a new water supply (Gibson & Mace 2006).
Omnipresent grandparents, however, are not inev itably
advantageous. Sear and Mace (2008) found evidence that
the presence of grandparents has been associated with
detrimental consequences for child survival. In 83% (10
of 12 studies) of cases, the presence of maternal grand-
fathers had no effect, and in 75% the paternal grandfather
had no effect or even a negative one on survival. There is
currently little evidence supporting the adaptive signifi-
cance of grandfathering (see Lahdenpera
¨
et al. 2007).
Negative effects have also been reported for paternal
grandmothers who may, in contrast to maternal grand-
mothers, increase their daughter-in-law’s fertility at the
expense of grandchild survival (Leonetti et al. 2005;
2007; Mace & Sear 2005). For example, in a historical
German population (1720–1874), the presence of a
paternal grandmother increased a grandchild’s risk of still-
birth by 35% (Voland & Beise 2005) and infant mortality
up to one month of age by 85% (Voland & Beise 2002).
The majority of these studies have been conducted in patri-
lineal societies; recent evidence from a matrilineal society
suggests that even maternal grandmothers may have a
negative effect under some circumstances (Sear 2008; but
see Leonetti et al. 2005). The beneficial influence of grand-
parents cannot be taken as given. But there is evidence that
the presence of maternal grandmothers in particular boosts
the survival chances of their grandchildren.
2.5. The issue of confounding effects
The grandparental investment literature consists predomi-
nantly of correlational studies showing associations, or lack
thereof, between grandparental presence and grandchild
survival, for example. Of course, these studies do not
show that this relationship is causal (Borgerhoff Mulder
2007; Sear & Mace 2008). Some argue that shared genes
or environment effects may equally account for the
observed associations. Grandparents who live under
good socioeconomic conditions or who have a fortuitous
genetic complement are more likely to live long and
healthy lives, and it would make sense that their grandchil-
dren are more likely to survive, are healthier, or have
better cognitive abilities (e.g., Modin & Fritzell 2009).
Although this possibility cannot be ruled out, several
pieces of evidence speak against it. First, if the effect is
due to shared genes or environment, one would expect
that all grandparents who share the same pro portion of
genes by common descent or share their residence with
a grandchild to have similar effects on grandchild survival.
Clearly, this is not the case, as we discuss in the next
section. Second, the detai led ethnographic accounts that
accompany many correlational studies not only corrobo-
rate associations between kin presence and survival,
growth, and develo pment, but also highlight potential
behaviors that promote these beneficial effects (e.g.,
Gibson & Mace 2005; Leonetti et al. 2007). Third, many
studies allow adjustment for a wide range of confounding
variables (e.g., grandparental health, age, socioeconomic
status, and residential di stance) that at least partially
Coall & Hertwig: Grandparental inve stment
4
BEHAVIORAL AND BRAIN SCIENCES (2010) 33:1
control for shared environment effects (e.g., Borgerhoff
Mulder 2007; Coall et al. 2009; Pollet et al. 2006). Taken
together, this evidence suggests that it is unlikely that
these effects can be exp lained away by shared gene or
shared environment effects.
3. Grandparental solicitude
Grandparental helping behavior will not occur invariably,
but, according to Hamilton’s rule, is moderated by oppor-
tunity costs that may differ across types of grandparents
(e.g., grandmother vs. grandfa ther). Indeed, one of the
most robust findings across the grandparental investment
literature is that maternal grandmothers invest the most,
have most contact, and the closest relationships with
their grandchildren, followed by maternal grandfathers,
paternal grandmothers, and, finally, paternal grand-
fathers.
4
Next to grandparent type, grandparental help is
also likely to be a function of ecological conditions that
determine the availability of grandparental resources,
including the communities’ subsistence strategy (e.g., the
efficiency of food production), availability of relatives,
marriage systems, residence patterns, inheritance pat-
terns, and the health of relatives (reviewed by Sear &
Mace 2008). In what follows, we review five evolutionary
factors that affect grandparents’ inclination to invest
their resources – genetic relatedness, paternity certainty,
sex-specific reproductive strategies , reproductive value,
and sex-biased grandparental investment – and two
factors that influence the availability of resources –
family size and birth order.
3.1. Genetic relatedness
Hamilton’s rule implies that the amount of resources
transferred to an individual should depend on the related-
ness of the provider (r, the coefficient of relatedness). The
closer the genetic relationship between two individuals,
the more likely helping behaviors will evolve and the
smaller the benefit needs to be for this altruism to be
maintained (Hamilton 1964). There has been a long
history of research suggesting that parents allocate their
resources to children according to their genetic related-
ness (e.g., Anderson 2005; Daly & Wilson 1980; but see
Hofferth & Anderson 2003). Evidence that grandparents
favor genetically related grandchildren also exists. The
investment that grandpar ents make in a family reduces
as the proportion of stepchildren in the family increases
(Eggebeen 1992). Relatedly, relationships with step-
grandparents are not considered to be as close as those
with biological grandparents (Aldous 1995). In lesbian-
mother families, grandchildren have more contact with
the biological grandparents compared with non-biological
grandparents (Patterson et al. 1998). Although the size of
the variance accounted for in these studies and the neces-
sity of a focus on genetic relatedness has been questioned
(Rose & Rose 2001), the patterns appear robust (see
Anderson 2005).
3.2. Paternity certainty
Paternity uncertainty is the (perceived) risk that a male is not
the biological father of his children. In contrast to males,
because of internal fertilization mammalian mothers know
categorically who their children are (Trivers 1972). In the
few mammalian species in which males provide parental
investment, they run the risk that their investment is being
wasted on another male’s offspring (Alexander 1974;
Clutton-Brock 1991; Geary 2000; Trivers 1972). Both theor-
etical and empirical studies suggest that lower levels of
paternity certainty are associated with reduced male par-
ental care (see Anderson et al. 2007).
This logic can be extended to grandparents and their
inclination to invest resources: Because the maternal
grandmother is certain of her relationship with her daugh-
ter and her daughter’s relationship with her grandchil-
dren, it is predicted that maternal grandmothers will
invest the most. Paternal grandfathers, on the other
hand, are predicted to invest the least because there are
two points of uncertainty between themselves and their
grandchildren: They are not 100% certain of their relation-
ship with their son nor of their son’s relationship with their
grandchildren. The maternal grandfather and paternal
grandmother are indistinguishable between these two
extremes and will both invest an intermediate amount,
because they both have one point at which their relation-
ship certainty with their grandchildren could be severed
(Smith 1987; 1988).
Consistent with these predictions, across human
societies maternal kin invest much more than paternal
kin during the prenatal, delivery, and postnatal periods
(Huber et al. 2004; Meehan 2005), and paternity uncer-
tainty seems to be associated with reduced intergenera-
tional investment from the father (Anderson et al. 2007;
Kurland 1979) and the father’s relatives (Gaulin & Schle-
gel 1980; Hartung 1985). Yet, some caution should be
taken in interpreting the effect of paternity uncertainty.
Paternity certainty varies considerably across human
societies (Anderson 2006). Moreover, throughout recent
human history paternity ce rtainty has increased substan-
tially in many cultures (Gaulin 1980). Although the rebut-
tal to this statement is usually that pa ternity certainty was
lower in our ancestral environment, and it is to this that
humans are tuned, the findings that investment varies by
paternity certainty within communities suggest that
humans respond to local conditions. Other authors state
that focusing on paternity certainty alone is inadequate
(Euler & Weitzel 1996; Pashos & McBurney 2008) and
that in patrilateral and patriloc al societies it is unable to
account for the higher levels of investment by paternal
relatives (Pashos 2000; but see Gibson & Mace 2005).
3.3. Sex-specific reproductive strategies
Several authors have addressed the limits of paternity cer-
tainty by incorporating sex-specific reproductive strategies
into their models of grandparental investment (Euler &
Michalski 2007; Euler & Weitzel 1996; Huber & Bree-
dlove 2007). Beca use of the large energetic investment
in the egg, as compared with the sperm, females are
physiologically obligated to provide higher levels of par-
ental investment than males (Bateman 1948). In
mammals, internal gestation and the production of milk
for offspring further increase this asymmetry (Clutton-
Brock 1989). Whereas females may focus on parental
investment, producing a few high-quality offspring,
males (because of their low levels of parental investment)
Coall & Hertwig: Grandparental investment
BEHAVIORAL AND BRAIN SCIENCES (2010) 33:1 5
