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86 VOLUME 8, NUMBER 3, JUNE 1999
Sex differences in behaviors tra-
ditionally have been attributed to
“social” factors, because of the pre-
sumed flexibility of human behav-
ior. Proposals that evolved mecha-
nisms explain why men and
women behave as they do have
been regarded warily because of
the belief that “biological” causa-
tion (including evolutionary caus-
es) implies biological determinism.
Modern evolutionary psychologi-
cal theory, however, makes no such
claims, but argues that contrasting
“social” with “evolutionary” expla-
nations is a false dichotomy (see
Tooby & Cosmides, 1992). From
this perspective, human nature in-
cludes evolved psychological
mechanisms that require input
such as social norms and cultural
beliefs for their operation, and dif-
ferent behavioral outcomes will re-
sult from these evolved mecha-
nisms in different environments. In
this article, we apply a theory moti-
vated by such evolutionary psy-
chological logic, parental invest-
ment theory (Trivers, 1972), to
interpret some well-documented
sex differences.
According to an evolutionary
psychological perspective (see
Buss, 1994), ancestral men and
women faced different adaptive
problems that threatened their sur-
vival and reproduction. As a result,
they evolved different psychologi-
cal mechanisms, and evidence of
this ancient heritage is apparent in
modern humans (for a review, see
Buss, 1994). Parental investment
theory accounts for many of these
differences. According to this theo-
ry, there is a conflict for both males
and females in how much time, ef-
fort, and resources to invest in mat-
ing versus parenting. For many
species, including humans, males
need to invest substantially less in
parenting than females to achieve
successful reproduction. In mam-
mals, fertilization and gestation
occur within the female, and, after
birth, mothers provide the primary
nutritional support for their off-
spring until they are weaned. Male
investment in offspring may be as
little as the sperm produced during
copulation. In humans, paternal in-
vestment is not essential for a
man’s offspring to reach adult-
hood, and although the amount of
time men spend in child care varies
across cultures, fathers spend less
time interacting with and caring for
their children than do mothers in
all cultures that have been studied
(see Geary, 1998).
Yet human males spend more
time caring for their offspring than
is typical for male mammals. One
factor contributing to this unusually
high level of paternal investment is
humans’ extended childhood.
Humans spend more time as juve-
niles than other primates, needing
many years to develop the brain
and the knowledge necessary to
navigate the complexities of society.
This prolonged immaturity requires
a supportive environment to which
both mothers and fathers ideally
contribute. Evidence from many
cultures and historical records indi-
cates that the death rate of offspring
increases as a function of father’s
absence, particularly in harsh envi-
ronments (see Geary, 1998). Thus,
whereas mothers opt to invest sub-
stantially in their children’s care
after birth in order to ensure their
offsprings’ survival, the case is not
as clear-cut for fathers, particularly
when one considers the number of
additional offspring men can have
by investing more effort in mating.
Women also may choose to invest
less in the care of a child and more
in mating, but the caloric costs and
duration of pregnancy and nursing
(in traditional societies today, and
surely in our evolutionary past,
nursing extends to the age of 3 or 4
years) reduce the number of chil-
dren a woman can expect to have in
her lifetime.
There is evidence that paternal
investment influences socialization
practices and the amount of
parental investment subsequent
generations devote to their off-
spring (Belsky, Steinberg, & Draper,
1991). In environments where fa-
thers are absent or where there is
marital discord, the resulting stress
produces harsh and inconsistent
child care and insecure attachment.
Published by Blackwell Publishers, Inc.
Differences in Parental Investment
Contribute to Important Differences
Between Men and Women
David F. Bjorklund and Todd K. Shackelford1
Department of Psychology, Florida Atlantic University, Boca Raton, Florida
Abstract
Parental investment theory
addresses sex differences that
result from the trade-off be-
tween parenting effort and
mating effort. For example,
relative to men, women spend
more time caring for offspring,
are more selective in assenting
to sexual intercourse, are more
upset by a partner’s emotional
infidelity than by a partner’s
sexual infidelity, and are better
able to inhibit their behaviors
in certain situations. These and
other sex differences are attrib-
utable to evolved mechanisms
that work in interaction with
the physical and social envi-
ronments.
Keywords
parental investment theory;
evolutionary psychology; sex-
ual strategies
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 87
Under such situations, children
reach puberty early, form short-
term and unstable relationships,
and invest relatively little in their
own offspring. In stressful and un-
certain environments, there is a ten-
dency to invest more in mating (for
both sexes) than in parenting. The
pattern is reversed for children
growing up in harmonious homes
and homes where the father is pres-
ent; such children mature later,
postpone sexual activity, and dis-
play greater investment in the fewer
children they produce. In sum, the
availability of resources, which is
related to paternal investment and
spousal harmony, leads to different
patterns of socialization, resulting
in differential parental investment
in the next generation.
The differential investment of
males and females in their off-
spring has resulted in the evolution
of different ways of behaving and
thinking in men and women.
Although these differences are gen-
erated by evolved psychological
mechanisms, they are more vari-
able than is typical in mammals.
Here, we discuss two areas in
which men and women behave and
think differently, as predicted by
parental investment theory: sexual
strategies and inhibitory abilities.
One class of sex differences that
can be understood in terms of
parental investment theory is sexu-
al strategies (Buss, 1994). Males in
most mammals can achieve
tremendous reproductive success
by inseminating many females,
making males relatively indiscrim-
inate when it comes to choosing a
sex partner. Females, in contrast,
have much more invested, at least
potentially, in a single copulation.
The possibility of pregnancy, and
the time and energy spent caring
for the resulting offspring, favored
ancestral females who were selec-
tive in mating. Because of the long
period of immaturity in humans,
ancient women’s reproductive in-
terests were often best served by
selecting a mate who not only
would provide good genes (e.g., as
signaled by facial symmetry;
Shackelford & Larsen, 1997), but
who also would invest in her and
her offspring. Over evolutionary
time, it was also in men’s reproduc-
tive interests to see to it that their
offspring received the support nec-
essary to survive to reproductive
age. But the amount and duration
of investment necessary to ensure
the survival of offspring was less
for men than for women. Thus, al-
though both men and women
shared a common reproductive
goal (getting their offspring to
adulthood), the optimal level of in-
vestment to achieve this goal was
unequal for the sexes, placing
males’ and females’ reproductive
interests in conflict.
Men are not indiscriminate
when it comes to selecting a mate,
especially when selecting a long-
term partner. Men around the
world want long-term partners
who are attractive, intelligent, and
kind. Women also desire attractive,
intelligent, and kind men as hus-
bands, but, as predicted by
parental investment theory, they
rate financial resources as more im-
portant in a mate than do men
(Buss, 1989). Despite the selectivity
that men and women universally
display in choosing a long-term
partner, and consistent with
parental investment theory, women
are more selective in granting sex,
and men are more eager to have ca-
sual sex (see Buss, 1994).
Men and women have evolved
different psychological mecha-
nisms as solutions to the adaptive
problems unique to their sex. For
example, fertilization occurs within
the female, and it is the female who
gestates and gives birth to the off-
spring. This greater prenatal in-
vestment of the female comes at
considerable cost but brings with it
certainty of maternity. In contrast,
males, who may invest only sperm
and the energy necessary to copu-
late, cannot be certain of paternity.
Furthermore, because women con-
ceal ovulation and are potentially
sexually receptive throughout their
menstrual cycle, it is difficult for a
man to know when copulation is
likely to result in pregnancy. As a
result, being the unwitting social
father to another man’s genetic off-
spring is a possibility for men. This
in fact occurs in between 2% and
30% of all births; the rate is similar
in traditional societies, and there is
no reason to believe that it differed
substantially for our ancestors (see
Baker & Bellis, 1995). Women, of
course, cannot so easily be fooled
into rearing another woman’s child
who they believe is their own, al-
though they do risk losing a mate’s
investment to another woman.
One consequence of this sex dif-
ference in the certainty of genetic
parenthood can be seen in men’s
and women’s reactions to a long-
term partner’s infidelity. When
asked to imagine that their long-
term partner is either (a) having ca-
sual sex with another person or (b)
developing a close emotional rela-
tionship with another person, men
and women respond differently.
Verbal reports and measures of
physiological arousal indicate that
women are more upset by a part-
ner’s emotional infidelity, which
could signal a loss of resources,
whereas men are more upset by a
partner’s sexual infidelity (Buss,
Larsen, Westen, & Semmelroth,
1992).
In conclusion, human sex differ-
ences in parental investment predict
sex differences in sexual strategies.
These sex differences include the
greater inclination of men to pursue
short-term casual sex; the greater
selectivity of women in choosing a
mate, especially in the context of
short-term mating; and the greater
SEXUAL STRATEGIES
Copyright © 1999 American Psychological Society
88 VOLUME 8, NUMBER 3, JUNE 1999
distress of men in response to a
long-term partner’s sexual infideli-
ty and of women in response to a
partner’s emotional infidelity.
Inhibition refers to the withhold-
ing of a response in situations in
which that response otherwise
would be made. The ability to in-
hibit inappropriate sexual and ag-
gressive responses is important for
the success of both men and women
in modern (and presumably ances-
tral) society and may have played a
role in the evolution of human intel-
ligence, particularly social intelli-
gence (Bjorklund & Harnishfeger,
1995). However, ancestral women
may have needed greater inhibitory
abilities than ancestral men in
certain contexts, because of their
different reproductive strategies
(Bjorklund & Kipp, 1996). For exam-
ple, because of ancestral women’s
greater investment in the potential
consequences of an act of copula-
tion, it might have been in their re-
productive interests to have greater
control of their sexual arousal and
related behaviors in order to more
closely evaluate the value of a man
before assenting to sex. Ancestral
women also may have needed sub-
stantial political skill in order to
keep sexual interests in other men
hidden from a mate. Male response
to suspected female infidelity can
be violent, and even when adultery
does not lead to aggression, it often
leads to divorce, which both histor-
ically and in contemporary societies
is more detrimental to a woman and
her offspring than to a man (see
Buss, 1994).
Similarly, the bulk of child-care
responsibilities falls to women, and
these also may require enhanced
inhibitory abilities. For example,
parents often must put the needs of
their infants ahead of their own,
delaying gratification of their own
desires and resisting distractions
that would take them away from
their infants. They also must inhib-
it aggression toward infants or
young children who may cry con-
tinuously, disobey, and damage
personal property.
In support of these hypotheses,
research has shown that women
are better able to control the ex-
pression of their emotions than are
men, despite the fact that women
are more emotionally expressive
than men. In studies in which peo-
ple are asked to display a positive
emotion after a negative experience
(e.g., pretending that a foul-tasting
drink tastes good) or vice versa, fe-
males (from the age of 4 years and
up) are better able to control their
emotional expressions (i.e., fool a
judge watching their reactions)
than are males (e.g., Cole, 1986). In
other research, there is evidence
that females are better than males
on tasks that involve resisting
temptation and delaying gratifica-
tion (e.g., Kochanska, Murray,
Jacques, Koenig, & Vandegeest,
1996), precisely the pattern one
would predict if selection pressures
associated with child care were
greater on ancestral females than
on ancestral males. And there is
limited evidence that women are
better able to inhibit sexual arousal
than are men (Cerny, 1978; Rosen,
1973). In contrast, there is no fe-
male advantage in inhibitory abili-
ties for cognitive tasks such as se-
lective attention, suggesting that
the sex differences are relatively
domain-specific, restricted to abili-
ties related to sexual and parenting
contexts, as predicted by parental
investment theory.
Different psychological mecha-
nisms in men and women, attribut-
able to sex differences in minimum
parental investment over evolu-
tionary history, provide the skeletal
features for adapted behaviors.
They represent biases that, in the
distant past, served men’s and
women’s reproductive fitness well.
However, humans exert greater in-
tentional control over sexual be-
havior than any other animal, and
the evolved sexual and child-care
strategies of men and women are
not invariantly manifested, but in-
stead are responsive to physical
and social environments over the
course of development. From this
perspective, nature and nurture are
not alternative explanations, but
instead are two sides of the same
explanatory coin.
Note
1. Address correspondence to either
David F. Bjorklund, Department of
Psychology, Florida Atlantic Uni-
versity, Boca Raton, FL 33431, e-mail:
dbjorklund@fau.edu, or Todd K.
Shackelford, Division of Science—
Psychology, Florida Atlantic Uni-
versity, 2912 College Ave., Davie, FL
33314, e-mail: tshackel@fau.edu.
References
Baker, R.R., & Bellis, M.A. (1995). Human sperm
competition. London: Chapman & Hall.
Belsky, J., Steinberg, L., & Draper, P. (1991).
Childhood experience, interpersonal develop-
ment, and reproductive strategy: An evolu-
Acknowledgments—We would like to
thank David Buss, David Geary, Erika
Hoff-Ginsberg, Martha Hubertz, Gregg
LeBlanc, Santo Tarantino, Robin
Vallacher, and Viviana Weekes, who pro-
vided helpful comments and sugges-
tions that improved this article.
CONCLUSIONS
INHIBITORY ABILITIES
Published by Blackwell Publishers, Inc.
Recommended Reading
Bjorklund, D.F., & Kipp, K. (1996).
(See References)
Buss, D.M. (1994). (See References)
Buss, D.M. (1999). Evolutionary psy-
chology: The new science of the
mind. Needham Heights, MA:
Allyn & Bacon.
Geary, D.C. (1998). (See References)
Trivers, R. (1972). (See References)
CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 89
tionary theory of socialization. Child
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Bjorklund, D.F., & Harnishfeger, K.K. (1995). The
role of inhibition mechanisms in the evolution
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and inhibition in cognition (pp. 141–173). New
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Copyright © 1999 American Psychological Society
The phenomena of attempted
and completed suicide are trou-
bling and mysterious enough in
themselves; the possibility that sui-
cide is socially contagious, even
more so. This article considers
whether suicide clusters exist, and
if so, whether “contagion” process-
es can account for them.
There is a potentially important
distinction between the terms sui-
cide cluster and suicide contagion.
A cluster refers to the factual oc-
currence of two or more complet-
ed or attempted suicides that are
nonrandomly “bunched” in space
or time (e.g., a series of suicide at-
tempts in the same high school or
a series of completed suicides in
response to the suicide of a
celebrity). The term cluster implies
nothing about why the cluster
came to be, only that it came to be.
By contrast, contagion refers to a
possible explanation (as I argue
later, a fairly vague explanation) of
why a cluster developed. Clusters
(of a sort) appear to occur, but the
status of contagion as the reason
for such occurrences is more
equivocal.
Given that attempted and com-
pleted suicides are relatively rare,
and given that they tend to be
more or less evenly distributed in
space and time (e.g., suicides occur
at roughly the same rate in various
regions of the United States and
occur at roughly the same rate re-
gardless of the day of the week or
the month), it is statistically un-
likely that suicides would cluster
by chance alone. Yet cluster they
do, at least under some circum-
stances. (Such clustering is often
termed the “Werther effect,” after
a fictional character of Goethe’s
whose suicide purportedly in-
spired actual suicides in 18th-cen-
tury Europe.) Two general types of
suicide cluster have been dis-
cussed in the literature: mass clus-
ters and point clusters. Mass clus-
ters are media related; point
clusters, local.
CLUSTERS—OF A SORT—
APPEAR TO OCCUR
The Clustering and Contagion
of Suicide
Thomas E. Joiner, Jr.1
Department of Psychology, Florida State University, Tallahassee, Florida
Abstract
Two general types of sui-
cide cluster have been dis-
cussed in the literature;
roughly, these can be classi-
fied as mass clusters and
point clusters. Mass clusters
are media related, and the ev-
idence for them is equivocal;
point clusters are local phe-
nomena, and these do appear
to occur. Contagion has not
been conceptually well devel-
oped nor empirically well
supported as an explanation
for suicide clusters. An alter-
native explanation for why
suicides sometimes cluster is
articulated: People who are
vulnerable to suicide may
cluster well before the occur-
rence of any overt suicidal
stimulus, and when they ex-
perience severe negative
events, including but not lim-
ited to the suicidal behavior of
one member of the cluster, all
members of the cluster are at
increased risk for suicidality
(a risk that may be offset by
good social support).
Keywords
suicide clusters; suicide con-
tagion
