86 VOLUME 8, NUMBER 3, JUNE 1999 time men spend in child care varies Differences in Parental Investment across cultures, fathers spend less Contribute to Important Differences time interacting with and caring for their children than do mothers in Between Men and Women all cultures that have been studied 1 (see Geary, 1998). David F. Bjorklund and Todd K. Shackelford Yet human males spend more Department of Psychology, Florida Atlantic University, Boca Raton, Florida time caring for their offspring than is typical for male mammals. One factor contributing to this unusually cludes evolved psychological Abstract high level of paternal investment is mechanisms that require input Parental investment theory humans’ extended childhood. such as social norms and cultural addresses sex differences that Humans spend more time as juve- beliefs for their operation, and dif- result from the trade-off be- niles than other primates, needing ferent behavioral outcomes will re- tween parenting effort and many years to develop the brain sult from these evolved mecha- mating effort. For example, and the knowledge necessary to nisms in different environments. In relative to men, women spend navigate the complexities of society. this article, we apply a theory moti- more time caring for offspring, This prolonged immaturity requires vated by such evolutionary psy- are more selective in assenting a supportive environment to which chological logic, parental invest- to sexual intercourse, are more both mothers and fathers ideally ment theory (Trivers, 1972), to upset by a partner’s emotional contribute. Evidence from many interpret some well-documented infidelity than by a partner’s cultures and historical records indi- sex differences. sexual infidelity, and are better cates that the death rate of offspring According to an evolutionary able to inhibit their behaviors increases as a function of father’s psychological perspective (see in certain situations. These and absence, particularly in harsh envi- Buss, 1994), ancestral men and other sex differences are attrib- ronments (see Geary, 1998). Thus, women faced different adaptive utable to evolved mechanisms whereas mothers opt to invest sub- problems that threatened their sur- that work in interaction with stantially in their children’s care vival and reproduction. As a result, the physical and social envi- after birth in order to ensure their they evolved different psychologi- ronments. offsprings’ survival, the case is not cal mechanisms, and evidence of as clear-cut for fathers, particularly this ancient heritage is apparent in Keywords when one considers the number of modern humans (for a review, see parental investment theory; additional offspring men can have Buss, 1994). Parental investment evolutionary psychology; sex- by investing more effort in mating. theory accounts for many of these ual strategies Women also may choose to invest differences. According to this theo- less in the care of a child and more ry, there is a conflict for both males in mating, but the caloric costs and Sex differences in behaviors tra- and females in how much time, ef- duration of pregnancy and nursing ditionally have been attributed to fort, and resources to invest in mat- (in traditional societies today, and “social” factors, because of the pre- ing versus parenting. For many surely in our evolutionary past, sumed flexibility of human behav- species, including humans, males nursing extends to the age of 3 or 4 ior. Proposals that evolved mecha- need to invest substantially less in years) reduce the number of chil- nisms explain why men and parenting than females to achieve dren a woman can expect to have in women behave as they do have successful reproduction. In mam- her lifetime. been regarded warily because of mals, fertilization and gestation There is evidence that paternal the belief that “biological” causa- occur within the female, and, after investment influences socialization tion (including evolutionary caus- birth, mothers provide the primary practices and the amount of es) implies biological determinism. nutritional support for their off- parental investment subsequent Modern evolutionary psychologi- spring until they are weaned. Male generations devote to their off- cal theory, however, makes no such investment in offspring may be as spring (Belsky, Steinberg, & Draper, claims, but argues that contrasting little as the sperm produced during 1991). In environments where fa- “social” with “evolutionary” expla- copulation. In humans, paternal in- thers are absent or where there is nations is a false dichotomy (see vestment is not essential for a marital discord, the resulting stress Tooby & Cosmides, 1992). From man’s offspring to reach adult- produces harsh and inconsistent this perspective, human nature in- hood, and although the amount of child care and insecure attachment. Published by Blackwell Publishers, Inc. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 87 Under such situations, children ancestral females who were selec- vestment of the female comes at reach puberty early, form short- tive in mating. Because of the long considerable cost but brings with it term and unstable relationships, period of immaturity in humans, certainty of maternity. In contrast, and invest relatively little in their ancient women’s reproductive in- males, who may invest only sperm own offspring. In stressful and un- terests were often best served by and the energy necessary to copu- certain environments, there is a ten- selecting a mate who not only late, cannot be certain of paternity. dency to invest more in mating (for would provide good genes (e.g., as Furthermore, because women con- both sexes) than in parenting. The signaled by facial symmetry; ceal ovulation and are potentially pattern is reversed for children Shackelford & Larsen, 1997), but sexually receptive throughout their growing up in harmonious homes who also would invest in her and menstrual cycle, it is difficult for a and homes where the father is pres- her offspring. Over evolutionary man to know when copulation is ent; such children mature later, time, it was also in men’s reproduc- likely to result in pregnancy. As a postpone sexual activity, and dis- tive interests to see to it that their result, being the unwitting social play greater investment in the fewer offspring received the support nec- father to another man’s genetic off- children they produce. In sum, the essary to survive to reproductive spring is a possibility for men. This availability of resources, which is age. But the amount and duration in fact occurs in between 2% and related to paternal investment and of investment necessary to ensure 30% of all births; the rate is similar spousal harmony, leads to different the survival of offspring was less in traditional societies, and there is patterns of socialization, resulting for men than for women. Thus, al- no reason to believe that it differed in differential parental investment though both men and women substantially for our ancestors (see in the next generation. shared a common reproductive Baker & Bellis, 1995). Women, of The differential investment of goal (getting their offspring to course, cannot so easily be fooled males and females in their off- adulthood), the optimal level of in- into rearing another woman’s child spring has resulted in the evolution vestment to achieve this goal was who they believe is their own, al- of different ways of behaving and unequal for the sexes, placing though they do risk losing a mate’s thinking in men and women. males’ and females’ reproductive investment to another woman. Although these differences are gen- interests in conflict. One consequence of this sex dif- erated by evolved psychological Men are not indiscriminate ference in the certainty of genetic mechanisms, they are more vari- when it comes to selecting a mate, parenthood can be seen in men’s able than is typical in mammals. especially when selecting a long- and women’s reactions to a long- Here, we discuss two areas in term partner. Men around the term partner’s infidelity. When which men and women behave and world want long-term partners asked to imagine that their long- think differently, as predicted by who are attractive, intelligent, and term partner is either (a) having ca- parental investment theory: sexual kind. Women also desire attractive, sual sex with another person or (b) strategies and inhibitory abilities. intelligent, and kind men as hus- developing a close emotional rela- bands, but, as predicted by tionship with another person, men parental investment theory, they and women respond differently. SEXUAL STRATEGIES rate financial resources as more im- Verbal reports and measures of portant in a mate than do men physiological arousal indicate that (Buss, 1989). Despite the selectivity women are more upset by a part- One class of sex differences that that men and women universally ner’s emotional infidelity, which can be understood in terms of display in choosing a long-term could signal a loss of resources, parental investment theory is sexu- partner, and consistent with whereas men are more upset by a al strategies (Buss, 1994). Males in parental investment theory, women partner’s sexual infidelity (Buss, most mammals can achieve are more selective in granting sex, Larsen, Westen, & Semmelroth, tremendous reproductive success and men are more eager to have ca- 1992). by inseminating many females, sual sex (see Buss, 1994). In conclusion, human sex differ- making males relatively indiscrim- Men and women have