MATING EFFORT The behavioral ecological study of human mating patterns has shed light on many classic topics in sociocultural and biological an- thropology (for recent reviews, see 17, 34, 88, 119). Topics covered here include the causes of polygyny and polyandry, variations in marriage transac- tions, and sociocultural change. Space limitations preclude a discussion of other important topics, including monogamy (21), divorce (20), physical sexual dimorphism (e.g. 85), sex differences in mate preferences and sexual behavior (39, 71a), and sex differences in spatial ability (156).
Mating systems and mate choice Mating systems are the outcome of in- teractions between male and female strategies. Studies of mating systems have tended to emphasize either female strategies for selecting males or male strategies for attracting females. Among humans, kin of the bride and groom also often have a great deal of influence on mate choice, and their influence on mating decisions is a little-explored but important area for new research.
HUMAN BEHAVIORAL ECOLOGY 35 The mating strategies of human males can sometimes produce spectacular results. For example, despotic rulers have routinely accumulated large harems and possessed extraordinary rights of sexual access to subject females (16). In politically less centralized societies force also is sometimes a factor in male mating success (e.g. 49, 78; see also 60, 168), but other factors such as manipulation of kin networks (48), hunting skill (112), religious status (140), and especially resource accumulation (30, 57, 116) are more frequently identified as the key variables. Even kinship terms can be the subject of male mating strategies. Yanomamo males have been shown to manipulate their Iroquois kinship terminology in an effort to obtain more mating opportunities, frequently moving women from nonmarriageable categories like "sister" and
"niece" to the marriageable category of cross-cousin (i.e. cousins linked by a brother and sister rather than by siblings of the same sex)(50).
Females may choose among males according to a variety of variables, including the resources they control. According to the polygyny threshold model (145), polygyny may result if males vary in terms of the resources they control enough to allow a female to raise as many or more offspring by mating with already mated males with high quality resources than with bachelors with poor resources. This helps explain polygyny among Kipsigis women, who prefer men who can offer them access to more land, even when controlling for the man's marital status and his overall wealth (33; see also 183).
Variations in male control of resources may also help to explain polyandry.
Among Tibetans, fraternal polyandry offers a way to deal with a shortage of arable land and a lack of economic options. The peasants of the Ladakh region can farm only in small fertile areas, and subdividing their estates to provide each son with a separate inheritance would lead to many small, nonviable farms. In some societies, this problem has been handled with rules of pri- mogeniture or ultimogeniture, but the Ladakhis developed instead a system of fraternal polyandry. Although polyandrous Tibetan families are highly fertile, younger brothers may suffer reproductively from the arrangement since they often do not reach sexual maturity until the wife they share is well into her reproductive career, and they often opt for monogamy over polyandry when economic development makes it possible (58).
Human females may also choose males based on their apparent resistance to pathogens, an idea supported by a cross-cultural correlation between degree of polygyny and pathogen stress (94, 134). Though intriguing, this can be further tested only with detailed studies of marital success, pathogen stress, and mate selection in individual societies. Furthermore, because resources, pathogen resistance, and other variables might influence mate choice simulta- neously, more studies are needed that measure the relative importance of such factors in specific societies.
The models discussed above may be inadequate with respect to many
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human societies owing to the influence of kin on marriage decisions. The preferences of kin are often expressed in the form of prohibitions, pre- scriptions, and preferences about sexual and marriage partners, including incest taboos (see 167) and cousin marriage preferences (47, 76; see also 65, 1 18). Even where marriage rules are few and marriages not arranged, kin can have important influences on mating decisions. For example, male Trinida- dian villagers have high rates of agonistic interactions with their daughters and their potential suitors. Such daughter guarding seems to help ensure the daughter's reputation of chastity, allaying suitors' fears of cuckoldry and helping the daughter to attract and retain a more desirable male. Young women with resident fathers are more than four times as likely to establish stable marriages with prosperous males than are young women without resident fathers (79). Because kin influences on mating decisions are crucial to understanding human mating patterns, they should have a high priority for future research, particularly in situations with clear conflicts of interest between the marriage partners and their kin.
Marriage transactions Behavioral ecologists have examined several types of marriage transactions, including direct exchange, bridewealth, and dowry.
A first step toward an analysis of direct exchange is Chagnon's (47, 48) work on how Yanomamo descent groups act as coalitions of related males, assisting one another in mate competition, with pairs of lineages building up reciprocal obligations to exchange mates (see also 118). This raises a number of questions for future research. Is there an optimum lineage size for the mating success of lineage members? What are the reproductive costs and benefits involved in the founding of new lineages? How are long-term exchange relationships between lineages begun and managed? Game theoretical models (see below) should prove useful in guiding research on such questions.
Borgerhoff Mulder (31) hypothesized that variations in bridewealth pay- ments among the Kipsigis might be explained by the bride's reproductive value, the value of her labor, and the relative status of the families involved.
Reproductive value was estimated by examining age at first menses, which correlates with lifetime reproductive success among Kipsigis women, and women's plumpness. The brides' labor value was estimated by dividing women into those who would and those who would not be living close enough to their natal homes after marriage to allow them to continue to contribute labor. Relative family status was estimated by means of wealth holdings.
Borgerhoff Mulder's analysis showed that both reproductive value and labor had measurable effects on bridewealth payments, with higher bridewealths being paid for earlier-maturing, plumper brides and for brides moving far from their natal households. The unimportance of relative family status may be due to the instability of wealth differentials among the Kipsigis. Dowry,
HUMAN BEHAVIORAL ECOLOGY 37 much less common than bridewealth, tends to appear among the upper levels of stratified societies. Gaulin & Boster (86; see also 65) conducted a success- ful holocultural test of the hypothesis that dowry is a mating strategy used by brides and their kin to attract the wealthiest bridegrooms in stratified societies.
Mating strategies and sociocultural change Adaptive changes in mating strategies may help explain sociocultural change. For example, the Mukogo- do of Kenya underwent a rapid shift from foraging to pastoralism early in this century as a result of changes in their regional marriage system. Before the 1920s, they lived in caves as foragers, spoke a unique language, married only among themselves, and paid beehives as bridewealth. Changes wrought by British colonization sparked a rapid increase in the rate of intermarriage between the Mukogodo and several groups of Maasai-speaking pastoralists;
but, because the Mukogodo had few livestock and the Maasai-speakers would not accept beehives as bridewealth, many more of their women married men from other groups than vice versa. Mukogodo men were under pressure either to become pastoralists or to risk reproductive failure, and their difficulties are reflected in a high rate of wifelessness: Among men who began looking for mates during the transition, about one third never married. The result was a rapid shift not only in subsistence patterns but also in language, religion, territorial organization, and social relations (54).
Cultural change also appears to enhance reproductive success among the Shipibo Indians of the Peruvian Amazon (106). Monogamy is becoming more common than polygyny among the Shipibo. As a result of shorter postpartum sexual abstinence and shorter interbirth intervals, women married monogam- ously have higher fertility than those married polygynously. Is monogamy therefore a reproductive strategy for the Shipibo, or is it just a reproductively fortuitous but unplanned result of culture change? What was discouraging monogamy before missionization? Pre-contact polygyny may have been due to variations among males in such favorable characteristics as pathogen resistance or economic success, or the Shipibo environment may have been changed in a crucial way by the increased availability of modern medicine or by economic development. Polygyny is associated with long interbirth in- tervals and lower child and maternal mortality. Before the introduction of modem medicines, monogamy may have been a worse strategy than polygyny since it could have increased child and maternal mortality. This idea remains to be tested, but it may be that the Shipibo are shifting reproductive strategies as their epidemiological environment changes.
PARENTAL EFFORT Major areas of research discussed here are sex-biased parental investment, inheritance practices, paternal behavior, and parent- offspring conflict. Other notable topics include acculturation practices (132)
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and adopting out as a parental strategy (18; for reviews, see 17, 34, 35, 88, 119).
Sex-biased parental investment Sex biases in parental solicitude, child abuse and neglect, and infanticide are well documented among both humans and nonhumans (e.g. 100 and below). The Trivers-Willard model is an often used explanation of this phenomenon (173). In its broadest formulation, the Trivers-Willard model predicts that if the condition of mothers during the period of parental investment correlates with the probable reproductive suc- cess of their offspring, natural selection should favor the ability of parents to adjust their investment in the sexes to favor the sex with the best reproductive prospects. More specifically, Trivers and Willard predicted that, when the reproductive success of males is more variable than that of females and males benefit more than females from good maternal condition and suffer more than females from poor maternal condition, males should be favored by mothers in good condition and females by mothers in poor condition. Among humans, Trivers and Willard suggested that socioeconomic status may be a good predictor of parental investment patterns if reproductive success of males is greater than that of females at the upper end of the hierarchy and lower than that of females at the lower end (see 88, 154).
The Trivers-Willard model has been used to explain biased sex ratios and parental investment patterns in a wide variety of historical and ethnographic settings (24, 29, 55, 56, 64, 141, 180). The focus of most of these studies is male bias among high-status groups. Voland (180), for example, found that in Schleswig-Holstein between 1720 and 1869 the reproductive prospects of males correlated with socioeconomic status, but those of females remained constant. High-status farmers appear to have responded to this by favoring their sons over their daughters. In the highest class, infant mortality was higher among females, while it was lower among females in all other classes (but see 154). The poor, low-status Mukogodo also fulfill the predictions of the Trivers-Willard model. Owing to their low status, Mukogodo males have poor reproductive prospects, but Mukogodo females have no trouble finding husbands and often marry hypergynously; and Mukogodo parents raise more daughters than sons. The sex ratio bias is probably produced after birth through better treatment of daughters, judging from visits to health facilities and nursing patterns (55).
Data from the Krummhmrn region of Germany in the 18th and 19th centuries provide an enlightening contrast with those from Schleswig- Holstein and the Mukogodo. In the Krummhorn, infant mortality rates sug- gest that high-status families biased their investment in favor of daughters, a pattern that corresponds with the relative reproductive prospects of sons and daughters. Due to land scarcity, only the youngest son of a farmer inherited
HUMAN BEHAVIORAL ECOLOGY 39 any land, and as a result sons of landed farmers were more likely to remain lifelong celibates than those of the landless. The daughters of landed farmers, however, were more likely to marry than those of the landless (184) . A much needed complement to the studies mentioned here would be analyses of the proximate psychological mechanisms involved in sex-biased parental be- havior, which would clarify the circumstances surrounding the evolution of such behavior and the reasons why particular models seem to work in some cases but not in others.
Inheritance patterns Sex biases are also common in inheritance practices, with inheritance by males being the most common pattern. Hartung (96) hypothesized that in polygynous societies sons are often favored over daught- ers in inheritance because they stand to benefit more reproductively from wealth than do their sisters. The males' advantage results from their ability to marry polygynously. Hartung's idea is supported by strong cross-cultural correlations between polygyny and male-biased inheritance (98). At first glance, the Kipsigis appear to fit Hartung's model: They are polygynous, pay bridewealth, and allow only sons to inherit land and livestock. However, Borgerhoff Mulder (32) has shown that although sons do benefit reproductive- ly more than daughters from their parents' ownership of cattle, both sons and daughters benefit from parental land ownership, raising the question of why the Kipsigis do not pass land on to both daughters and sons. One problem is that land inheritance by daughters would lead to the breakup of the patrilineal corporate landowning group and also conflict with their custom of virilocal postmarital residence. Kipsigis parents also have other ways of benefiting their daughters, such as through gifts of food and through marriage negotia- tions. Cultural inertia may be involved. Until the 1930s, the Kipsigis held land communally, and only livestock were inherited. While it was simple to extend the sons-only inheritance rule to land, the idea of allowing daughters to inherit land seems preposterous to Kipsigis parents. Recently, however, some Kipsigis have begun to give or sell land to their matrilocally resident sons-in- law, allowing them to maintain a male-biased fa?ade while giving their daughters access to additional real property (T. MacMillan, personal com- munication).
Another influence on inheritance patterns may be paternity certainty. In societies where paternity certainty tends to be low, a typical man's genetically closest kin in the next generation are likely to be his sisters' children rather than his wife's. The idea that this might help explain matriliny and the avunculate has existed for centuries. The Arab chronicler al-Bakri, for ex- ample, writing in the 11th century, noted that the throne of ancient Ghana was inherited by the king's sister's son because the king "has no doubt that his successor is a son of his sister, while he is not certain that his son is in fact his
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own" (1, p. 79). Cross-cultural tests (76, 87, 97, 99; see also 88) of this idea have had mixed results and are beset by methodological and theoretical problems. Society-wide levels of paternity certainty are difficult to measure with the existing cross-cultural databases, and their meanings are difficult to interpret. Furthermore, rarely does a society have the extremely low level of paternity certainty necessary to make any man's sisters' children his geneti- cally closest descendants (see 88). This model also does not address the causes of variations in paternity certainty. In an effort to get around some of these problems, Hartung (99) has examined matrilineality as a female in- heritance strategy, pointing out that matrilineal inheritance is to females' advantage whenever paternity is at all in doubt because anything inherited through males may be wasted owing to cuckoldry. All these models leave open the question of how conflicts of interest among kin over inheritance patterns are settled, a question that may best be answered through the use of game theoretical models and detailed studies in specific societies.
Paternal behavior Despite the importance of male parental investment among humans, few detailed studies exist of paternal behavior in traditional societies. Hewlett's (108) work among the Aka pygmies of central Africa is a notable exception. He found that lower-status Aka men with few resources to offer their wives and children compensated by spending more time in direct child care than higher-status men, suggesting that even in simple, superficial- ly homogenous societies individuals can be expected to follow a variety of different reproductive strategies (see also 107, 112, 148).
Parent-offspring conflict Although the reproductive interests of parents and offspring overlap greatly, they also may conflict (172). Flinn (80) has documented such reproductive conflict between mothers and daughters in a Trinidadian village (see also 175). Because both mothers and daughters benefit from help from co-resident nonreproductive females, when both are potentially reproductive, both should want to reproduce directly and receive undivided help from the other. The rate of agonistic interactions between daughters and their parents and daughters' reproductive rates reflect this conflict: Daughters aged 14-25 with reproductive mothers experienced more such interactions and lower fertility than daughters with nonreproductive mothers. It is unclear exactly how daughters' fertility is suppressed, but the possibility that it is the result of stress induced by agonistic interactions should be explored, perhaps by using salivary steroids to assess stress levels and ovarian function. Many other opportunities exist for studies of parent-off- spring conflict. Gerontocratic societies offer especially clear opportunities to study reproductive conflicts between fathers and sons.
HUMAN BEHAVIORAL ECOLOGY 41 Indirect reproduction Because related individuals share genes with their codescendant kin, they can reproduce not only directly but also indirectly by helping their kin to reproduce (93). Selection will favor such behavior when the costs to the actor in terms of reproduction are less than the benefits to the recipient, devalued by the coefficient of relatedness (e.g. 0.5 for full siblings and 0.125 for first cousins), an inequality known as Hamilton's rule. Hamil- ton's theory of indirect reproduction is also known as kin selection, inclusive fitness theory, and nepotism theory. Many studies have demonstrated that humans are generally nepotistic-i.e. that they tend to help others in accor- dance with genetic relatedness (e.g. 9, 23, 48, 60, 74, 89, 101, 112, 115, 155; see 34, 88, 95, 119), but full tests of the theory require measurements of the reproductive costs and benefits involved in helping behaviors as well.
Steps toward this goal have been taken by Flinn (80; see above), Berte' (15), and Turke (175). Berte (15) showed that success of maize cultivation among the K'ekchi' Maya depends on household heads' abilities to secure the aid of locally resident genealogical kin. Men with more such kin had higher crop yields, and household heads who were highly related to the local pool of potential laborers had higher reproductive success. Turke (175) demonstrated the reproductive benefits of aid to kin in a study of daughters and grandparents as helpers at the nest on Ifaluk. Ifalukese women who bear daughters first have higher reproductive success than those who bear sons first, and in- dividuals' reproductive success correlates with their numbers of living par- ents. Daughters, however, pay a price in that their reproductive success correlates negatively with their numbers of siblings. The question of whether daughters are compensated for their loss of direct reproduction by an increase in indirect reproduction remains to be answered.
Hamilton's theory may also be applied to situations where aid to con- sanguineal kin is channeled through intermediaries, such as affinal kin, which may help to explain the importance of aid among affines in many societies (67, 114). Although affines do not share genes by common descent, they share a genetic stake in their common descendants and should therefore be expected to cooperate. Dow (67) has quantified such relationships and reanalyzed Hawkes's (101) data on gardening aid among the Binumarien of New Guinea. Further development of this idea may eventually provide a useful extension of Hamilton's rule.